SSRs Solid State Relays.

J

John Crighton

Guest
Hello All,
a friend of mine wishes to replace four heavy duty relays
that control a 24 VDC electric motor.
The motor is either off, runs forward or runs in reverse.
My friend tells me the normal running current is 10 amps
lightly loaded, but at start up he thinks the current is
around 50 amps.

He cant get the correct relays anymore, the equipment
is old. Suitable electrical replacement types are expensive
and physically don't fit. If modifications have to be made
he thought SSRs would be good.
Something like this.
http://www.crydom.com/pdf/crydom_D06D.pdf

Those particular SSRs look great.
The only problem is the price. 150 dollars locally
here in Australia. Too expensive for my friend.

As a hobbyist, I am even more of a cheapskate
than my friend is, so I am interested to know what
is in these things to make them so expensive.

We want to make something similar to the Crydom
relay, specification wise, with locally easy to obtain
parts and a lot cheaper than 600 dollars for four units.
All ideas welcome.

Regards,
John Crighton
Sydney
 
On Sat, 08 May 2004 03:56:15 GMT, john_c@tpg.com.au (John Crighton)
wrote:

Hello All,
a friend of mine wishes to replace four heavy duty relays
that control a 24 VDC electric motor.
The motor is either off, runs forward or runs in reverse.
My friend tells me the normal running current is 10 amps
lightly loaded, but at start up he thinks the current is
around 50 amps.
Click to expand...
Since the motor is DC, could you use some simple mosfets in parallel?

I built a telescope mirror grinding machine using a 24vdc wheelchair
motor to drive the 24" diameter turntable. The motor is controlled by
six IRFZ40N mosfets in parallel. So far I have 60+ hours on it with
no troubles. Here's a link to some images of the grinder.. (nobody
laugh at my ugly electronics :)

http://lerch.no-ip.com/atm/Projects/Polisher/page_01.htm


Take Care,
James Lerch
http://lerch.no-ip.com/atm (My telescope construction, Testing, and Coating site)

Press on: nothing in the world can take the place of perseverance.
Talent will not; nothing is more common than unsuccessful men with talent.
Genius will not; unrewarded genius is almost a proverb.
Education will not; the world is full of educated derelicts.
Persistence and determination alone are omnipotent.
Calvin Coolidge
 
James Lerch wrote...
I built a telescope mirror grinding machine using a 24vdc wheelchair
motor to drive the 24" diameter turntable. The motor is controlled by
six IRFZ40N mosfets in parallel. So far I have 60+ hours on it with
no troubles. Here's a link to some images of the grinder.. (nobody
laugh at my ugly electronics :)

http://lerch.no-ip.com/atm/Projects/Polisher/page_01.htm
Click to expand...
Doesn't look so bad, apparently gets the job done. We see a
little open-frame switching supply, and a small breadboard
with FETs, no heat sinks but with a big fan blowing on them?

http://lerch.no-ip.com/atm/Projects/Polisher/Dsc05941.jpg
http://lerch.no-ip.com/atm/Projects/Polisher/Dsc05934.jpg

Thanks,
- Win

(email: use hill_at_rowland-dot-org for now)
 
On Sat, 08 May 2004 03:56:15 GMT, john_c@tpg.com.au (John Crighton)
wrote:

Hello All,
a friend of mine wishes to replace four heavy duty relays
that control a 24 VDC electric motor.
The motor is either off, runs forward or runs in reverse.
My friend tells me the normal running current is 10 amps
lightly loaded, but at start up he thinks the current is
around 50 amps.

He cant get the correct relays anymore, the equipment
is old. Suitable electrical replacement types are expensive
and physically don't fit. If modifications have to be made
he thought SSRs would be good.
Something like this.
http://www.crydom.com/pdf/crydom_D06D.pdf

Those particular SSRs look great.
The only problem is the price. 150 dollars locally
here in Australia. Too expensive for my friend.

As a hobbyist, I am even more of a cheapskate
than my friend is, so I am interested to know what
is in these things to make them so expensive.

