zener diode question

[klem kedidelhopper]

I need to come up with a zener diode that will handle 1.0 amp
of current at 12V. I currently am using two 6.8V 10 watt stud
mount zeners in series mounted with insulators on a large
heatsink but it's pushing it. When the current spikes to 1.0 amp the
diodes get pretty hot.
Since I have a bag of these diodes Would it be a problem to
employ four of them in a series/parallel arrangement to attain
13.60V at 20watts? I realize that the exact voltage that each
diode "clips" at might be a bit different but would that really
matter? It would be a simple job to simply add one diode in parallel
with each one presently there. Thanks, Lenny

 

[mike]

On 4/15/2013 8:12 AM, klem kedidelhopper wrote:

I need to come up with a zener diode that will handle 1.0 amp
of current at 12V. I currently am using two 6.8V 10 watt stud
mount zeners in series mounted with insulators on a large
heatsink but it's pushing it. When the current spikes to 1.0 amp the
diodes get pretty hot.
Since I have a bag of these diodes Would it be a problem to
employ four of them in a series/parallel arrangement to attain
13.60V at 20watts? I realize that the exact voltage that each
diode "clips" at might be a bit different but would that really
matter? It would be a simple job to simply add one diode in parallel
with each one presently there. Thanks, Lenny

Depends on exactly what the spec says, but...
You're asking a 10W diode to dissipate 6.8W. That shouldn't
be a problem if you keep the stud cool.
Mounting twice as many diodes on the same heat sink won't change
the heat sink
temperature much at all. You'll save half of the junction to case
thermal resistance in the package and the chip runs a little cooler.
But the package will still get "pretty hot" whatever that means.

At low current, a 6.8V zener has a slight positive voltage temp
coefficient, but the actual voltage may be all over the place.
I'd carefully match them at rated current.

Better solution would be a bigger heat sink or a fan.
Better insulators?
Two heat sinks and no insulators?

 

[Arfa Daily]

"klem kedidelhopper" <captainvideo462009@gmail.com> wrote in message
news:85e12e55-6f89-418e-9292-89a68c5a75ad@hc4g2000pbb.googlegroups.com...

I need to come up with a zener diode that will handle 1.0 amp
of current at 12V. I currently am using two 6.8V 10 watt stud
mount zeners in series mounted with insulators on a large
heatsink but it's pushing it. When the current spikes to 1.0 amp the
diodes get pretty hot.
Since I have a bag of these diodes Would it be a problem to
employ four of them in a series/parallel arrangement to attain
13.60V at 20watts? I realize that the exact voltage that each
diode "clips" at might be a bit different but would that really
matter? It would be a simple job to simply add one diode in parallel
with each one presently there. Thanks, Lenny

This is to do with the question from a couple of months back, where I
suggested using a couple of series zeners to drop a fixed amount in your
power supply, yes ?

I'm not a fan of putting zeners in parallel to up the power rating, as it's
almost impossible to guarantee that they will both have exactly the same
knee voltage, resulting in the current not dividing equally between them, if
at all. At 1 amp, you are not actually exceeding the 10 watt rating of the
zeners, as long as your heatsink is managing to keep the junction
temperatures below the quoted maximum.

I think I would be more inclined to either change to a larger heatsink, or a
matt black one if it's plain ally, or paint the existing one black, or add a
fan, or whatever combination of any of those. As you have 13.6v across the
zeners, you could just 'steal' a bit of that to run a small 12v fan. Just
connect directly across the zeners, "+" terminal to the upstream side and
"-" to the downstream. That would be my preferred solution.

Arfa

 

[Jamie]

Arfa Daily wrote:

"klem kedidelhopper" <captainvideo462009@gmail.com> wrote in message
news:85e12e55-6f89-418e-9292-89a68c5a75ad@hc4g2000pbb.googlegroups.com...

I need to come up with a zener diode that will handle 1.0 amp
of current at 12V. I currently am using two 6.8V 10 watt stud
mount zeners in series mounted with insulators on a large
heatsink but it's pushing it. When the current spikes to 1.0 amp the
diodes get pretty hot.
Since I have a bag of these diodes Would it be a problem to
employ four of them in a series/parallel arrangement to attain
13.60V at 20watts? I realize that the exact voltage that each
diode "clips" at might be a bit different but would that really
matter? It would be a simple job to simply add one diode in parallel
with each one presently there. Thanks, Lenny


This is to do with the question from a couple of months back, where I
suggested using a couple of series zeners to drop a fixed amount in your
power supply, yes ?

