Triac Efficiency...

[boB]

On Wed, 07 Jun 2023 10:57:48 -0700, John Larkin
<jlarkin@highlandSNIPMEtechnology.com> wrote:

On Wed, 07 Jun 2023 10:33:01 -0700, boB <boB@K7IQ.com> wrote:

On Wed, 07 Jun 2023 07:09:09 -0700, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Tue, 06 Jun 2023 20:36:01 -0700, boB <boB@K7IQ.com> wrote:

On Mon, 05 Jun 2023 19:33:17 +0100, Cursitor Doom <cd@notformail.com
wrote:

On Mon, 05 Jun 2023 11:04:03 -0700, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Mon, 05 Jun 2023 11:01:33 -0700, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Mon, 05 Jun 2023 18:54:16 +0100, Cursitor Doom <cd@notformail.com
wrote:

On Mon, 05 Jun 2023 18:50:38 +0100, Cursitor Doom <cd@notformail.com
wrote:

On Mon, 05 Jun 2023 09:21:32 -0700, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Mon, 5 Jun 2023 12:10:05 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2023-06-04 23:05, John Larkin wrote:
On Mon, 05 Jun 2023 00:03:20 +0100, Cursitor Doom <cd@notformail.com
wrote:

On Sun, 4 Jun 2023 15:45:44 -0700 (PDT), Ricky
gnuarm.deletethisbit@gmail.com> wrote:

On Sunday, June 4, 2023 at 6:13:39?PM UTC-4, John Larkin wrote:
On Sun, 04 Jun 2023 22:34:00 +0100, Cursitor Doom <c...@notformail.com
wrote:
Greetings, gentlemen,

As we all know, to get maximum efficiency out of a MOSFET, it needs to
be driven hard and fast and spend as little time between fully on and
fully off as possible. Does that same principle extend to Triacs? And
if so, how can that best be implemented when one has to use AC to
drive the gate?

CD.
Once a triac fires, the gate drive doesn\'t matter any more.

But they do need a hard, fast drive to fire properly, long enough to
let the drain current rise above the latch limit.

AC gate drive will be slow-rise, which could just possibly damage the
triac if the load current is high.

That should be on the data sheets.

I believe that is why a diac is typically used in the control circuit.

I thought diacs were sometimes used because triacs don\'t fire
symetrically, but I could be wrong.
So, as regards AC drive, is a higher frequency drive signal a safer
bet? Also, the load is AC too, so the thing will turn off at every
zero-crossing point, which makes life more complicated. :-(

With an AC load, an AC gate drive is probably OK. The triac will turn
on before the voltage gets too high, so the spreading damage mechanism
won\'t happen much.

The bad case is when there\'s a lot of voltage across the device with a
lot of instantaneous current behind it, and it gets a slow, wimpy
trigger.

Post your proposed circuit and we can discuss it.

I still call the terminals anode, gate, cathode. I can never remember
what\'s MT1 and MT2. Top and bottom would be OK too.


The triac is one of those parts where folks get confused by fossilized
analogies.

A triac is a bidirectional thyristor, but it does not behave like two
SCRs in antiparallel. SCR triggering only works in one quadrant, for a
start, so two antiparallel SCRs would be a two-quadrant device, whereas
triacs trigger in all four quadrants.

The triggering also isn\'t that symmetrical, because the structures
aren\'t--the second and fourth quadrant triggering mechanisms are fairly
different, and the four-layer stacks aren\'t well separated, as they
would be in two SCRs.

Triacs are also very slow even compared with SCRs, which are pretty poky
devices by modern standards.

Someplace I have a paper that goes into the details of real triac
operation--iirc it\'s a good read, but I\'m not laying my hands on it at
the moment.

(I haven\'t actually used a triac since I was a teenager, so this is just
out of general interest.)

Cheers

Phil Hobbs

I haven\'t designed in a triac for maybe 15 years now. I used a couple
of triacs to soft-start a giant power supply that had a 60 Hz power
transformer. Switch in a giant resistor on the primary side, wait a
second, then short it out. That was hard on resistors but prevented
the occasional 1000 amp startup surge. And allowed a little C&K power
switch on the front panel.

The customer insisted that we not use a switching power supply!

Real relays are usually cheaper than triacs, easier to drive, and
don\'t need heat sinks.

Yes, but you can\'t fire them on and off anything like as fast and they
don\'t have the same longevity AFAIK.

Plus a relay capable of switching 16A is going to be a hog, take up
loads of space compared to a triac and cost a hell of a lot more, I
would imagine.

It\'s easy to look up. Digikey has 15 amp relays starting around 60
cents.

The triac will need a heat sink, which will be big and cost more than
the triac.

Relays don\'t need isolated gate drivers, TVS/MOV protection, or
snubbers either.


Well, you got me there!


I have seen and used snubber across relays. To extinguish DC arcs
across the contacts. Sometimes you have to increase contact life.

Do you know why a snubber extinguishes a DC arc?

Yes. Of course... An R-C-D snubber.

The C has been discharged by the R when the relay opens.
Then, the C, an electrolytic typically, (large-ish C) charges up
instead of the contacts taking the arc. The C is a short across the
contacts when the contacts open.

Then, the R discharges the Cap so it is ready for the next opening.

So the snubber prevents the arc but doesn\'t extinguish one. The R-C-D
will be polarity sensitive. And if the contacts cycle before the
large-ish C discharges, the snubber won\'t work.

What sort of RC values are good?

The R has to be small enough to discharge the capacitor in enough time
for the next trigger which is plenty long.

The capacitor is fairly large. Several microfarads as well we several
hundred volts DC in this case.

It does work and worked very well.

boB

 

[John Larkin]

On Thu, 08 Jun 2023 13:46:05 -0700, boB <boB@K7IQ.com> wrote:

On Wed, 07 Jun 2023 10:57:48 -0700, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Wed, 07 Jun 2023 10:33:01 -0700, boB <boB@K7IQ.com> wrote:

On Wed, 07 Jun 2023 07:09:09 -0700, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Tue, 06 Jun 2023 20:36:01 -0700, boB <boB@K7IQ.com> wrote:

On Mon, 05 Jun 2023 19:33:17 +0100, Cursitor Doom <cd@notformail.com
wrote:

On Mon, 05 Jun 2023 11:04:03 -0700, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Mon, 05 Jun 2023 11:01:33 -0700, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Mon, 05 Jun 2023 18:54:16 +0100, Cursitor Doom <cd@notformail.com
wrote:

On Mon, 05 Jun 2023 18:50:38 +0100, Cursitor Doom <cd@notformail.com
wrote:

On Mon, 05 Jun 2023 09:21:32 -0700, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Mon, 5 Jun 2023 12:10:05 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2023-06-04 23:05, John Larkin wrote:
On Mon, 05 Jun 2023 00:03:20 +0100, Cursitor Doom <cd@notformail.com
wrote:

On Sun, 4 Jun 2023 15:45:44 -0700 (PDT), Ricky
gnuarm.deletethisbit@gmail.com> wrote:

On Sunday, June 4, 2023 at 6:13:39?PM UTC-4, John Larkin wrote:
On Sun, 04 Jun 2023 22:34:00 +0100, Cursitor Doom <c...@notformail.com
wrote:
Greetings, gentlemen,

As we all know, to get maximum efficiency out of a MOSFET, it needs to
be driven hard and fast and spend as little time between fully on and
fully off as possible. Does that same principle extend to Triacs? And
if so, how can that best be implemented when one has to use AC to
drive the gate?

CD.
Once a triac fires, the gate drive doesn\'t matter any more.

But they do need a hard, fast drive to fire properly, long enough to
let the drain current rise above the latch limit.

AC gate drive will be slow-rise, which could just possibly damage the
triac if the load current is high.

That should be on the data sheets.

I believe that is why a diac is typically used in the control circuit.

I thought diacs were sometimes used because triacs don\'t fire
symetrically, but I could be wrong.
So, as regards AC drive, is a higher frequency drive signal a safer
bet? Also, the load is AC too, so the thing will turn off at every
zero-crossing point, which makes life more complicated. :-(

With an AC load, an AC gate drive is probably OK. The triac will turn
on before the voltage gets too high, so the spreading damage mechanism
won\'t happen much.

The bad case is when there\'s a lot of voltage across the device with a
lot of instantaneous current behind it, and it gets a slow, wimpy
trigger.