We want to make something similar to the Crydom
relay, specification wise, with locally easy to obtain
parts and a lot cheaper than 600 dollars for four units.
All ideas welcome.

Regards,
John Crighton
Sydney
Click to expand...

There are some incredibly cheap automotive relays around, rated at
10-30 amps continuous and only costing a few dollars. Mouser and
Digikey have some. They all have 12-volt coils, so you'd need a lower
control voltage or a series resistor, but the contacts should be OK at
24. You could buy a lifetime supply of spares for $40 or so.

John
 
On 8 May 2004 06:59:54 -0700, Winfield Hill
<Winfield_member@newsguy.com> wrote:

James Lerch wrote...

I built a telescope mirror grinding machine using a 24vdc wheelchair
motor to drive the 24" diameter turntable. The motor is controlled by
six IRFZ40N mosfets in parallel. So far I have 60+ hours on it with
no troubles. Here's a link to some images of the grinder.. (nobody
laugh at my ugly electronics :)

http://lerch.no-ip.com/atm/Projects/Polisher/page_01.htm

Doesn't look so bad, apparently gets the job done.
Click to expand...
So far :)

We see a little open-frame switching supply,
Click to expand...
Stolen from an old external HD box, and really in need of a cover.
BTW, if you drop something inside it, turn the power off before
reaching in to retrieve it (Yea, you'd think I was smarter than
that..)

and a small breadboard
with FETs, no heat sinks but with a big fan blowing on them?
Click to expand...
Yea, those eight FETs run two stepper motors off 12vdc. Unfortunately
the original steppers didn't have enough guts to move the over-arm
with confidence, so I replaced the steppers with larger versions. Now
the FETs run rather warm and I've exhausted my supply of to-220 mica
wafers. The big fan was one of those "temporary" fixes that remain
after 6 months of use.. :)


Take Care,
James Lerch
http://lerch.no-ip.com/atm (My telescope construction, Testing, and Coating site)

Press on: nothing in the world can take the place of perseverance.
Talent will not; nothing is more common than unsuccessful men with talent.
Genius will not; unrewarded genius is almost a proverb.
Education will not; the world is full of educated derelicts.
Persistence and determination alone are omnipotent.
Calvin Coolidge
 
On Sat, 08 May 2004 08:28:08 -0700, John Larkin
<jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote:

On Sat, 08 May 2004 03:56:15 GMT, john_c@tpg.com.au (John Crighton)
wrote:

Hello All,
a friend of mine wishes to replace four heavy duty relays
that control a 24 VDC electric motor.
The motor is either off, runs forward or runs in reverse.
My friend tells me the normal running current is 10 amps
lightly loaded, but at start up he thinks the current is
around 50 amps.

He cant get the correct relays anymore, the equipment
is old. Suitable electrical replacement types are expensive
and physically don't fit. If modifications have to be made
he thought SSRs would be good.
Something like this.
http://www.crydom.com/pdf/crydom_D06D.pdf

Those particular SSRs look great.
The only problem is the price. 150 dollars locally
here in Australia. Too expensive for my friend.

As a hobbyist, I am even more of a cheapskate
than my friend is, so I am interested to know what
is in these things to make them so expensive.

We want to make something similar to the Crydom
relay, specification wise, with locally easy to obtain
parts and a lot cheaper than 600 dollars for four units.
All ideas welcome.

Regards,
John Crighton
Sydney


There are some incredibly cheap automotive relays around, rated at
10-30 amps continuous and only costing a few dollars. Mouser and
Digikey have some. They all have 12-volt coils, so you'd need a lower
control voltage or a series resistor, but the contacts should be OK at
24. You could buy a lifetime supply of spares for $40 or so.

John
Click to expand...
My friend is considering that idea. The 24V DC motor is
used in a fishing boats auto pilot. In use, the motor is
continually starting and stopping, running one way then
the other and the relay contacts arc when they open up.
The motor works hard in rough seas so there would be
a heavy current flowing when the contacts open. Much
more than the ten amps measured at light load.