I'm not a fan of putting zeners in parallel to up the power rating, as
it's almost impossible to guarantee that they will both have exactly the
same knee voltage, resulting in the current not dividing equally between
them, if at all. At 1 amp, you are not actually exceeding the 10 watt
rating of the zeners, as long as your heatsink is managing to keep the
junction temperatures below the quoted maximum.

I think I would be more inclined to either change to a larger heatsink,
or a matt black one if it's plain ally, or paint the existing one black,
or add a fan, or whatever combination of any of those. As you have 13.6v
across the zeners, you could just 'steal' a bit of that to run a small
12v fan. Just connect directly across the zeners, "+" terminal to the
upstream side and "-" to the downstream. That would be my preferred
solution.

Arfa

It is a lot easier to find and mount power transistor on a heat sink
than it is finding high power zeners. You can create a high current
zener this way.

But, I only use that type of circuit as an active clamp mostly for
protection, where the current sink will be high at times.

I was going to suggest this to Klem earlier but I seems from past
post and replies that he does not seem to always understand some
concepts and explanations of things. It's like quantum physics.

Jamie

 

[Phil Allison]

"klem kedidelhopper"

I need to come up with a zener diode that will handle 1.0 amp
of current at 12V.

** All you need is a NPN power transistor and a 6.2 V zener:

http://sound.westhost.com/appnotes/an007.htm

Got any old 2N3055s lying about ?

Easily make a 50W zener with one.



.... Phil

 

My question is why use shunt regulation in the first place.

Also true about the "pass" transistor, any transistor of suitable ratings will work and most likely help solve any thermal problems. It doesn't even matter if it is NPN or PNP. Works both ways.

The only reason to use shunt regulation is if you expect spikes which should be clamped. This can happen in an automotive application for example by just having loose or dirty battery cables, or any electrical fittings between the alternator (generator) and the load. This porblem is compunded these days by the fact that a modern fuel injected car with multiple coils (a coil pack)pulls a hell of alot of current to just run. Have any idea what a heated O2 sensor pulls ? what's more a car might have three or four of them. That is on top of an injector for every cylider maybe more.

The thing is I am am having a hard time imagining another situation where such a regulation scheme would be appropriate. If it's automotive then it is, but that has not been revealed.

 

[mike]

On 4/16/2013 2:21 AM, klem kedidelhopper wrote:

On Apr 16, 2:12 am, jurb6...@gmail.com wrote:
My question is why use shunt regulation in the first place.

Also true about the "pass" transistor, any transistor of suitable ratings will work and most likely help solve any thermal problems. It doesn't even matter if it is NPN or PNP. Works both ways.

The only reason to use shunt regulation is if you expect spikes which should be clamped. This can happen in an automotive application for example by just having loose or dirty battery cables, or any electrical fittings between the alternator (generator) and the load. This porblem is compunded these days by the fact that a modern fuel injected car with multiple coils (a coil pack)pulls a hell of alot of current to just run. Have any idea what a heated O2 sensor pulls ? what's more a car might have three or four of them. That is on top of an injector for every cylider maybe more.

The thing is I am am having a hard time imagining another situation where such a regulation scheme would be appropriate. If it's automotive then it is, but that has not been revealed.

Yes this is related to my post of several months ago. I employed
Arfa's suggestion of using two of my 6.8V 10 watt zeners in series to
make a 13.6 V diode. And from my previous discussion this was then
put in series with the rectified 24VAC, (39VDC) to drop the voltage to
the input of my 12V regulator to a safer level. These two diodes were
mounted with insulators and silicone grease on a large old black
aluminum heat sink that I pulled out of a 1960's Delco automobile
radio. With the diodes in the circuit the voltage to the input of the
regulator was 19VDC, and under full load it was 16.5VDC, which is just
about where I wanted it to be. The only problem is that the diodes as
well as the heat sink were getting extremely hot when the regulator
was fully loaded, (1.0Amp). In the interest of survivability
assessment for my test I ran the unit overnight. So I added more
aluminum and beefed up the contact of the heat sink to the mounting
surface. Now the heat sink runs cooler however the diodes are still
too hot to hold a finger on at full load. This surprises me. If I had
good contact with the heat sink shouldn't IT be warmer than the
semiconductor mounted to it? The thought now occurs to me that since I
had to use plastic insulators from the junk box between the diodes and
the heat sink they may not be providing suitable thermal conductivity,
and this could also be a problem. I tried to buy a couple of suitable
insulating diode mounting kits for DO-4 devices that would provide
good thermal contact with the heat sink but I couldn't find a source
for them. My distributor couldn't find any and NTE doesn't seem to
sell them separately. I remember in my previous post that you had
suggested a transistor arrangement last time Jamie, but you're correct
in that I didn't fully understand what you were referring to at the
time. Now that Phil has provided the schematic it seems quite clear.
Thanks Phil. I could certainly try that arrangement too but after
putting this all down on paper, (so to speak), now I guess I'm
wondering. What if thermal conductivity between my diodes and my now
massive heat sink is my problem? My junk box mounting hardware
certainly could be suspect. Perhaps I should try to obtain the correct
insulators and then reassess this heat issue. Does anyone have a
source for them? The diodes are 1N2970B stud mount. Thanks, Lenny