Post your proposed circuit and we can discuss it.

I still call the terminals anode, gate, cathode. I can never remember
what\'s MT1 and MT2. Top and bottom would be OK too.


The triac is one of those parts where folks get confused by fossilized
analogies.

A triac is a bidirectional thyristor, but it does not behave like two
SCRs in antiparallel. SCR triggering only works in one quadrant, for a
start, so two antiparallel SCRs would be a two-quadrant device, whereas
triacs trigger in all four quadrants.

The triggering also isn\'t that symmetrical, because the structures
aren\'t--the second and fourth quadrant triggering mechanisms are fairly
different, and the four-layer stacks aren\'t well separated, as they
would be in two SCRs.

Triacs are also very slow even compared with SCRs, which are pretty poky
devices by modern standards.

Someplace I have a paper that goes into the details of real triac
operation--iirc it\'s a good read, but I\'m not laying my hands on it at
the moment.

(I haven\'t actually used a triac since I was a teenager, so this is just
out of general interest.)

Cheers

Phil Hobbs

I haven\'t designed in a triac for maybe 15 years now. I used a couple
of triacs to soft-start a giant power supply that had a 60 Hz power
transformer. Switch in a giant resistor on the primary side, wait a
second, then short it out. That was hard on resistors but prevented
the occasional 1000 amp startup surge. And allowed a little C&K power
switch on the front panel.

The customer insisted that we not use a switching power supply!

Real relays are usually cheaper than triacs, easier to drive, and
don\'t need heat sinks.

Yes, but you can\'t fire them on and off anything like as fast and they
don\'t have the same longevity AFAIK.

Plus a relay capable of switching 16A is going to be a hog, take up
loads of space compared to a triac and cost a hell of a lot more, I
would imagine.

It\'s easy to look up. Digikey has 15 amp relays starting around 60
cents.

The triac will need a heat sink, which will be big and cost more than
the triac.

Relays don\'t need isolated gate drivers, TVS/MOV protection, or
snubbers either.


Well, you got me there!


I have seen and used snubber across relays. To extinguish DC arcs
across the contacts. Sometimes you have to increase contact life.

Do you know why a snubber extinguishes a DC arc?

Yes. Of course... An R-C-D snubber.

The C has been discharged by the R when the relay opens.
Then, the C, an electrolytic typically, (large-ish C) charges up
instead of the contacts taking the arc. The C is a short across the
contacts when the contacts open.

Then, the R discharges the Cap so it is ready for the next opening.

So the snubber prevents the arc but doesn\'t extinguish one. The R-C-D
will be polarity sensitive. And if the contacts cycle before the
large-ish C discharges, the snubber won\'t work.

What sort of RC values are good?


The R has to be small enough to discharge the capacitor in enough time
for the next trigger which is plenty long.

The capacitor is fairly large. Several microfarads as well we several
hundred volts DC in this case.

It does work and worked very well.

boB

Sure, it works to prevent arcs. Buy why does an RC extinguish a DC
arc? Actually, does it?

One hint is that DC arcs are associated with inductance in the loop.
That\'s why you can arc weld with a few car batteries, but you need a
big inductor.

 

[boB]

On Thu, 08 Jun 2023 15:22:03 -0700, John Larkin
<jlarkin@highlandSNIPMEtechnology.com> wrote:

On Thu, 08 Jun 2023 13:46:05 -0700, boB <boB@K7IQ.com> wrote:

On Wed, 07 Jun 2023 10:57:48 -0700, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Wed, 07 Jun 2023 10:33:01 -0700, boB <boB@K7IQ.com> wrote:

On Wed, 07 Jun 2023 07:09:09 -0700, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Tue, 06 Jun 2023 20:36:01 -0700, boB <boB@K7IQ.com> wrote:

On Mon, 05 Jun 2023 19:33:17 +0100, Cursitor Doom <cd@notformail.com
wrote:

On Mon, 05 Jun 2023 11:04:03 -0700, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Mon, 05 Jun 2023 11:01:33 -0700, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Mon, 05 Jun 2023 18:54:16 +0100, Cursitor Doom <cd@notformail.com
wrote:

On Mon, 05 Jun 2023 18:50:38 +0100, Cursitor Doom <cd@notformail.com
wrote:

On Mon, 05 Jun 2023 09:21:32 -0700, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Mon, 5 Jun 2023 12:10:05 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2023-06-04 23:05, John Larkin wrote:
On Mon, 05 Jun 2023 00:03:20 +0100, Cursitor Doom <cd@notformail.com
wrote:

On Sun, 4 Jun 2023 15:45:44 -0700 (PDT), Ricky
gnuarm.deletethisbit@gmail.com> wrote:

On Sunday, June 4, 2023 at 6:13:39?PM UTC-4, John Larkin wrote:
On Sun, 04 Jun 2023 22:34:00 +0100, Cursitor Doom <c...@notformail.com
wrote:
Greetings, gentlemen,

As we all know, to get maximum efficiency out of a MOSFET, it needs to
be driven hard and fast and spend as little time between fully on and
fully off as possible. Does that same principle extend to Triacs? And
if so, how can that best be implemented when one has to use AC to
drive the gate?

CD.
Once a triac fires, the gate drive doesn\'t matter any more.

But they do need a hard, fast drive to fire properly, long enough to
let the drain current rise above the latch limit.

AC gate drive will be slow-rise, which could just possibly damage the
triac if the load current is high.

That should be on the data sheets.

I believe that is why a diac is typically used in the control circuit.

I thought diacs were sometimes used because triacs don\'t fire
symetrically, but I could be wrong.
So, as regards AC drive, is a higher frequency drive signal a safer
bet? Also, the load is AC too, so the thing will turn off at every
zero-crossing point, which makes life more complicated. :-(

With an AC load, an AC gate drive is probably OK. The triac will turn
on before the voltage gets too high, so the spreading damage mechanism
won\'t happen much.

The bad case is when there\'s a lot of voltage across the device with a
lot of instantaneous current behind it, and it gets a slow, wimpy
trigger.

Post your proposed circuit and we can discuss it.

I still call the terminals anode, gate, cathode. I can never remember
what\'s MT1 and MT2. Top and bottom would be OK too.


The triac is one of those parts where folks get confused by fossilized
analogies.

A triac is a bidirectional thyristor, but it does not behave like two
SCRs in antiparallel. SCR triggering only works in one quadrant, for a
start, so two antiparallel SCRs would be a two-quadrant device, whereas
triacs trigger in all four quadrants.

The triggering also isn\'t that symmetrical, because the structures
aren\'t--the second and fourth quadrant triggering mechanisms are fairly
different, and the four-layer stacks aren\'t well separated, as they
would be in two SCRs.

Triacs are also very slow even compared with SCRs, which are pretty poky
devices by modern standards.

Someplace I have a paper that goes into the details of real triac
operation--iirc it\'s a good read, but I\'m not laying my hands on it at
the moment.

(I haven\'t actually used a triac since I was a teenager, so this is just
out of general interest.)

Cheers

Phil Hobbs

I haven\'t designed in a triac for maybe 15 years now. I used a couple
of triacs to soft-start a giant power supply that had a 60 Hz power
transformer. Switch in a giant resistor on the primary side, wait a
second, then short it out. That was hard on resistors but prevented
the occasional 1000 amp startup surge. And allowed a little C&K power
switch on the front panel.

The customer insisted that we not use a switching power supply!

Real relays are usually cheaper than triacs, easier to drive, and
don\'t need heat sinks.

Yes, but you can\'t fire them on and off anything like as fast and they
don\'t have the same longevity AFAIK.

Plus a relay capable of switching 16A is going to be a hog, take up
loads of space compared to a triac and cost a hell of a lot more, I
would imagine.

It\'s easy to look up. Digikey has 15 amp relays starting around 60
cents.

The triac will need a heat sink, which will be big and cost more than
the triac.

Relays don\'t need isolated gate drivers, TVS/MOV protection, or
snubbers either.


Well, you got me there!


I have seen and used snubber across relays. To extinguish DC arcs
across the contacts. Sometimes you have to increase contact life.

Do you know why a snubber extinguishes a DC arc?

Yes. Of course... An R-C-D snubber.

The C has been discharged by the R when the relay opens.
Then, the C, an electrolytic typically, (large-ish C) charges up
instead of the contacts taking the arc. The C is a short across the
contacts when the contacts open.

Then, the R discharges the Cap so it is ready for the next opening.