My friend mentioned the solid state relays and wondered
what was in them. The SSRs would do a much better job
in the long term than the ordinary relays.

I am still interested to know how they work also. I am
guessing they have mosfets and some sort of photo voltaic
device to generate several volts or so to turn the mosfets on.
I did read about such a device similar in size to small
opto coupler but I have not been able to find anything
on the net. Anyway, thanks for the reply.

Regards
John Crighton
Sydney
 
On Sat, 08 May 2004 04:23:07 GMT, jlerch@no.spam.tampabay.rr.com
(James Lerch) wrote:

On Sat, 08 May 2004 03:56:15 GMT, john_c@tpg.com.au (John Crighton)
wrote:

Hello All,
a friend of mine wishes to replace four heavy duty relays
that control a 24 VDC electric motor.
The motor is either off, runs forward or runs in reverse.
My friend tells me the normal running current is 10 amps
lightly loaded, but at start up he thinks the current is
around 50 amps.


Since the motor is DC, could you use some simple mosfets in parallel?

Yes, we are looking into doing something like that also. An H bridge
Click to expand...
arrangement. Is that what you are using? Are you switching your
motor reverse also? Thanks for the reply.
Regards,
John Crighton
Sydney
 
"John Crighton" <john_c@tpg.com.au> wrote in message
news:409d7a9f.3515496@News.individual.net...
On Sat, 08 May 2004 08:28:08 -0700, John Larkin

There are some incredibly cheap automotive relays around, rated at
10-30 amps continuous and only costing a few dollars. Mouser and
Digikey have some. They all have 12-volt coils, so you'd need a lower
control voltage or a series resistor, but the contacts should be OK at
24. You could buy a lifetime supply of spares for $40 or so.

John


My friend is considering that idea. The 24V DC motor is
used in a fishing boats auto pilot. In use, the motor is
continually starting and stopping, running one way then
the other and the relay contacts arc when they open up.
The motor works hard in rough seas so there would be
a heavy current flowing when the contacts open. Much
more than the ten amps measured at light load.

My friend mentioned the solid state relays and wondered
what was in them. The SSRs would do a much better job
in the long term than the ordinary relays.
Click to expand...
It's typically a triac. SSRs are used for AC - they
won't work on DC - that is, once it's turned on, with DC,
it can't be turned off, except with a commutating circuit
that about doubles the complexity of the circuit.

So let go of the Solid-State Relay idea, OK? :)

But a solid-state switch, like a bank of transistors, or
an industrial H-bridge, could do the job. But it'd be a
circuit that somebody'd have to design (or copy) - I
don't think there's such a thing as an off-the-shelf
"solid-state-switch" for DC. (or that's not what it'd
be called.)

Good Luck!
Rich
 
"Rich Grise" <null@example.net> wrote in message
news:tdjnc.98389$G_.87582@nwrddc02.gnilink.net...
"John Crighton" <john_c@tpg.com.au> wrote in message
news:409d7a9f.3515496@News.individual.net...
On Sat, 08 May 2004 08:28:08 -0700, John Larkin

There are some incredibly cheap automotive relays around, rated at
10-30 amps continuous and only costing a few dollars. Mouser and
Digikey have some. They all have 12-volt coils, so you'd need a lower
control voltage or a series resistor, but the contacts should be OK at
24. You could buy a lifetime supply of spares for $40 or so.

John


My friend is considering that idea. The 24V DC motor is
used in a fishing boats auto pilot. In use, the motor is
continually starting and stopping, running one way then
the other and the relay contacts arc when they open up.
The motor works hard in rough seas so there would be
a heavy current flowing when the contacts open. Much
more than the ten amps measured at light load.

My friend mentioned the solid state relays and wondered
what was in them. The SSRs would do a much better job
in the long term than the ordinary relays.

It's typically a triac. SSRs are used for AC - they
won't work on DC - that is, once it's turned on, with DC,
it can't be turned off, except with a commutating circuit
that about doubles the complexity of the circuit.