http://www.datasheetcatalog.org/datasheet2/a/0a1whup4cfhdaygr9zxyqtaki7yy.pdf

I've lost the original context.
If the transformer has the secondary on the outside, removing almost
half the turns is a better solution in several dimensions.

A heat sink that's actually sinking heat is always colder than
the device it's sinking.
Measuring the temperature is often more helpful than "extremely hot".
My calculator doesn't have a key for that.

Bottom line is that it appears that every part of your system
is unsuitable for the function it needs to perform. Hang them
together and you get something that sorta works. There is no
magic fix.

 

[Phil Allison]

"klem kedidelhopper"

Now that Phil has provided the schematic it seems quite clear.
Thanks Phil. I could certainly try that arrangement too but after
putting this all down on paper, (so to speak), now I guess I'm
wondering. What if thermal conductivity between my diodes and my now
massive heat sink is my problem? My junk box mounting hardware
certainly could be suspect. Perhaps I should try to obtain the correct
insulators and then reassess this heat issue.


** WTF !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Wot sort of TOTAL BLOODY IDIOT tries to *insulate* the threaded stud
of a diode, zener or thyristor from the fucking heatsink ?

ROTFLMAO !!!!!!!!!!!!!!!!!!!!!



..... Phil

 

[mike]

On 4/16/2013 3:56 AM, Phil Allison wrote:

"klem kedidelhopper"

Now that Phil has provided the schematic it seems quite clear.
Thanks Phil. I could certainly try that arrangement too but after
putting this all down on paper, (so to speak), now I guess I'm
wondering. What if thermal conductivity between my diodes and my now
massive heat sink is my problem? My junk box mounting hardware
certainly could be suspect. Perhaps I should try to obtain the correct
insulators and then reassess this heat issue.


** WTF !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

Wot sort of TOTAL BLOODY IDIOT tries to *insulate* the threaded stud
of a diode, zener or thyristor from the fucking heatsink ?

ROTFLMAO !!!!!!!!!!!!!!!!!!!!!



.... Phil


Probably the same sort of person who thinks about what he wrote

before pressing that send button. ;-)

 

[klem kedidelhopper]

On Apr 16, 2:12 am, jurb6...@gmail.com wrote:

My question is why use shunt regulation in the first place.

Also true about the "pass" transistor, any transistor of suitable ratings will work and most likely help solve any thermal problems. It doesn't even matter if it is NPN or PNP. Works both ways.

The only reason to use shunt regulation is if you expect spikes which should be clamped. This can happen in an automotive application for example by just having loose or dirty battery cables, or any electrical fittings between the alternator (generator) and the load. This porblem is compunded these days by the fact that a modern fuel injected car with multiple coils (a coil pack)pulls a hell of alot of current to just run. Have any idea what a heated O2 sensor pulls ? what's more a car might have three or four of them.. That is on top of an injector for every cylider maybe more.

The thing is I am am having a hard time imagining another situation where such a regulation scheme would be appropriate. If it's automotive then it is, but that has not been revealed.