So the snubber prevents the arc but doesn\'t extinguish one. The R-C-D
will be polarity sensitive. And if the contacts cycle before the
large-ish C discharges, the snubber won\'t work.

What sort of RC values are good?


The R has to be small enough to discharge the capacitor in enough time
for the next trigger which is plenty long.

The capacitor is fairly large. Several microfarads as well we several
hundred volts DC in this case.

It does work and worked very well.

boB

Sure, it works to prevent arcs. Buy why does an RC extinguish a DC
arc? Actually, does it?

It prevents the arc from starting in the first place when the contact
opens. As long as there was at least a few amps of current and some
minimal Voc, an arc can be drawn.

The capacitor is the only component necessary to stop the arc as long
as it was discharged when the contact opens.

DC arcs are super easy to make. Just take a power supply with over,
say, 50V and with a short circuit current (Isc) of a few amps.

Short the leads together or, if needed, add some resistance to limit
voltage across the power supply and pull those leads apart. You will
get an arc that you can sustain until the wire disintegrates and the
arc stops.

Better yet, take a solar panel (in the sun) of 100 to 450 watts and
short the leads together. Then pull the leads apart. Makes very nice
arcs.

Pull them apart quickly and FAR apart and minimal heat and wire damage
will occur but for a relay, that won\'t work. Not without something to
mitigate.

Now, take a newer PV system of several thousand watts and several
hundred volts and do the same thing. Might not be able to even hold
onto the wires in addition to being blinded by the UV.

One hint is that DC arcs are associated with inductance in the loop.
That\'s why you can arc weld with a few car batteries, but you need a
big inductor.

No inductor needed if there is enough voltage. You would need at least
3 car batteries from my experience to get a sustaine arc.

An inductive energy storage arc could also benefit from a snubber of
course but is easier to stop cuz it doesn\'t last long. Those are more
like sparks and will quickly extinguish because of limited energy
stored in the inductor. But enough current and voltage will have the
same problem I am talking about.

We also detect DC arcs and mitigate those. Called arc fault in the PV
industry.

Try drawing DC arcs. They\'re fun ! As long as you don\'t get burned
or blinded.

boB

 

[boB]

On Thu, 08 Jun 2023 18:09:16 -0700, boB <boB@K7IQ.com> wrote:

On Thu, 08 Jun 2023 15:22:03 -0700, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Thu, 08 Jun 2023 13:46:05 -0700, boB <boB@K7IQ.com> wrote:

On Wed, 07 Jun 2023 10:57:48 -0700, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Wed, 07 Jun 2023 10:33:01 -0700, boB <boB@K7IQ.com> wrote:

On Wed, 07 Jun 2023 07:09:09 -0700, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Tue, 06 Jun 2023 20:36:01 -0700, boB <boB@K7IQ.com> wrote:

On Mon, 05 Jun 2023 19:33:17 +0100, Cursitor Doom <cd@notformail.com
wrote:

On Mon, 05 Jun 2023 11:04:03 -0700, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Mon, 05 Jun 2023 11:01:33 -0700, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Mon, 05 Jun 2023 18:54:16 +0100, Cursitor Doom <cd@notformail.com
wrote:

On Mon, 05 Jun 2023 18:50:38 +0100, Cursitor Doom <cd@notformail.com
wrote:

On Mon, 05 Jun 2023 09:21:32 -0700, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Mon, 5 Jun 2023 12:10:05 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2023-06-04 23:05, John Larkin wrote:
On Mon, 05 Jun 2023 00:03:20 +0100, Cursitor Doom <cd@notformail.com
wrote:

On Sun, 4 Jun 2023 15:45:44 -0700 (PDT), Ricky
gnuarm.deletethisbit@gmail.com> wrote:

On Sunday, June 4, 2023 at 6:13:39?PM UTC-4, John Larkin wrote:
On Sun, 04 Jun 2023 22:34:00 +0100, Cursitor Doom <c...@notformail.com
wrote:
Greetings, gentlemen,

As we all know, to get maximum efficiency out of a MOSFET, it needs to
be driven hard and fast and spend as little time between fully on and
fully off as possible. Does that same principle extend to Triacs? And
if so, how can that best be implemented when one has to use AC to
drive the gate?

CD.
Once a triac fires, the gate drive doesn\'t matter any more.

But they do need a hard, fast drive to fire properly, long enough to
let the drain current rise above the latch limit.

AC gate drive will be slow-rise, which could just possibly damage the
triac if the load current is high.

That should be on the data sheets.

I believe that is why a diac is typically used in the control circuit.

I thought diacs were sometimes used because triacs don\'t fire
symetrically, but I could be wrong.
So, as regards AC drive, is a higher frequency drive signal a safer
bet? Also, the load is AC too, so the thing will turn off at every
zero-crossing point, which makes life more complicated. :-(

With an AC load, an AC gate drive is probably OK. The triac will turn
on before the voltage gets too high, so the spreading damage mechanism
won\'t happen much.

The bad case is when there\'s a lot of voltage across the device with a
lot of instantaneous current behind it, and it gets a slow, wimpy
trigger.

Post your proposed circuit and we can discuss it.

I still call the terminals anode, gate, cathode. I can never remember
what\'s MT1 and MT2. Top and bottom would be OK too.


The triac is one of those parts where folks get confused by fossilized
analogies.

A triac is a bidirectional thyristor, but it does not behave like two
SCRs in antiparallel. SCR triggering only works in one quadrant, for a
start, so two antiparallel SCRs would be a two-quadrant device, whereas
triacs trigger in all four quadrants.

The triggering also isn\'t that symmetrical, because the structures
aren\'t--the second and fourth quadrant triggering mechanisms are fairly
different, and the four-layer stacks aren\'t well separated, as they
would be in two SCRs.

Triacs are also very slow even compared with SCRs, which are pretty poky
devices by modern standards.

Someplace I have a paper that goes into the details of real triac
operation--iirc it\'s a good read, but I\'m not laying my hands on it at
the moment.

(I haven\'t actually used a triac since I was a teenager, so this is just
out of general interest.)

Cheers

Phil Hobbs

I haven\'t designed in a triac for maybe 15 years now. I used a couple
of triacs to soft-start a giant power supply that had a 60 Hz power
transformer. Switch in a giant resistor on the primary side, wait a
second, then short it out. That was hard on resistors but prevented
the occasional 1000 amp startup surge. And allowed a little C&K power
switch on the front panel.

The customer insisted that we not use a switching power supply!

Real relays are usually cheaper than triacs, easier to drive, and
don\'t need heat sinks.

Yes, but you can\'t fire them on and off anything like as fast and they
don\'t have the same longevity AFAIK.

Plus a relay capable of switching 16A is going to be a hog, take up
loads of space compared to a triac and cost a hell of a lot more, I
would imagine.

It\'s easy to look up. Digikey has 15 amp relays starting around 60
cents.

The triac will need a heat sink, which will be big and cost more than
the triac.

Relays don\'t need isolated gate drivers, TVS/MOV protection, or
snubbers either.


Well, you got me there!


I have seen and used snubber across relays. To extinguish DC arcs
across the contacts. Sometimes you have to increase contact life.

Do you know why a snubber extinguishes a DC arc?

Yes. Of course... An R-C-D snubber.

The C has been discharged by the R when the relay opens.
Then, the C, an electrolytic typically, (large-ish C) charges up
instead of the contacts taking the arc. The C is a short across the
contacts when the contacts open.

Then, the R discharges the Cap so it is ready for the next opening.

So the snubber prevents the arc but doesn\'t extinguish one. The R-C-D
will be polarity sensitive. And if the contacts cycle before the
large-ish C discharges, the snubber won\'t work.

What sort of RC values are good?


The R has to be small enough to discharge the capacitor in enough time
for the next trigger which is plenty long.

The capacitor is fairly large. Several microfarads as well we several
hundred volts DC in this case.

It does work and worked very well.

boB

Sure, it works to prevent arcs. Buy why does an RC extinguish a DC
arc? Actually, does it?

It prevents the arc from starting in the first place when the contact
opens. As long as there was at least a few amps of current and some
minimal Voc, an arc can be drawn.

The capacitor is the only component necessary to stop the arc as long
as it was discharged when the contact opens.

DC arcs are super easy to make. Just take a power supply with over,
say, 50V and with a short circuit current (Isc) of a few amps.