So let go of the Solid-State Relay idea, OK? :)

But a solid-state switch, like a bank of transistors, or
an industrial H-bridge, could do the job. But it'd be a
circuit that somebody'd have to design (or copy) - I
don't think there's such a thing as an off-the-shelf
"solid-state-switch" for DC. (or that's not what it'd
be called.)

Good Luck!
Rich
Click to expand...
http://www.crydom.com/product/producttree2.asp?SwitchingVoltage=DC,Control%20-%20Volts%20DC ....Paul
 
On Sun, 09 May 2004 08:15:49 GMT, john_c@tpg.com.au (John Crighton)
wrote:

Hello Rich,
No, I am still hanging on to the DIY solid state relay idea. :)

This is what I am aiming for.
http://www.crydom.com/pdf/crydom_D06D.pdf

I found a data sheet for the little photo voltaic devices
http://www.irf.com/product-info/datasheets/data/pvin.pdf

I will ring up the usual electronic parts places here in Sydney
to-morrow and see if I can find any.

Regards,
John Crighton
Sydney
Click to expand...

The PVs are cute, but they have tiny output currents and fairly low
voltages, usually. A small potted DC-DC converter makes a good gate
driver. 5-volt to 15-volt converters are common and cheap. Their
output caps are small, so a modest load resistor will discharge them
pretty fast at turnoff, and they have lots of gate drive capability.
Power mosfets like lots of gate drive for low Ron, so a dc/dc works
well for fairly slow switching drive.

John
 
On Sun, 09 May 2004 01:23:49 GMT, john_c@tpg.com.au (John Crighton)
wrote:

Yes, we are looking into doing something like that also. An H bridge
arrangement. Is that what you are using? Are you switching your
motor reverse also? Thanks for the reply.
Click to expand...
Hi John,

No H-Bridge here, just 6 simple Fets in parallel to switch the ground
leg on/off to the motor via the PWM signal from a laptop.

However, an H-bridge arrangement shouldn't be terrible more difficult
to put together. Heck, you could probably get creative with a 556
timer and make a 'soft start' option :)


Take Care,
James Lerch
http://lerch.no-ip.com/atm (My telescope construction, Testing, and Coating site)

Press on: nothing in the world can take the place of perseverance.
Talent will not; nothing is more common than unsuccessful men with talent.
Genius will not; unrewarded genius is almost a proverb.
Education will not; the world is full of educated derelicts.
Persistence and determination alone are omnipotent.
Calvin Coolidge
 
On Sun, 09 May 2004 05:21:29 GMT, "Rich Grise" <null@example.net>
wrote:

"John Crighton" <john_c@tpg.com.au> wrote in message
news:409d7a9f.3515496@News.individual.net...
On Sat, 08 May 2004 08:28:08 -0700, John Larkin

There are some incredibly cheap automotive relays around, rated at
10-30 amps continuous and only costing a few dollars. Mouser and
Digikey have some. They all have 12-volt coils, so you'd need a lower
control voltage or a series resistor, but the contacts should be OK at
24. You could buy a lifetime supply of spares for $40 or so.

John


My friend is considering that idea. The 24V DC motor is
used in a fishing boats auto pilot. In use, the motor is
continually starting and stopping, running one way then
the other and the relay contacts arc when they open up.
The motor works hard in rough seas so there would be
a heavy current flowing when the contacts open. Much
more than the ten amps measured at light load.

My friend mentioned the solid state relays and wondered
what was in them. The SSRs would do a much better job
in the long term than the ordinary relays.

It's typically a triac. SSRs are used for AC - they
won't work on DC - that is, once it's turned on, with DC,
it can't be turned off, except with a commutating circuit
that about doubles the complexity of the circuit.

So let go of the Solid-State Relay idea, OK? :)
Click to expand...
There are lots of DC ssr's around nowadays. The small ones (surfmount
and dips) are photofets, and the bigger brick-types have various sorts
of isolated drive thingies. They come in unidirectional and
bidirectional versions, so the bi-versions can switch AC too.