Yes this is related to my post of several months ago. I employed
Arfa's suggestion of using two of my 6.8V 10 watt zeners in series to
make a 13.6 V diode. And from my previous discussion this was then
put in series with the rectified 24VAC, (39VDC) to drop the voltage to
the input of my 12V regulator to a safer level. These two diodes were
mounted with insulators and silicone grease on a large old black
aluminum heat sink that I pulled out of a 1960's Delco automobile
radio. With the diodes in the circuit the voltage to the input of the
regulator was 19VDC, and under full load it was 16.5VDC, which is just
about where I wanted it to be. The only problem is that the diodes as
well as the heat sink were getting extremely hot when the regulator
was fully loaded, (1.0Amp). In the interest of survivability
assessment for my test I ran the unit overnight. So I added more
aluminum and beefed up the contact of the heat sink to the mounting
surface. Now the heat sink runs cooler however the diodes are still
too hot to hold a finger on at full load. This surprises me. If I had
good contact with the heat sink shouldn't IT be warmer than the
semiconductor mounted to it? The thought now occurs to me that since I
had to use plastic insulators from the junk box between the diodes and
the heat sink they may not be providing suitable thermal conductivity,
and this could also be a problem. I tried to buy a couple of suitable
insulating diode mounting kits for DO-4 devices that would provide
good thermal contact with the heat sink but I couldn't find a source
for them. My distributor couldn't find any and NTE doesn't seem to
sell them separately. I remember in my previous post that you had
suggested a transistor arrangement last time Jamie, but you're correct
in that I didn't fully understand what you were referring to at the
time. Now that Phil has provided the schematic it seems quite clear.
Thanks Phil. I could certainly try that arrangement too but after
putting this all down on paper, (so to speak), now I guess I'm
wondering. What if thermal conductivity between my diodes and my now
massive heat sink is my problem? My junk box mounting hardware
certainly could be suspect. Perhaps I should try to obtain the correct
insulators and then reassess this heat issue. Does anyone have a
source for them? The diodes are 1N2970B stud mount. Thanks, Lenny

http://www.datasheetcatalog.org/datasheet2/a/0a1whup4cfhdaygr9zxyqtaki7yy.pdf

 

[Wild_Bill]

The stud package devices are supposed to be directly mounted to the heatsink
metal (if the heatsink is electrically isolated) or mounted on mica
insulators to transfer heat.
The plastic insulators I'm familiar with (look like nylon or PE/PP material)
don't transfer heat.
Maybe you could use some TO-220 or other insulators to cut out some discs.
The glass-impregnated rubber insulators would be better than the present
plastic ones.
Use plastic bushing washers under the nuts if isolation is required.

It seems like a good idea to use parts that one has lying around, but there
are 3-terminal regulators that are capable of 5A readily available in most
common voltages.
Dropping 39V to 13.6V is a large span for a 3-terminal regulator, so a lower
output transformer would be more practical.

Transistor regulator circuits have been used for a long time, because they
were effective and cheaper, even after 3-terminal regulators became
available.
Everything from hobbiest PSUs to large industrial equipment commonly had
transistor regulators.

--
Cheers,
WB
..............


"klem kedidelhopper" <captainvideo462009@gmail.com> wrote in message
news:0599c63a-bc72-457f-85ee-71b25a7fe71c@r4g2000vbf.googlegroups.com...

Yes this is related to my post of several months ago. I employed
Arfa's suggestion of using two of my 6.8V 10 watt zeners in series to
make a 13.6 V diode. And from my previous discussion this was then
put in series with the rectified 24VAC, (39VDC) to drop the voltage to
the input of my 12V regulator to a safer level. These two diodes were
mounted with insulators and silicone grease on a large old black
aluminum heat sink that I pulled out of a 1960's Delco automobile
radio. With the diodes in the circuit the voltage to the input of the
regulator was 19VDC, and under full load it was 16.5VDC, which is just
about where I wanted it to be. The only problem is that the diodes as
well as the heat sink were getting extremely hot when the regulator
was fully loaded, (1.0Amp). In the interest of survivability
assessment for my test I ran the unit overnight. So I added more
aluminum and beefed up the contact of the heat sink to the mounting
surface. Now the heat sink runs cooler however the diodes are still
too hot to hold a finger on at full load. This surprises me. If I had
good contact with the heat sink shouldn't IT be warmer than the
semiconductor mounted to it? The thought now occurs to me that since I
had to use plastic insulators from the junk box between the diodes and
the heat sink they may not be providing suitable thermal conductivity,
and this could also be a problem. I tried to buy a couple of suitable
insulating diode mounting kits for DO-4 devices that would provide
good thermal contact with the heat sink but I couldn't find a source
for them. My distributor couldn't find any and NTE doesn't seem to
sell them separately. I remember in my previous post that you had
suggested a transistor arrangement last time Jamie, but you're correct
in that I didn't fully understand what you were referring to at the
time. Now that Phil has provided the schematic it seems quite clear.
Thanks Phil. I could certainly try that arrangement too but after
putting this all down on paper, (so to speak), now I guess I'm
wondering. What if thermal conductivity between my diodes and my now
massive heat sink is my problem? My junk box mounting hardware
certainly could be suspect. Perhaps I should try to obtain the correct
insulators and then reassess this heat issue. Does anyone have a
source for them? The diodes are 1N2970B stud mount. Thanks, Lenny