Short the leads together or, if needed, add some resistance to limit
voltage across the power supply and pull those leads apart. You will
get an arc that you can sustain until the wire disintegrates and the
arc stops.

Better yet, take a solar panel (in the sun) of 100 to 450 watts and
short the leads together. Then pull the leads apart. Makes very nice
arcs.

Pull them apart quickly and FAR apart and minimal heat and wire damage
will occur but for a relay, that won\'t work. Not without something to
mitigate.

Now, take a newer PV system of several thousand watts and several
hundred volts and do the same thing. Might not be able to even hold
onto the wires in addition to being blinded by the UV.



One hint is that DC arcs are associated with inductance in the loop.
That\'s why you can arc weld with a few car batteries, but you need a
big inductor.



No inductor needed if there is enough voltage. You would need at least
3 car batteries from my experience to get a sustaine arc.

An inductive energy storage arc could also benefit from a snubber of
course but is easier to stop cuz it doesn\'t last long. Those are more
like sparks and will quickly extinguish because of limited energy
stored in the inductor. But enough current and voltage will have the
same problem I am talking about.

We also detect DC arcs and mitigate those. Called arc fault in the PV
industry.

Try drawing DC arcs. They\'re fun ! As long as you don\'t get burned
or blinded.

boB

Here is someone demonstrating a small arc...

https://youtu.be/S9a2oPCIMr0?t=16

 

[Ricky]

On Thursday, June 8, 2023 at 9:09:39 PM UTC-4, boB wrote:

On Thu, 08 Jun 2023 15:22:03 -0700, John Larkin
jla...@highlandSNIPMEtechnology.com> wrote:

On Thu, 08 Jun 2023 13:46:05 -0700, boB <b...@K7IQ.com> wrote:

On Wed, 07 Jun 2023 10:57:48 -0700, John Larkin
jla...@highlandSNIPMEtechnology.com> wrote:

On Wed, 07 Jun 2023 10:33:01 -0700, boB <b...@K7IQ.com> wrote:

On Wed, 07 Jun 2023 07:09:09 -0700, John Larkin
jla...@highlandSNIPMEtechnology.com> wrote:

On Tue, 06 Jun 2023 20:36:01 -0700, boB <b...@K7IQ.com> wrote:

On Mon, 05 Jun 2023 19:33:17 +0100, Cursitor Doom <c...@notformail.com
wrote:

On Mon, 05 Jun 2023 11:04:03 -0700, John Larkin
jla...@highlandSNIPMEtechnology.com> wrote:

On Mon, 05 Jun 2023 11:01:33 -0700, John Larkin
jla...@highlandSNIPMEtechnology.com> wrote:

On Mon, 05 Jun 2023 18:54:16 +0100, Cursitor Doom <c...@notformail.com
wrote:

On Mon, 05 Jun 2023 18:50:38 +0100, Cursitor Doom <c...@notformail.com
wrote:

On Mon, 05 Jun 2023 09:21:32 -0700, John Larkin
jla...@highlandSNIPMEtechnology.com> wrote:

On Mon, 5 Jun 2023 12:10:05 -0400, Phil Hobbs
pcdhSpamM...@electrooptical.net> wrote:

On 2023-06-04 23:05, John Larkin wrote:
On Mon, 05 Jun 2023 00:03:20 +0100, Cursitor Doom <c...@notformail.com
wrote:

On Sun, 4 Jun 2023 15:45:44 -0700 (PDT), Ricky
gnuarm.del...@gmail.com> wrote:

On Sunday, June 4, 2023 at 6:13:39?PM UTC-4, John Larkin wrote:
On Sun, 04 Jun 2023 22:34:00 +0100, Cursitor Doom <c...@notformail.com
wrote:
Greetings, gentlemen,

As we all know, to get maximum efficiency out of a MOSFET, it needs to
be driven hard and fast and spend as little time between fully on and
fully off as possible. Does that same principle extend to Triacs? And
if so, how can that best be implemented when one has to use AC to
drive the gate?

CD.
Once a triac fires, the gate drive doesn\'t matter any more.

But they do need a hard, fast drive to fire properly, long enough to
let the drain current rise above the latch limit.

AC gate drive will be slow-rise, which could just possibly damage the
triac if the load current is high.

That should be on the data sheets.

I believe that is why a diac is typically used in the control circuit.

I thought diacs were sometimes used because triacs don\'t fire
symetrically, but I could be wrong.
So, as regards AC drive, is a higher frequency drive signal a safer
bet? Also, the load is AC too, so the thing will turn off at every
zero-crossing point, which makes life more complicated. :-(

With an AC load, an AC gate drive is probably OK. The triac will turn
on before the voltage gets too high, so the spreading damage mechanism
won\'t happen much.

The bad case is when there\'s a lot of voltage across the device with a
lot of instantaneous current behind it, and it gets a slow, wimpy
trigger.

Post your proposed circuit and we can discuss it.

I still call the terminals anode, gate, cathode. I can never remember
what\'s MT1 and MT2. Top and bottom would be OK too.


The triac is one of those parts where folks get confused by fossilized
analogies.

A triac is a bidirectional thyristor, but it does not behave like two
SCRs in antiparallel. SCR triggering only works in one quadrant, for a
start, so two antiparallel SCRs would be a two-quadrant device, whereas
triacs trigger in all four quadrants.

The triggering also isn\'t that symmetrical, because the structures
aren\'t--the second and fourth quadrant triggering mechanisms are fairly
different, and the four-layer stacks aren\'t well separated, as they
would be in two SCRs.

Triacs are also very slow even compared with SCRs, which are pretty poky
devices by modern standards.

Someplace I have a paper that goes into the details of real triac
operation--iirc it\'s a good read, but I\'m not laying my hands on it at
the moment.

(I haven\'t actually used a triac since I was a teenager, so this is just
out of general interest.)

Cheers

Phil Hobbs

I haven\'t designed in a triac for maybe 15 years now. I used a couple
of triacs to soft-start a giant power supply that had a 60 Hz power
transformer. Switch in a giant resistor on the primary side, wait a
second, then short it out. That was hard on resistors but prevented
the occasional 1000 amp startup surge. And allowed a little C&K power
switch on the front panel.

The customer insisted that we not use a switching power supply!

Real relays are usually cheaper than triacs, easier to drive, and
don\'t need heat sinks.

Yes, but you can\'t fire them on and off anything like as fast and they
don\'t have the same longevity AFAIK.

Plus a relay capable of switching 16A is going to be a hog, take up
loads of space compared to a triac and cost a hell of a lot more, I
would imagine.

It\'s easy to look up. Digikey has 15 amp relays starting around 60
cents.

The triac will need a heat sink, which will be big and cost more than
the triac.

Relays don\'t need isolated gate drivers, TVS/MOV protection, or
snubbers either.


Well, you got me there!


I have seen and used snubber across relays. To extinguish DC arcs
across the contacts. Sometimes you have to increase contact life.

Do you know why a snubber extinguishes a DC arc?

Yes. Of course... An R-C-D snubber.

The C has been discharged by the R when the relay opens.
Then, the C, an electrolytic typically, (large-ish C) charges up
instead of the contacts taking the arc. The C is a short across the
contacts when the contacts open.

Then, the R discharges the Cap so it is ready for the next opening.

So the snubber prevents the arc but doesn\'t extinguish one. The R-C-D
will be polarity sensitive. And if the contacts cycle before the
large-ish C discharges, the snubber won\'t work.

What sort of RC values are good?


The R has to be small enough to discharge the capacitor in enough time
for the next trigger which is plenty long.

The capacitor is fairly large. Several microfarads as well we several
hundred volts DC in this case.

It does work and worked very well.

boB

Sure, it works to prevent arcs. Buy why does an RC extinguish a DC
arc? Actually, does it?
It prevents the arc from starting in the first place when the contact
opens. As long as there was at least a few amps of current and some
minimal Voc, an arc can be drawn.

The capacitor is the only component necessary to stop the arc as long
as it was discharged when the contact opens.

When you say discharged when the contact opens, how could it not be discharged? It\'s across the contacts, no? Or are we visualizing different circuits? If you mean it should be discharged *while* the contacts are open, at that time, the cap will end up with the power supply voltage across them.

--

Rick C.