The little ones make cool floating signal-level switches; offsets are
zero.

Unfortunately, they all seem to have schmitt action, so they don't
work for linear control. That eliminates a number of cute
applications, like HV power amp output stages.

But a solid-state switch, like a bank of transistors, or
an industrial H-bridge, could do the job. But it'd be a
circuit that somebody'd have to design (or copy) - I
don't think there's such a thing as an off-the-shelf
"solid-state-switch" for DC. (or that's not what it'd
be called.)
Click to expand...
They are called DC SSRs. Mouser and Digikey stock them.

John
 
On Sun, 09 May 2004 16:42:26 GMT, jlerch@no.spam.tampabay.rr.com
(James Lerch) wrote:

On Sun, 09 May 2004 01:23:49 GMT, john_c@tpg.com.au (John Crighton)
wrote:

Yes, we are looking into doing something like that also. An H bridge
arrangement. Is that what you are using? Are you switching your
motor reverse also? Thanks for the reply.

Hi John,

No H-Bridge here, just 6 simple Fets in parallel to switch the ground
leg on/off to the motor via the PWM signal from a laptop.

However, an H-bridge arrangement shouldn't be terrible more difficult
to put together. Heck, you could probably get creative with a 556
timer and make a 'soft start' option :)
Click to expand...
Or PWM linear speed control.

John
 
"John Larkin" <jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote in
message
There are lots of DC ssr's around nowadays. The small ones (surfmount
and dips) are photofets, and the bigger brick-types have various sorts
of isolated drive thingies. They come in unidirectional and
bidirectional versions, so the bi-versions can switch AC too.
...
They are called DC SSRs. Mouser and Digikey stock them.
Click to expand...
<kick><scream><drag>Whaddaya mean, Twenty-first century!!?!?!??!?!?!!!!!
</drag></scream></kick>

;-)
Rich
 
On Sun, 09 May 2004 22:18:31 GMT, "Rich Grise" <null@example.net>
wrote:

"John Larkin" <jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote in
message

There are lots of DC ssr's around nowadays. The small ones (surfmount
and dips) are photofets, and the bigger brick-types have various sorts
of isolated drive thingies. They come in unidirectional and
bidirectional versions, so the bi-versions can switch AC too.
...
They are called DC SSRs. Mouser and Digikey stock them.

kick><scream><drag>Whaddaya mean, Twenty-first century!!?!?!??!?!?!!!!!
/drag></scream></kick

;-)
Rich
Click to expand...

OK, do you remember liquid-state relays?

John
 
On Sun, 09 May 2004 09:40:29 -0700, John Larkin
<jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote:

The PVs are cute, but they have tiny output currents and fairly low
voltages, usually. A small potted DC-DC converter makes a good gate
driver. 5-volt to 15-volt converters are common and cheap. Their
output caps are small, so a modest load resistor will discharge them
pretty fast at turnoff, and they have lots of gate drive capability.
Power mosfets like lots of gate drive for low Ron, so a dc/dc works
well for fairly slow switching drive.

John
Click to expand...
Hello John,
thanks for that suggestion, the dc/dc isolated supply
could run continuously and the negative output side
to the source of the mosfet could be switched with a
normal opto coupler.

Having to provide a separate supply is not as nice as
the expensive Crydom relay but this method is cheap.
Great! Thanks again! :)

Regards,
John Crighton
Sydney
 
On Sun, 09 May 2004 15:40:43 -0700, the renowned John Larkin
<jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote:

On Sun, 09 May 2004 22:18:31 GMT, "Rich Grise" <null@example.net
wrote:

"John Larkin" <jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote in
message

There are lots of DC ssr's around nowadays. The small ones (surfmount
and dips) are photofets, and the bigger brick-types have various sorts
of isolated drive thingies. They come in unidirectional and
bidirectional versions, so the bi-versions can switch AC too.
...
They are called DC SSRs. Mouser and Digikey stock them.

kick><scream><drag>Whaddaya mean, Twenty-first century!!?!?!??!?!?!!!!!
/drag></scream></kick

;-)
Rich



OK, do you remember liquid-state relays?