http://www.datasheetcatalog.org/datasheet2/a/0a1whup4cfhdaygr9zxyqtaki7yy.pdf

 

[Arfa Daily]

I've lost the original context.
If the transformer has the secondary on the outside, removing almost
half the turns is a better solution in several dimensions.

A heat sink that's actually sinking heat is always colder than
the device it's sinking.
Measuring the temperature is often more helpful than "extremely hot".
My calculator doesn't have a key for that.

Bottom line is that it appears that every part of your system
is unsuitable for the function it needs to perform. Hang them
together and you get something that sorta works. There is no
magic fix.

The original thread got very long and convoluted. There were many
suggestions involving series regulators, and wiring the secondary side of
the transformer in a different way that produced half the voltage input for
the (existing) regulator whilst still maintaining the full voltage that was
required for the rest of the circuitry - at least that's what I recall. The
current arrangement is not a shunt regulator. It is a series 'dropper' based
on items that Klem had to hand, and was suggestd as a 'quick fix' that he
understood, for the immediate problem that he had. It does do the job just
fine, but the mounting method to the heatsinks appears to be wrong.

The correct way to mount them is exactly the same as you would a TO3
transistor. That is with a mica washer between the flat face of the stud end
and the heatsink, and a plastic top hat washer on the other side of the
heatsink, with the hole being large enough to accommodate the narrow part of
the insulator. I have mounted stud diodes like this many times, and never
had a problem.

Arfa

 

[Phil Allison]

"Arfa Daily"

The correct way to mount them is exactly the same as you would a TO3
transistor. That is with a mica washer between the flat face of the stud
end and the heatsink, and a plastic top hat washer on the other side of
the heatsink, with the hole being large enough to accommodate the narrow
part of the insulator. I have mounted stud diodes like this many times,
and never had a problem.

** A TO3 device has a large surface area.

A DO4 stud device has almost none unless you include the thread.

So it is blindingly obvious that for best heatsinking, you screw them into a
threaded hole.

The heatsink may then need to be insulated from other metal work.

This is how all high power stud diodes zeners must be mounted, if you wish
to get anything like the maker's power ratings.

Fuck head.



.... Phil

 

[Phil Allison]

"mike" <ham789@netzero.net>


** Fuck off you BULLSHITTING bloody radio ham !!!!!!

 

[Phil Allison]

"Wild_Bill"

The stud package devices are supposed to be directly mounted to the
heatsink metal (if the heatsink is electrically isolated) or mounted on
mica insulators to transfer heat.

** The difference is HUGE.



.... Phil

 

[Arfa Daily]

"Phil Allison" <phil_a@tpg.com.au> wrote in message
news:at4u9eFdjtnU1@mid.individual.net...

"Arfa Daily"

The correct way to mount them is exactly the same as you would a TO3
transistor. That is with a mica washer between the flat face of the stud
end and the heatsink, and a plastic top hat washer on the other side of
the heatsink, with the hole being large enough to accommodate the narrow
part of the insulator. I have mounted stud diodes like this many times,
and never had a problem.


** A TO3 device has a large surface area.

A DO4 stud device has almost none unless you include the thread.

So it is blindingly obvious that for best heatsinking, you screw them into
a threaded hole.

The heatsink may then need to be insulated from other metal work.

This is how all high power stud diodes zeners must be mounted, if you wish
to get anything like the maker's power ratings.

Fuck head.



... Phil

Many extremely high power computer linear power supplies that I worked on
some years ago, would disagree with you. As would the construction of the
rectifier packs in a lot of arc welding machines. But as ever, I expect this
will set you off into one of your expletive peppered rants where only you
are right, and everyone else, despite their real-world experiences, is wrong
....

Arfa

 

[Arfa Daily]

"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
news:0abbt.399$Pj4.339@fx14.fr7...