-+- Get 1,000 miles of free Supercharging
-+- Tesla referral code - https://ts.la/richard11209

 

[boB]

On Thu, 8 Jun 2023 19:36:41 -0700 (PDT), Ricky
<gnuarm.deletethisbit@gmail.com> wrote:

On Thursday, June 8, 2023 at 9:09:39?PM UTC-4, boB wrote:
On Thu, 08 Jun 2023 15:22:03 -0700, John Larkin
jla...@highlandSNIPMEtechnology.com> wrote:

On Thu, 08 Jun 2023 13:46:05 -0700, boB <b...@K7IQ.com> wrote:

On Wed, 07 Jun 2023 10:57:48 -0700, John Larkin
jla...@highlandSNIPMEtechnology.com> wrote:

On Wed, 07 Jun 2023 10:33:01 -0700, boB <b...@K7IQ.com> wrote:

On Wed, 07 Jun 2023 07:09:09 -0700, John Larkin
jla...@highlandSNIPMEtechnology.com> wrote:

On Tue, 06 Jun 2023 20:36:01 -0700, boB <b...@K7IQ.com> wrote:

On Mon, 05 Jun 2023 19:33:17 +0100, Cursitor Doom <c...@notformail.com
wrote:

On Mon, 05 Jun 2023 11:04:03 -0700, John Larkin
jla...@highlandSNIPMEtechnology.com> wrote:

On Mon, 05 Jun 2023 11:01:33 -0700, John Larkin
jla...@highlandSNIPMEtechnology.com> wrote:

On Mon, 05 Jun 2023 18:54:16 +0100, Cursitor Doom <c...@notformail.com
wrote:

On Mon, 05 Jun 2023 18:50:38 +0100, Cursitor Doom <c...@notformail.com
wrote:

On Mon, 05 Jun 2023 09:21:32 -0700, John Larkin
jla...@highlandSNIPMEtechnology.com> wrote:

On Mon, 5 Jun 2023 12:10:05 -0400, Phil Hobbs
pcdhSpamM...@electrooptical.net> wrote:

On 2023-06-04 23:05, John Larkin wrote:
On Mon, 05 Jun 2023 00:03:20 +0100, Cursitor Doom <c...@notformail.com
wrote:

On Sun, 4 Jun 2023 15:45:44 -0700 (PDT), Ricky
gnuarm.del...@gmail.com> wrote:

On Sunday, June 4, 2023 at 6:13:39?PM UTC-4, John Larkin wrote:
On Sun, 04 Jun 2023 22:34:00 +0100, Cursitor Doom <c...@notformail.com
wrote:
Greetings, gentlemen,

As we all know, to get maximum efficiency out of a MOSFET, it needs to
be driven hard and fast and spend as little time between fully on and
fully off as possible. Does that same principle extend to Triacs? And
if so, how can that best be implemented when one has to use AC to
drive the gate?

CD.
Once a triac fires, the gate drive doesn\'t matter any more.

But they do need a hard, fast drive to fire properly, long enough to
let the drain current rise above the latch limit.

AC gate drive will be slow-rise, which could just possibly damage the
triac if the load current is high.

That should be on the data sheets.

I believe that is why a diac is typically used in the control circuit.

I thought diacs were sometimes used because triacs don\'t fire
symetrically, but I could be wrong.
So, as regards AC drive, is a higher frequency drive signal a safer
bet? Also, the load is AC too, so the thing will turn off at every
zero-crossing point, which makes life more complicated. :-(

With an AC load, an AC gate drive is probably OK. The triac will turn
on before the voltage gets too high, so the spreading damage mechanism
won\'t happen much.

The bad case is when there\'s a lot of voltage across the device with a
lot of instantaneous current behind it, and it gets a slow, wimpy
trigger.

Post your proposed circuit and we can discuss it.

I still call the terminals anode, gate, cathode. I can never remember
what\'s MT1 and MT2. Top and bottom would be OK too.


The triac is one of those parts where folks get confused by fossilized
analogies.

A triac is a bidirectional thyristor, but it does not behave like two
SCRs in antiparallel. SCR triggering only works in one quadrant, for a
start, so two antiparallel SCRs would be a two-quadrant device, whereas
triacs trigger in all four quadrants.

The triggering also isn\'t that symmetrical, because the structures
aren\'t--the second and fourth quadrant triggering mechanisms are fairly
different, and the four-layer stacks aren\'t well separated, as they
would be in two SCRs.

Triacs are also very slow even compared with SCRs, which are pretty poky
devices by modern standards.

Someplace I have a paper that goes into the details of real triac
operation--iirc it\'s a good read, but I\'m not laying my hands on it at
the moment.

(I haven\'t actually used a triac since I was a teenager, so this is just
out of general interest.)

Cheers

Phil Hobbs

I haven\'t designed in a triac for maybe 15 years now. I used a couple
of triacs to soft-start a giant power supply that had a 60 Hz power
transformer. Switch in a giant resistor on the primary side, wait a
second, then short it out. That was hard on resistors but prevented
the occasional 1000 amp startup surge. And allowed a little C&K power
switch on the front panel.

The customer insisted that we not use a switching power supply!

Real relays are usually cheaper than triacs, easier to drive, and
don\'t need heat sinks.

Yes, but you can\'t fire them on and off anything like as fast and they
don\'t have the same longevity AFAIK.

Plus a relay capable of switching 16A is going to be a hog, take up
loads of space compared to a triac and cost a hell of a lot more, I
would imagine.

It\'s easy to look up. Digikey has 15 amp relays starting around 60
cents.

The triac will need a heat sink, which will be big and cost more than
the triac.

Relays don\'t need isolated gate drivers, TVS/MOV protection, or
snubbers either.


Well, you got me there!


I have seen and used snubber across relays. To extinguish DC arcs
across the contacts. Sometimes you have to increase contact life.

Do you know why a snubber extinguishes a DC arc?

Yes. Of course... An R-C-D snubber.

The C has been discharged by the R when the relay opens.
Then, the C, an electrolytic typically, (large-ish C) charges up
instead of the contacts taking the arc. The C is a short across the
contacts when the contacts open.

Then, the R discharges the Cap so it is ready for the next opening.

So the snubber prevents the arc but doesn\'t extinguish one. The R-C-D
will be polarity sensitive. And if the contacts cycle before the
large-ish C discharges, the snubber won\'t work.

What sort of RC values are good?


The R has to be small enough to discharge the capacitor in enough time
for the next trigger which is plenty long.

The capacitor is fairly large. Several microfarads as well we several
hundred volts DC in this case.

It does work and worked very well.

boB

Sure, it works to prevent arcs. Buy why does an RC extinguish a DC
arc? Actually, does it?
It prevents the arc from starting in the first place when the contact
opens. As long as there was at least a few amps of current and some
minimal Voc, an arc can be drawn.

The capacitor is the only component necessary to stop the arc as long
as it was discharged when the contact opens.

When you say discharged when the contact opens, how could it not be discharged? It\'s across the contacts, no? Or are we visualizing different circuits? If you mean it should be discharged *while* the contacts are open, at that time, the cap will end up with the power supply voltage across them.

Yes, you\'re right.
But the capacitor goes through that diode of the RCD and then the
resistor would be the discharge path.

You don\'t want to discharge the capacitor\'s energy immediately across
the contacts as they close, otherwise the contacts might weld closed.