John
Click to expand...
Remember? I've got some right here. 60A 3-phase.

Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
speff@interlog.com Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog Info for designers: http://www.speff.com
 
"John Larkin" <jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote in
message
On Sun, 09 May 2004 22:18:31 GMT, "Rich Grise" <null@example.net
wrote:

"John Larkin" <jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote in
message

They are called DC SSRs. Mouser and Digikey stock them.

kick><scream><drag>Whaddaya mean, Twenty-first century!!?!?!??!?!?!!!!!
/drag></scream></kick

;-)
Rich



OK, do you remember liquid-state relays?
Click to expand...
Not unless you mean the ones where there's actually a little tank of
mercury and they have to be operated upright.

Thanks,
Rich
 
On Sun, 09 May 2004 05:21:29 GMT, "Rich Grise" <null@example.net>
wrote:

"John Crighton" <john_c@tpg.com.au> wrote in message
news:409d7a9f.3515496@News.individual.net...
On Sat, 08 May 2004 08:28:08 -0700, John Larkin

There are some incredibly cheap automotive relays around, rated at
10-30 amps continuous and only costing a few dollars. Mouser and
Digikey have some. They all have 12-volt coils, so you'd need a lower
control voltage or a series resistor, but the contacts should be OK at
24. You could buy a lifetime supply of spares for $40 or so.

John


My friend is considering that idea. The 24V DC motor is
used in a fishing boats auto pilot. In use, the motor is
continually starting and stopping, running one way then
the other and the relay contacts arc when they open up.
The motor works hard in rough seas so there would be
a heavy current flowing when the contacts open. Much
more than the ten amps measured at light load.

My friend mentioned the solid state relays and wondered
what was in them. The SSRs would do a much better job
in the long term than the ordinary relays.

It's typically a triac. SSRs are used for AC - they
won't work on DC - that is, once it's turned on, with DC,
it can't be turned off, except with a commutating circuit
that about doubles the complexity of the circuit.

So let go of the Solid-State Relay idea, OK? :)

But a solid-state switch, like a bank of transistors, or
an industrial H-bridge, could do the job. But it'd be a
circuit that somebody'd have to design (or copy) - I
don't think there's such a thing as an off-the-shelf
"solid-state-switch" for DC. (or that's not what it'd
be called.)

Good Luck!
Rich
Click to expand...

Hello Rich,
No, I am still hanging on to the DIY solid state relay idea. :)

This is what I am aiming for.
http://www.crydom.com/pdf/crydom_D06D.pdf

I found a data sheet for the little photo voltaic devices
http://www.irf.com/product-info/datasheets/data/pvin.pdf

I will ring up the usual electronic parts places here in Sydney
to-morrow and see if I can find any.

Regards,
John Crighton
Sydney
 
In message <cqct9054mgsoubp1fo3iiocnhqcthihcbd@4ax.com>, John Larkin
<jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> writes
On Sun, 09 May 2004 22:18:31 GMT, "Rich Grise" <null@example.net
wrote:

"John Larkin" <jjlarkin@highlandSNIPtechTHISnologyPLEASE.com> wrote in
message

There are lots of DC ssr's around nowadays. The small ones (surfmount
and dips) are photofets, and the bigger brick-types have various sorts
of isolated drive thingies. They come in unidirectional and
bidirectional versions, so the bi-versions can switch AC too.
...
They are called DC SSRs. Mouser and Digikey stock them.

kick><scream><drag>Whaddaya mean, Twenty-first century!!?!?!??!?!?!!!!!
/drag></scream></kick

;-)
Rich



OK, do you remember liquid-state relays?
Click to expand...
Unfortunately, yes. Except these ones were solid state only a few
microseconds earlier and they quickly settled back to a different solid
state after the fuse blew,
--
Graham
You can't call yourself an engineer until you have smoked a few designs.
(author unknown)
 
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