I've lost the original context.
If the transformer has the secondary on the outside, removing almost
half the turns is a better solution in several dimensions.

A heat sink that's actually sinking heat is always colder than
the device it's sinking.
Measuring the temperature is often more helpful than "extremely hot".
My calculator doesn't have a key for that.

Bottom line is that it appears that every part of your system
is unsuitable for the function it needs to perform. Hang them
together and you get something that sorta works. There is no
magic fix.

The original thread got very long and convoluted. There were many
suggestions involving series regulators, and wiring the secondary side of
the transformer in a different way that produced half the voltage input
for the (existing) regulator whilst still maintaining the full voltage
that was required for the rest of the circuitry - at least that's what I
recall. The current arrangement is not a shunt regulator. It is a series
'dropper' based on items that Klem had to hand, and was suggestd as a
'quick fix' that he understood, for the immediate problem that he had. It
does do the job just fine, but the mounting method to the heatsinks
appears to be wrong.

The correct way to mount them is exactly the same as you would a TO3
transistor. That is with a mica washer between the flat face of the stud
end and the heatsink, and a plastic top hat washer on the other side of
the heatsink, with the hole being large enough to accommodate the narrow
part of the insulator. I have mounted stud diodes like this many times,
and never had a problem.

Arfa

Here is a mounting kit

http://www.surplussales.com/semiconductors/SemiC-MountingKits.html

For reference of what the parts look like, here is another

http://shop.rays-tek.com/transistor-diode-stud-mounting-kit-for-10-32-unf-thread-3525-p.asp

Arfa

 

[Ian Field]

"Jamie" <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote in message
news:SG1bt.540231$J13.57137@newsfe08.iad...

Arfa Daily wrote:



"klem kedidelhopper" <captainvideo462009@gmail.com> wrote in message
news:85e12e55-6f89-418e-9292-89a68c5a75ad@hc4g2000pbb.googlegroups.com...

I need to come up with a zener diode that will handle 1.0 amp
of current at 12V. I currently am using two 6.8V 10 watt stud
mount zeners in series mounted with insulators on a large
heatsink but it's pushing it. When the current spikes to 1.0 amp the
diodes get pretty hot.
Since I have a bag of these diodes Would it be a problem to
employ four of them in a series/parallel arrangement to attain
13.60V at 20watts? I realize that the exact voltage that each
diode "clips" at might be a bit different but would that really
matter? It would be a simple job to simply add one diode in parallel
with each one presently there. Thanks, Lenny


This is to do with the question from a couple of months back, where I
suggested using a couple of series zeners to drop a fixed amount in your
power supply, yes ?

I'm not a fan of putting zeners in parallel to up the power rating, as
it's almost impossible to guarantee that they will both have exactly the
same knee voltage, resulting in the current not dividing equally between
them, if at all. At 1 amp, you are not actually exceeding the 10 watt
rating of the zeners, as long as your heatsink is managing to keep the
junction temperatures below the quoted maximum.

I think I would be more inclined to either change to a larger heatsink,
or a matt black one if it's plain ally, or paint the existing one black,
or add a fan, or whatever combination of any of those. As you have 13.6v
across the zeners, you could just 'steal' a bit of that to run a small
12v fan. Just connect directly across the zeners, "+" terminal to the
upstream side and "-" to the downstream. That would be my preferred
solution.

Arfa

It is a lot easier to find and mount power transistor on a heat sink
than it is finding high power zeners. You can create a high current
zener this way.

I was thinking of suggesting something involving power transistors, but
there seems to be a growing opinion that the OP isn't using a big enough HS,
if space is an issue - maybe drill & tap for the zener studs in a surplus
CPU cooler.

Along the power transistor route, the TL431 programmable zener is a good
place to start - most of the manufacturers that source this part have
published application examples of adding an external booster transistor.

 

[Ian Field]

"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
news:yN0bt.3612$pP5.1814@fx24.fr7...