Same reason you wouldn\'t want to discharge a large HV capacitor
directly across a FET or it would blow up the FET... Or, certainly
would not be good for it.

boB

 

[Ricky]

On Friday, June 9, 2023 at 1:58:58 AM UTC-4, boB wrote:

On Thu, 8 Jun 2023 19:36:41 -0700 (PDT), Ricky
gnuarm.del...@gmail.com> wrote:
On Thursday, June 8, 2023 at 9:09:39?PM UTC-4, boB wrote:
On Thu, 08 Jun 2023 15:22:03 -0700, John Larkin
jla...@highlandSNIPMEtechnology.com> wrote:

On Thu, 08 Jun 2023 13:46:05 -0700, boB <b...@K7IQ.com> wrote:

On Wed, 07 Jun 2023 10:57:48 -0700, John Larkin
jla...@highlandSNIPMEtechnology.com> wrote:

On Wed, 07 Jun 2023 10:33:01 -0700, boB <b...@K7IQ.com> wrote:

On Wed, 07 Jun 2023 07:09:09 -0700, John Larkin
jla...@highlandSNIPMEtechnology.com> wrote:

On Tue, 06 Jun 2023 20:36:01 -0700, boB <b...@K7IQ.com> wrote:

On Mon, 05 Jun 2023 19:33:17 +0100, Cursitor Doom <c...@notformail.com
wrote:

On Mon, 05 Jun 2023 11:04:03 -0700, John Larkin
jla...@highlandSNIPMEtechnology.com> wrote:

On Mon, 05 Jun 2023 11:01:33 -0700, John Larkin
jla...@highlandSNIPMEtechnology.com> wrote:

On Mon, 05 Jun 2023 18:54:16 +0100, Cursitor Doom <c...@notformail.com
wrote:

On Mon, 05 Jun 2023 18:50:38 +0100, Cursitor Doom <c...@notformail.com
wrote:

On Mon, 05 Jun 2023 09:21:32 -0700, John Larkin
jla...@highlandSNIPMEtechnology.com> wrote:

On Mon, 5 Jun 2023 12:10:05 -0400, Phil Hobbs
pcdhSpamM...@electrooptical.net> wrote:

On 2023-06-04 23:05, John Larkin wrote:
On Mon, 05 Jun 2023 00:03:20 +0100, Cursitor Doom <c...@notformail.com
wrote:

On Sun, 4 Jun 2023 15:45:44 -0700 (PDT), Ricky
gnuarm.del...@gmail.com> wrote:

On Sunday, June 4, 2023 at 6:13:39?PM UTC-4, John Larkin wrote:
On Sun, 04 Jun 2023 22:34:00 +0100, Cursitor Doom <c....@notformail.com
wrote:
Greetings, gentlemen,

As we all know, to get maximum efficiency out of a MOSFET, it needs to
be driven hard and fast and spend as little time between fully on and
fully off as possible. Does that same principle extend to Triacs? And
if so, how can that best be implemented when one has to use AC to
drive the gate?

CD.
Once a triac fires, the gate drive doesn\'t matter any more.

But they do need a hard, fast drive to fire properly, long enough to
let the drain current rise above the latch limit.

AC gate drive will be slow-rise, which could just possibly damage the
triac if the load current is high.

That should be on the data sheets.

I believe that is why a diac is typically used in the control circuit.

I thought diacs were sometimes used because triacs don\'t fire
symetrically, but I could be wrong.
So, as regards AC drive, is a higher frequency drive signal a safer
bet? Also, the load is AC too, so the thing will turn off at every
zero-crossing point, which makes life more complicated.. :-(

With an AC load, an AC gate drive is probably OK. The triac will turn
on before the voltage gets too high, so the spreading damage mechanism
won\'t happen much.

The bad case is when there\'s a lot of voltage across the device with a
lot of instantaneous current behind it, and it gets a slow, wimpy
trigger.

Post your proposed circuit and we can discuss it.

I still call the terminals anode, gate, cathode. I can never remember
what\'s MT1 and MT2. Top and bottom would be OK too.


The triac is one of those parts where folks get confused by fossilized
analogies.

A triac is a bidirectional thyristor, but it does not behave like two
SCRs in antiparallel. SCR triggering only works in one quadrant, for a
start, so two antiparallel SCRs would be a two-quadrant device, whereas
triacs trigger in all four quadrants.

The triggering also isn\'t that symmetrical, because the structures
aren\'t--the second and fourth quadrant triggering mechanisms are fairly
different, and the four-layer stacks aren\'t well separated, as they
would be in two SCRs.

Triacs are also very slow even compared with SCRs, which are pretty poky
devices by modern standards.

Someplace I have a paper that goes into the details of real triac
operation--iirc it\'s a good read, but I\'m not laying my hands on it at
the moment.

(I haven\'t actually used a triac since I was a teenager, so this is just
out of general interest.)

Cheers

Phil Hobbs

I haven\'t designed in a triac for maybe 15 years now. I used a couple
of triacs to soft-start a giant power supply that had a 60 Hz power
transformer. Switch in a giant resistor on the primary side, wait a
second, then short it out. That was hard on resistors but prevented
the occasional 1000 amp startup surge. And allowed a little C&K power
switch on the front panel.

The customer insisted that we not use a switching power supply!

Real relays are usually cheaper than triacs, easier to drive, and
don\'t need heat sinks.

Yes, but you can\'t fire them on and off anything like as fast and they
don\'t have the same longevity AFAIK.

Plus a relay capable of switching 16A is going to be a hog, take up
loads of space compared to a triac and cost a hell of a lot more, I
would imagine.

It\'s easy to look up. Digikey has 15 amp relays starting around 60
cents.

The triac will need a heat sink, which will be big and cost more than
the triac.

Relays don\'t need isolated gate drivers, TVS/MOV protection, or
snubbers either.


Well, you got me there!


I have seen and used snubber across relays. To extinguish DC arcs
across the contacts. Sometimes you have to increase contact life..

Do you know why a snubber extinguishes a DC arc?

Yes. Of course... An R-C-D snubber.

The C has been discharged by the R when the relay opens.
Then, the C, an electrolytic typically, (large-ish C) charges up
instead of the contacts taking the arc. The C is a short across the
contacts when the contacts open.

Then, the R discharges the Cap so it is ready for the next opening..

So the snubber prevents the arc but doesn\'t extinguish one. The R-C-D
will be polarity sensitive. And if the contacts cycle before the
large-ish C discharges, the snubber won\'t work.

What sort of RC values are good?


The R has to be small enough to discharge the capacitor in enough time
for the next trigger which is plenty long.

The capacitor is fairly large. Several microfarads as well we several
hundred volts DC in this case.

It does work and worked very well.

boB

Sure, it works to prevent arcs. Buy why does an RC extinguish a DC
arc? Actually, does it?
It prevents the arc from starting in the first place when the contact
opens. As long as there was at least a few amps of current and some
minimal Voc, an arc can be drawn.

The capacitor is the only component necessary to stop the arc as long
as it was discharged when the contact opens.

When you say discharged when the contact opens, how could it not be discharged? It\'s across the contacts, no? Or are we visualizing different circuits? If you mean it should be discharged *while* the contacts are open, at that time, the cap will end up with the power supply voltage across them.
Yes, you\'re right.
But the capacitor goes through that diode of the RCD and then the
resistor would be the discharge path.

You don\'t want to discharge the capacitor\'s energy immediately across
the contacts as they close, otherwise the contacts might weld closed.

Same reason you wouldn\'t want to discharge a large HV capacitor
directly across a FET or it would blow up the FET... Or, certainly
would not be good for it.

You\'ve lost me. I thought you were talking about a DC circuit. The resistor would need to be in series with the cap to prevent a surge current through the contact, but then that limits the protection from the capacitor.

With a FET, you can control the current through it. It doesn\'t need to be a surge.

--

Rick C.

-++ Get 1,000 miles of free Supercharging
-++ Tesla referral code - https://ts.la/richard11209

 

[boB]

On Thu, 8 Jun 2023 23:55:04 -0700 (PDT), Ricky
<gnuarm.deletethisbit@gmail.com> wrote:

On Friday, June 9, 2023 at 1:58:58?AM UTC-4, boB wrote:
On Thu, 8 Jun 2023 19:36:41 -0700 (PDT), Ricky
gnuarm.del...@gmail.com> wrote:
On Thursday, June 8, 2023 at 9:09:39?PM UTC-4, boB wrote:
On Thu, 08 Jun 2023 15:22:03 -0700, John Larkin
jla...@highlandSNIPMEtechnology.com> wrote:

On Thu, 08 Jun 2023 13:46:05 -0700, boB <b...@K7IQ.com> wrote:

On Wed, 07 Jun 2023 10:57:48 -0700, John Larkin
jla...@highlandSNIPMEtechnology.com> wrote:

On Wed, 07 Jun 2023 10:33:01 -0700, boB <b...@K7IQ.com> wrote:

On Wed, 07 Jun 2023 07:09:09 -0700, John Larkin
jla...@highlandSNIPMEtechnology.com> wrote:

On Tue, 06 Jun 2023 20:36:01 -0700, boB <b...@K7IQ.com> wrote:

On Mon, 05 Jun 2023 19:33:17 +0100, Cursitor Doom <c...@notformail.com
wrote:

On Mon, 05 Jun 2023 11:04:03 -0700, John Larkin
jla...@highlandSNIPMEtechnology.com> wrote:

On Mon, 05 Jun 2023 11:01:33 -0700, John Larkin
jla...@highlandSNIPMEtechnology.com> wrote:

