| Author |
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Tim M
Guest
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 Posted:
Sun Feb 13, 2005 2:01 am Post subject:
Re: Zener Heat |
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Zeners are in series, and to respond to another question, I do realize
the 12V diodes will dissipate about twice as much power as the 6.2V ones.
I'm feeding it with approx. 155 VDC, and the string of zeners adds up to
146.4 (at test conditions there's 149 V across them, which is close
enough for me). I only have 1/4-W resistors handy, so the test setup
has two 330-ohm resistors in parallel to give 165-ohm at 0.5 W.
-Tim
amdx wrote:
| Quote: | "Tim M" <tym1@spammenot.cwru.edu> wrote in message
news:cuk9dh$fmq$1@eeyore.INS.cwru.edu...
I'm designing a power supply that involves putting a bunch of zeners in
series, specifically 6 1N4742As (12V, 1W) and 12 1N4735As (6.2V, 1W).
Hi Tim,
Let's see if we can understand what you are doing.
1) Are the zeners in Series?
This would meaning you would have 98 volts at the top of the string.
2) What is the voltage you are feeding the string with ?
3) What is the value of your voltage dropping resistor?
If you could answer these three questions, then we can probably help.
Mike
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amdx
Guest
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| Posted:
Sun Feb 13, 2005 2:55 am Post subject:
Re: Zener Heat |
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| Quote: | I'm feeding it with approx. 155 VDC, and the string of >zeners adds up to
146.4 (at test conditions there's 149 >V across them, which is close enough |
for me).
Oh!, 12-6volt and 6-12 volt = 144volt
| Quote: | I only have 1/4-W resistors handy, so the test setup
has two 330-ohm resistors in parallel to give 165-ohm at 0.5 W.
|
So you have about 6v across 165 ohms or about 3.6 milliamps through the
string at no load.
If you connected a 40ma load at the 72 volt junction
the voltage at the top of the string would start to drop below the 155volt
supply voltage and the string would stop functioning properly. I'm sure your
running this close to keep heat low in the string.This would happen even if
you spread out the load among several zener junctions.
The 12volt zener at the top of the string would have about a half watt
across it, which is ok.
Well Tim, now that I've went through this I think you need to find a way
to cool your zeners (sorry, I guess you new that ;-).
If it's a one of unit, you might bend a thin u shaped piece of alum.
around each diode. Such as,
poor ascii art. l_o_l The vertical sections will help get rid of
the heat.
I'm curious why you need such high voltages for ICs.
I don't have any ideas other than heat sinking.
Mike |
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R.Lewis
Guest
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| Posted:
Sun Feb 13, 2005 3:02 am Post subject:
Re: Zener Heat |
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"Tim M" <tym1@spammenot.cwru.edu> wrote in message
news:culna6$i65$1@eeyore.INS.cwru.edu...
| Quote: | Zeners are in series, and to respond to another question, I do realize
the 12V diodes will dissipate about twice as much power as the 6.2V ones.
I'm feeding it with approx. 155 VDC, and the string of zeners adds up to
146.4 (at test conditions there's 149 V across them, which is close
enough for me). I only have 1/4-W resistors handy, so the test setup
has two 330-ohm resistors in parallel to give 165-ohm at 0.5 W.
-Tim
snip
|
From what you have told us so far this is either a top secret product for
the military or you have but a limited idea of what you are doing.
Be careful - 150Vdc is dangerous - and get someone in to do whatever it is
you are playing with before you harm yourself.
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Tim M
Guest
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| Posted:
Sun Feb 13, 2005 3:22 am Post subject:
Re: Zener Heat |
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Yep...I'm in the process of making some copper "wings" as another has
suggested and soldering them to the leads as they enter the device.
Each fin is about 12mmX25mm in area, which should give a good surface
area for cooling.
-Tim
amdx wrote:
| Quote: | I'm feeding it with approx. 155 VDC, and the string of >zeners adds up to
146.4 (at test conditions there's 149 >V across them, which is close enough
for me).
Oh!, 12-6volt and 6-12 volt = 144volt
I only have 1/4-W resistors handy, so the test setup
has two 330-ohm resistors in parallel to give 165-ohm at 0.5 W.
So you have about 6v across 165 ohms or about 3.6 milliamps through the
string at no load.
If you connected a 40ma load at the 72 volt junction
the voltage at the top of the string would start to drop below the 155volt
supply voltage and the string would stop functioning properly. I'm sure your
running this close to keep heat low in the string.This would happen even if
you spread out the load among several zener junctions.
The 12volt zener at the top of the string would have about a half watt
across it, which is ok.
Well Tim, now that I've went through this I think you need to find a way
to cool your zeners (sorry, I guess you new that ;-).
If it's a one of unit, you might bend a thin u shaped piece of alum.
around each diode. Such as,
poor ascii art. l_o_l The vertical sections will help get rid of
the heat.