"klem kedidelhopper" <captainvideo462009@gmail.com> wrote in message
news:85e12e55-6f89-418e-9292-89a68c5a75ad@hc4g2000pbb.googlegroups.com...
I need to come up with a zener diode that will handle 1.0 amp
of current at 12V. I currently am using two 6.8V 10 watt stud
mount zeners in series mounted with insulators on a large
heatsink but it's pushing it. When the current spikes to 1.0 amp the
diodes get pretty hot.
Since I have a bag of these diodes Would it be a problem to
employ four of them in a series/parallel arrangement to attain
13.60V at 20watts? I realize that the exact voltage that each
diode "clips" at might be a bit different but would that really
matter? It would be a simple job to simply add one diode in parallel
with each one presently there. Thanks, Lenny


This is to do with the question from a couple of months back, where I
suggested using a couple of series zeners to drop a fixed amount in your
power supply, yes ?

I'm not a fan of putting zeners in parallel to up the power rating, as
it's almost impossible to guarantee that they will both have exactly the
same knee voltage, resulting in the current not dividing equally between
them, if at all. At 1 amp, you are not actually exceeding the 10 watt
rating of the zeners, as long as your heatsink is managing to keep the
junction temperatures below the quoted maximum.

I'm not a fan of parallel zeners either - but the parallel combination of 2
pairs of series connected zeners is slightly less dodgy than the series
connection of 2 parallel pairs of zeners.

 

[Gareth Magennis]

"klem kedidelhopper" wrote in message
news:0599c63a-bc72-457f-85ee-71b25a7fe71c@r4g2000vbf.googlegroups.com...

On Apr 16, 2:12 am, jurb6...@gmail.com wrote:

My question is why use shunt regulation in the first place.

Also true about the "pass" transistor, any transistor of suitable ratings
will work and most likely help solve any thermal problems. It doesn't even
matter if it is NPN or PNP. Works both ways.

The only reason to use shunt regulation is if you expect spikes which
should be clamped. This can happen in an automotive application for
example by just having loose or dirty battery cables, or any electrical
fittings between the alternator (generator) and the load. This porblem is
compunded these days by the fact that a modern fuel injected car with
multiple coils (a coil pack)pulls a hell of alot of current to just run.
Have any idea what a heated O2 sensor pulls ? what's more a car might have
three or four of them. That is on top of an injector for every cylider
maybe more.

The thing is I am am having a hard time imagining another situation where
such a regulation scheme would be appropriate. If it's automotive then it
is, but that has not been revealed.

Yes this is related to my post of several months ago. I employed
Arfa's suggestion of using two of my 6.8V 10 watt zeners in series to
make a 13.6 V diode. And from my previous discussion this was then
put in series with the rectified 24VAC, (39VDC) to drop the voltage to
the input of my 12V regulator to a safer level. These two diodes were
mounted with insulators and silicone grease on a large old black
aluminum heat sink that I pulled out of a 1960's Delco automobile
radio. With the diodes in the circuit the voltage to the input of the
regulator was 19VDC, and under full load it was 16.5VDC, which is just
about where I wanted it to be. The only problem is that the diodes as
well as the heat sink were getting extremely hot when the regulator
was fully loaded, (1.0Amp). In the interest of survivability
assessment for my test I ran the unit overnight. So I added more
aluminum and beefed up the contact of the heat sink to the mounting
surface. Now the heat sink runs cooler however the diodes are still
too hot to hold a finger on at full load. This surprises me. If I had
good contact with the heat sink shouldn't IT be warmer than the
semiconductor mounted to it? The thought now occurs to me that since I
had to use plastic insulators from the junk box between the diodes and
the heat sink they may not be providing suitable thermal conductivity,
and this could also be a problem. I tried to buy a couple of suitable
insulating diode mounting kits for DO-4 devices that would provide
good thermal contact with the heat sink but I couldn't find a source
for them. My distributor couldn't find any and NTE doesn't seem to
sell them separately. I remember in my previous post that you had
suggested a transistor arrangement last time Jamie, but you're correct
in that I didn't fully understand what you were referring to at the
time. Now that Phil has provided the schematic it seems quite clear.
Thanks Phil. I could certainly try that arrangement too but after
putting this all down on paper, (so to speak), now I guess I'm
wondering. What if thermal conductivity between my diodes and my now
massive heat sink is my problem? My junk box mounting hardware
certainly could be suspect. Perhaps I should try to obtain the correct
insulators and then reassess this heat issue. Does anyone have a
source for them? The diodes are 1N2970B stud mount. Thanks, Lenny

http://www.datasheetcatalog.org/datasheet2/a/0a1whup4cfhdaygr9zxyqtaki7yy.pdf




You might be better off using 2 heatsinks, bolting the zeners straight onto
them, and then isolating the heatsinks rather than the Zeners.



Gareth.