On Mon, 05 Jun 2023 18:54:16 +0100, Cursitor Doom <c...@notformail.com
wrote:

On Mon, 05 Jun 2023 18:50:38 +0100, Cursitor Doom <c...@notformail.com
wrote:

On Mon, 05 Jun 2023 09:21:32 -0700, John Larkin
jla...@highlandSNIPMEtechnology.com> wrote:

On Mon, 5 Jun 2023 12:10:05 -0400, Phil Hobbs
pcdhSpamM...@electrooptical.net> wrote:

On 2023-06-04 23:05, John Larkin wrote:
On Mon, 05 Jun 2023 00:03:20 +0100, Cursitor Doom <c...@notformail.com
wrote:

On Sun, 4 Jun 2023 15:45:44 -0700 (PDT), Ricky
gnuarm.del...@gmail.com> wrote:

On Sunday, June 4, 2023 at 6:13:39?PM UTC-4, John Larkin wrote:
On Sun, 04 Jun 2023 22:34:00 +0100, Cursitor Doom <c...@notformail.com
wrote:
Greetings, gentlemen,

As we all know, to get maximum efficiency out of a MOSFET, it needs to
be driven hard and fast and spend as little time between fully on and
fully off as possible. Does that same principle extend to Triacs? And
if so, how can that best be implemented when one has to use AC to
drive the gate?

CD.
Once a triac fires, the gate drive doesn\'t matter any more.

But they do need a hard, fast drive to fire properly, long enough to
let the drain current rise above the latch limit.

AC gate drive will be slow-rise, which could just possibly damage the
triac if the load current is high.

That should be on the data sheets.

I believe that is why a diac is typically used in the control circuit.

I thought diacs were sometimes used because triacs don\'t fire
symetrically, but I could be wrong.
So, as regards AC drive, is a higher frequency drive signal a safer
bet? Also, the load is AC too, so the thing will turn off at every
zero-crossing point, which makes life more complicated. :-(

With an AC load, an AC gate drive is probably OK. The triac will turn
on before the voltage gets too high, so the spreading damage mechanism
won\'t happen much.

The bad case is when there\'s a lot of voltage across the device with a
lot of instantaneous current behind it, and it gets a slow, wimpy
trigger.

Post your proposed circuit and we can discuss it.

I still call the terminals anode, gate, cathode. I can never remember
what\'s MT1 and MT2. Top and bottom would be OK too.


The triac is one of those parts where folks get confused by fossilized
analogies.

A triac is a bidirectional thyristor, but it does not behave like two
SCRs in antiparallel. SCR triggering only works in one quadrant, for a
start, so two antiparallel SCRs would be a two-quadrant device, whereas
triacs trigger in all four quadrants.

The triggering also isn\'t that symmetrical, because the structures
aren\'t--the second and fourth quadrant triggering mechanisms are fairly
different, and the four-layer stacks aren\'t well separated, as they
would be in two SCRs.

Triacs are also very slow even compared with SCRs, which are pretty poky
devices by modern standards.

Someplace I have a paper that goes into the details of real triac
operation--iirc it\'s a good read, but I\'m not laying my hands on it at
the moment.

(I haven\'t actually used a triac since I was a teenager, so this is just
out of general interest.)

Cheers

Phil Hobbs

I haven\'t designed in a triac for maybe 15 years now. I used a couple
of triacs to soft-start a giant power supply that had a 60 Hz power
transformer. Switch in a giant resistor on the primary side, wait a
second, then short it out. That was hard on resistors but prevented
the occasional 1000 amp startup surge. And allowed a little C&K power
switch on the front panel.

The customer insisted that we not use a switching power supply!

Real relays are usually cheaper than triacs, easier to drive, and
don\'t need heat sinks.

Yes, but you can\'t fire them on and off anything like as fast and they
don\'t have the same longevity AFAIK.

Plus a relay capable of switching 16A is going to be a hog, take up
loads of space compared to a triac and cost a hell of a lot more, I
would imagine.

It\'s easy to look up. Digikey has 15 amp relays starting around 60
cents.

The triac will need a heat sink, which will be big and cost more than
the triac.

Relays don\'t need isolated gate drivers, TVS/MOV protection, or
snubbers either.


Well, you got me there!


I have seen and used snubber across relays. To extinguish DC arcs
across the contacts. Sometimes you have to increase contact life.

Do you know why a snubber extinguishes a DC arc?

Yes. Of course... An R-C-D snubber.

The C has been discharged by the R when the relay opens.
Then, the C, an electrolytic typically, (large-ish C) charges up
instead of the contacts taking the arc. The C is a short across the
contacts when the contacts open.

Then, the R discharges the Cap so it is ready for the next opening.

So the snubber prevents the arc but doesn\'t extinguish one. The R-C-D
will be polarity sensitive. And if the contacts cycle before the
large-ish C discharges, the snubber won\'t work.

What sort of RC values are good?


The R has to be small enough to discharge the capacitor in enough time
for the next trigger which is plenty long.

The capacitor is fairly large. Several microfarads as well we several
hundred volts DC in this case.

It does work and worked very well.

boB

Sure, it works to prevent arcs. Buy why does an RC extinguish a DC
arc? Actually, does it?
It prevents the arc from starting in the first place when the contact
opens. As long as there was at least a few amps of current and some
minimal Voc, an arc can be drawn.

The capacitor is the only component necessary to stop the arc as long
as it was discharged when the contact opens.

When you say discharged when the contact opens, how could it not be discharged? It\'s across the contacts, no? Or are we visualizing different circuits? If you mean it should be discharged *while* the contacts are open, at that time, the cap will end up with the power supply voltage across them.
Yes, you\'re right.
But the capacitor goes through that diode of the RCD and then the
resistor would be the discharge path.

You don\'t want to discharge the capacitor\'s energy immediately across
the contacts as they close, otherwise the contacts might weld closed.

Same reason you wouldn\'t want to discharge a large HV capacitor
directly across a FET or it would blow up the FET... Or, certainly
would not be good for it.

You\'ve lost me. I thought you were talking about a DC circuit. The resistor would need to be in series with the cap to prevent a surge current through the contact, but then that limits the protection from the capacitor.

With a FET, you can control the current through it. It doesn\'t need to be a surge.

Yes, DC.

I suppose a FET could have been made to work as well but a FET would
also need to have a drive circuit that limits how long it is on and
sufficient over-current protection so it doesn\'t blow up. But since
the on time would have been very short, a heat sink could be either
minimal or not even used.

I like simple but effective fixes like an electrolytic capacitor that
does all this pretty much by itself. BTW, this particular circuit was
for multiple relays and PV circuits and was in the mid 1990s where
really good low RdsOn FETs were not so plentiful and low cost. We
used the best FETs at the time in our inverters back at Trace
Engineering.

And you are absolutely correct about not wanting a series resistor
with the capacitor if at all possible. The capacitor was really an
elegant solution. I should take a picture of this device. I will do
that sooner than later if I run across the PV GFP board which I
still have one of.

boB

 

[John S]

On 6/4/2023 4:34 PM, Cursitor Doom wrote:

Greetings, gentlemen,

As we all know, to get maximum efficiency out of a MOSFET, it needs to
be driven hard and fast and spend as little time between fully on and
fully off as possible. Does that same principle extend to Triacs? And
if so, how can that best be implemented when one has to use AC to
drive the gate?

CD.

BTW, guys, it\'s not the diminishing voltage across the triac that stops
its conduction, it is the diminishing current through it. Imagine an
inductive load. It will start to turn off when the current goes goes
below the holding threshold. If the rate of rise of voltage (due to the
inductance) across it is low enough, it will completely turn off.

 

[Phil Allison]

John S wrote:
------------------------

BTW, guys, it\'s not the diminishing voltage across the triac that stops
its conduction, it is the diminishing current through it. Imagine an
inductive load.

** Imagine a resistive load, eg heater or lamp.

When the supply voltage drops to under 1volt, a triac will turn off.
It can be re triggered when the voltage rises sufficiently again.

FYI: For a 240V 50Hz supply, this takes about 20 microseconds.



...... Phil

 

[John S]

On 6/10/2023 12:18 AM, Phil Allison wrote:

John S wrote:
------------------------

BTW, guys, it\'s not the diminishing voltage across the triac that stops
its conduction, it is the diminishing current through it. Imagine an
inductive load.