I'm curious why you need such high voltages for ICs.
I don't have any ideas other than heat sinking.
Mike
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Tim M
Guest
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| Posted:
Sun Feb 13, 2005 3:29 am Post subject:
Re: Zener Heat |
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Well, the project is for a Nixie clock, and I'm using a Villard voltage
doubler and line current to power the tubes, as 1.414*110 V is below the
required ionization voltage. To keep things simple, I'm tapping across
one of the capacitors that only sees 1.414*110 V and using the Zeners to
drop the voltage to a low enough level for a 5V voltage regulator.
Some of the old TTL chips used in my desgin require a relatively high
current (e.g. up to 25 mA for the 7441, though I will be using its
output to switch transistors as opposed to driving the Nixies directly,
so I expect to be well below this).
I've been a bit incomplete about the rest because I haven't finished
designing the logic and tube driver stages and haven't nailed down exact
parameters beyond a maximum current drain specification.
I'm quite aware of the dangers of high voltage--line AC especially--but
thank you for the reminder.
-Tim
R.Lewis wrote:
| Quote: | "Tim M" <tym1@spammenot.cwru.edu> wrote in message
news:culna6$i65$1@eeyore.INS.cwru.edu...
Zeners are in series, and to respond to another question, I do realize
the 12V diodes will dissipate about twice as much power as the 6.2V ones.
I'm feeding it with approx. 155 VDC, and the string of zeners adds up to
146.4 (at test conditions there's 149 V across them, which is close
enough for me). I only have 1/4-W resistors handy, so the test setup
has two 330-ohm resistors in parallel to give 165-ohm at 0.5 W.
-Tim
snip
From what you have told us so far this is either a top secret product for
the military or you have but a limited idea of what you are doing.
Be careful - 150Vdc is dangerous - and get someone in to do whatever it is
you are playing with before you harm yourself.
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John Woodgate
Guest
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| Posted:
Sun Feb 13, 2005 3:59 am Post subject:
Re: Zener Heat |
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I read in sci.electronics.design that Tim M <tym1@spammenot.cwru.edu>
wrote (in <culsfj$rf6$1@eeyore.INS.cwru.edu>) about 'Zener Heat', on
Sat, 12 Feb 2005:
| Quote: | To keep things simple, I'm tapping across
one of the capacitors that only sees 1.414*110 V and using the Zeners to
drop the voltage to a low enough level for a 5V voltage regulator.
|
This is not a good design. You would do far better to derive your 5 V
supply from a 9 V wall-wart.
If you are putting the zener string in series with your regulator, it
simply won't work, nor will it work if you put the regulator across a 12
V zener at the ground end of the string. Your zener current of 3.6 mA is
the maximum you can draw from the regulator.
--
Regards, John Woodgate, OOO - Own Opinions Only.
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk |
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Tim M
Guest
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| Posted:
Sun Feb 13, 2005 4:19 am Post subject:
Re: Zener Heat |
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36 mA?
-Tim
amdx wrote:
| Quote: | I'm feeding it with approx. 155 VDC, and the string of >zeners adds up to
146.4 (at test conditions there's 149 >V across them, which is close enough
for me).
Oh!, 12-6volt and 6-12 volt = 144volt
I only have 1/4-W resistors handy, so the test setup
has two 330-ohm resistors in parallel to give 165-ohm at 0.5 W.
So you have about 6v across 165 ohms or about 3.6 milliamps through the
string at no load.
If you connected a 40ma load at the 72 volt junction
the voltage at the top of the string would start to drop below the 155volt
supply voltage and the string would stop functioning properly. I'm sure your
running this close to keep heat low in the string.This would happen even if
you spread out the load among several zener junctions.
The 12volt zener at the top of the string would have about a half watt
across it, which is ok.
Well Tim, now that I've went through this I think you need to find a way
to cool your zeners (sorry, I guess you new that ;-).
If it's a one of unit, you might bend a thin u shaped piece of alum.
around each diode. Such as,
poor ascii art. l_o_l The vertical sections will help get rid of
the heat.
I'm curious why you need such high voltages for ICs.
I don't have any ideas other than heat sinking.
Mike
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Tim M
Guest
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| Posted:
Sun Feb 13, 2005 4:21 am Post subject:
Re: Zener Heat |
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Actually, I think it should be 36 mA and not 3.6 mA.
I wish to do thing sans transformers for size and weight issues, plus I
don't want the finished project to require two separate electrical outlets.
The complete power supply circuit works under 36 mA load except for the
heating of the zeners.
-Tim
John Woodgate wrote:
| Quote: | I read in sci.electronics.design that Tim M <tym1@spammenot.cwru.edu
wrote (in <culsfj$rf6$1@eeyore.INS.cwru.edu>) about 'Zener Heat', on
Sat, 12 Feb 2005:
To keep things simple, I'm tapping across
one of the capacitors that only sees 1.414*110 V and using the Zeners to
drop the voltage to a low enough level for a 5V voltage regulator.