** Imagine a resistive load, eg heater or lamp.

When the supply voltage drops to under 1volt, a triac will turn off.
It can be re triggered when the voltage rises sufficiently again.

FYI: For a 240V 50Hz supply, this takes about 20 microseconds.



..... Phil

Of course it will turn off because the current drops to zero at 1V.

 

[Phil Allison]

John S wrote:
-----------------------

BTW, guys, it\'s not the diminishing voltage across the triac that stops
its conduction, it is the diminishing current through it. Imagine an
inductive load.

** Imagine a resistive load, eg heater or lamp.

When the supply voltage drops to under 1volt, a triac will turn off.
It can be re triggered when the voltage rises sufficiently again.

FYI: For a 240V 50Hz supply, this takes about 20 microseconds.



Of course it will turn off because the current drops to zero at 1V.

** So you think Ohm\'s Law can be defied ?



........ Phil

 

[Ricky]

On Monday, June 12, 2023 at 2:13:03 AM UTC-4, Phil Allison wrote:

John S wrote:
-----------------------


BTW, guys, it\'s not the diminishing voltage across the triac that stops
its conduction, it is the diminishing current through it. Imagine an
inductive load.

** Imagine a resistive load, eg heater or lamp.

When the supply voltage drops to under 1volt, a triac will turn off.
It can be re triggered when the voltage rises sufficiently again.

FYI: For a 240V 50Hz supply, this takes about 20 microseconds.



Of course it will turn off because the current drops to zero at 1V.
** So you think Ohm\'s Law can be defied ?



....... Phil

Of course. Ohm\'s law only applies to materials that it applies to. Circular reasoning, true. But it\'s realistic. Ohm\'s laws predicts a linear relationship between current and voltage, which is not true for many devices, in particular, semiconductors.

You should know that.

--

Rick C.

+-- Get 1,000 miles of free Supercharging
+-- Tesla referral code - https://ts.la/richard11209

 

[boB]

On Thu, 08 Jun 2023 13:42:28 -0700, boB <boB@K7IQ.com> wrote:

On Wed, 07 Jun 2023 22:50:31 +0100, Cursitor Doom <cd@notformail.com
wrote:

On Wed, 07 Jun 2023 10:57:48 -0700, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Wed, 07 Jun 2023 10:33:01 -0700, boB <boB@K7IQ.com> wrote:

On Wed, 07 Jun 2023 07:09:09 -0700, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Tue, 06 Jun 2023 20:36:01 -0700, boB <boB@K7IQ.com> wrote:

On Mon, 05 Jun 2023 19:33:17 +0100, Cursitor Doom <cd@notformail.com
wrote:

On Mon, 05 Jun 2023 11:04:03 -0700, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Mon, 05 Jun 2023 11:01:33 -0700, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Mon, 05 Jun 2023 18:54:16 +0100, Cursitor Doom <cd@notformail.com
wrote:

On Mon, 05 Jun 2023 18:50:38 +0100, Cursitor Doom <cd@notformail.com
wrote:

On Mon, 05 Jun 2023 09:21:32 -0700, John Larkin
jlarkin@highlandSNIPMEtechnology.com> wrote:

On Mon, 5 Jun 2023 12:10:05 -0400, Phil Hobbs
pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2023-06-04 23:05, John Larkin wrote:
On Mon, 05 Jun 2023 00:03:20 +0100, Cursitor Doom <cd@notformail.com
wrote:

On Sun, 4 Jun 2023 15:45:44 -0700 (PDT), Ricky
gnuarm.deletethisbit@gmail.com> wrote:

On Sunday, June 4, 2023 at 6:13:39?PM UTC-4, John Larkin wrote:
On Sun, 04 Jun 2023 22:34:00 +0100, Cursitor Doom <c...@notformail.com
wrote:
Greetings, gentlemen,

As we all know, to get maximum efficiency out of a MOSFET, it needs to
be driven hard and fast and spend as little time between fully on and
fully off as possible. Does that same principle extend to Triacs? And
if so, how can that best be implemented when one has to use AC to
drive the gate?

CD.
Once a triac fires, the gate drive doesn\'t matter any more.

But they do need a hard, fast drive to fire properly, long enough to
let the drain current rise above the latch limit.

AC gate drive will be slow-rise, which could just possibly damage the
triac if the load current is high.

That should be on the data sheets.

I believe that is why a diac is typically used in the control circuit.

I thought diacs were sometimes used because triacs don\'t fire
symetrically, but I could be wrong.
So, as regards AC drive, is a higher frequency drive signal a safer
bet? Also, the load is AC too, so the thing will turn off at every
zero-crossing point, which makes life more complicated. :-(

With an AC load, an AC gate drive is probably OK. The triac will turn
on before the voltage gets too high, so the spreading damage mechanism
won\'t happen much.

The bad case is when there\'s a lot of voltage across the device with a
lot of instantaneous current behind it, and it gets a slow, wimpy
trigger.

Post your proposed circuit and we can discuss it.

I still call the terminals anode, gate, cathode. I can never remember
what\'s MT1 and MT2. Top and bottom would be OK too.


The triac is one of those parts where folks get confused by fossilized
analogies.

A triac is a bidirectional thyristor, but it does not behave like two
SCRs in antiparallel. SCR triggering only works in one quadrant, for a
start, so two antiparallel SCRs would be a two-quadrant device, whereas
triacs trigger in all four quadrants.

The triggering also isn\'t that symmetrical, because the structures
aren\'t--the second and fourth quadrant triggering mechanisms are fairly
different, and the four-layer stacks aren\'t well separated, as they
would be in two SCRs.

Triacs are also very slow even compared with SCRs, which are pretty poky
devices by modern standards.

Someplace I have a paper that goes into the details of real triac
operation--iirc it\'s a good read, but I\'m not laying my hands on it at
the moment.

(I haven\'t actually used a triac since I was a teenager, so this is just
out of general interest.)

Cheers

Phil Hobbs

I haven\'t designed in a triac for maybe 15 years now. I used a couple
of triacs to soft-start a giant power supply that had a 60 Hz power
transformer. Switch in a giant resistor on the primary side, wait a
second, then short it out. That was hard on resistors but prevented
the occasional 1000 amp startup surge. And allowed a little C&K power
switch on the front panel.

The customer insisted that we not use a switching power supply!

Real relays are usually cheaper than triacs, easier to drive, and
don\'t need heat sinks.

Yes, but you can\'t fire them on and off anything like as fast and they
don\'t have the same longevity AFAIK.

Plus a relay capable of switching 16A is going to be a hog, take up
loads of space compared to a triac and cost a hell of a lot more, I
would imagine.

It\'s easy to look up. Digikey has 15 amp relays starting around 60
cents.

The triac will need a heat sink, which will be big and cost more than
the triac.

Relays don\'t need isolated gate drivers, TVS/MOV protection, or
snubbers either.


Well, you got me there!


I have seen and used snubber across relays. To extinguish DC arcs
across the contacts. Sometimes you have to increase contact life.

Do you know why a snubber extinguishes a DC arc?

Yes. Of course... An R-C-D snubber.

The C has been discharged by the R when the relay opens.
Then, the C, an electrolytic typically, (large-ish C) charges up
instead of the contacts taking the arc. The C is a short across the
contacts when the contacts open.

Then, the R discharges the Cap so it is ready for the next opening.

So the snubber prevents the arc but doesn\'t extinguish one. The R-C-D
will be polarity sensitive. And if the contacts cycle before the
large-ish C discharges, the snubber won\'t work.

What sort of RC values are good?

All very well for switching DC currents, but how to prolong contact
life when the current is AC?



The are only happens when the contacts open and that is when the
capacitor shorts out the voltage to keep the arc from even happening.
I\'m talking BIG continuous arcing if this is not snubbed.

It WILL work and it DOES work.

Of course it is polarity sensitive ! This was used on a solar PV
array disconnect for ground fault protection in the 1990s. It worked
great.

No AC. Just DC here.

There are other ways of stopping relay arcs for AC too but involves
triacs across the contacts.

boB

I finally put back my dropbox app back. Here is a picture of that old
PV GFP unit I did back around 1995. The stand-up electrolytics are
the snubber caps. Not huge, but not small either.

https://www.dropbox.com/scl/fi/k0huquqfn7qrd63wdf7qg/DC-PV-GFP.PNG?dl=0&rlkey=ad6lf9yfwuib0ezmps3na8cq2

boB