This is not a good design. You would do far better to derive your 5 V
supply from a 9 V wall-wart.
If you are putting the zener string in series with your regulator, it
simply won't work, nor will it work if you put the regulator across a 12
V zener at the ground end of the string. Your zener current of 3.6 mA is
the maximum you can draw from the regulator. |
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Rich Grise
Guest
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| Posted:
Sun Feb 13, 2005 5:44 am Post subject:
Re: Zener Heat |
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On Sat, 12 Feb 2005 16:21:37 +0000, amdx wrote:
| Quote: | "Tim M" <tym1@spammenot.cwru.edu> wrote in message
news:cuk9dh$fmq$1@eeyore.INS.cwru.edu...
I'm designing a power supply that involves putting a bunch of zeners in
series, specifically 6 1N4742As (12V, 1W) and 12 1N4735As (6.2V, 1W).
Hi Tim,
Let's see if we can understand what you are doing.
1) Are the zeners in Series?
This would meaning you would have 98 volts at the top of the string.
|
I get (6 * 12) + (12 * 6.2) = 146.4V.
At 50 mA (which you seem to have snipped), that's 146.4 * 0.05 = 7.32
watts. Each 6.2V zener dissipates 6.2 * 0.05 = .31W, and each 12V zener
dissipates 12 * .05 = .6 W.
| Quote: | 2) What is the voltage you are feeding the string with ?
3) What is the value of your voltage dropping resistor?
If you could answer these three questions, then we can probably help.
|
Yeah, those two question. ;-)
Good Luck!
Rich |
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Rich Grise
Guest
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| Posted:
Sun Feb 13, 2005 6:11 am Post subject:
Re: Zener Heat |
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On Sun, 13 Feb 2005 01:12:04 +0000, Fred Bloggs wrote:
| Quote: |
Well, the project is for a Nixie clock, and I'm using a Villard voltage
doubler and line current to power the tubes, as 1.414*110 V is below the
required ionization voltage. To keep things simple, I'm tapping across
one of the capacitors that only sees 1.414*110 V and using the Zeners to
drop the voltage to a low enough level for a 5V voltage regulator.
Some of the old TTL chips used in my desgin require a relatively high
current (e.g. up to 25 mA for the 7441, though I will be using its
output to switch transistors as opposed to driving the Nixies directly,
so I expect to be well below this).
You need to get rid of the zeners and put that power dissipation in a
component built to take it, like a power resistor,
|
He needs to find a different way of getting a 5V logic supply than by
subtracting 144V through a zener stack from a nixie supply.
Good Luck!
Rich |
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Rich Grise
Guest
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| Posted:
Sun Feb 13, 2005 6:11 am Post subject:
Re: Zener Heat |
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On Sun, 13 Feb 2005 01:55:49 +0000, Fred Bloggs wrote:
| Quote: |
Adding copper fins to the leads of the zeners does dissipate heat nicely
so that the temperature is within spec,
Wonderful- nothing like lots of exposed metal at high ungrounded voltage.
but I agree that this design is
a bit flaky for continuous use.
I will give the doubler-capacitor and capacitor-rectifier tricks a try.
Will have to see what I have in the parts bin.
That's even more foolish than your original idea.
|
Bloggs, don't be an idiot.
It is _not_ "more" foolish. In fact, I'd argue that it's not _as_ foolish.
But I use the logic that, since he already has an unisolated 170V supply,
the addition of a couple of capacitors and a couple of diodes and ONE
zener (depending on what voltage he actually wants) gets rid of the hot
exposed string, reduces the current requirements of the 170V supply almost
a hundredfold, and - well, if that's not enough, then oh well.
Oh, yeah. It gives him the LV supply at arbitrary current, if he's clever
enough to copy the design properly:
http://ww1.microchip.com/downloads/en/AppNotes/00954A.pdf
(three minute google search) ;-p
Thanks,
Rich |
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Fred Bloggs
Guest
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| Posted:
Sun Feb 13, 2005 6:11 am Post subject:
Re: Zener Heat |
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| Quote: | Adding copper fins to the leads of the zeners does dissipate heat nicely
so that the temperature is within spec,
|
Wonderful- nothing like lots of exposed metal at high ungrounded voltage.
| Quote: | but I agree that this design is
a bit flaky for continuous use.
I will give the doubler-capacitor and capacitor-rectifier tricks a try.
Will have to see what I have in the parts bin.
|
That's even more foolish than your original idea. |
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John Popelish
Guest
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| Posted:
Sun Feb 13, 2005 6:11 am Post subject:
Re: Zener Heat |
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Tim M wrote:
| Quote: |
I've noticed, and I haven't fallen into the deathtrap yet. ;)
Adding copper fins to the leads of the zeners does dissipate heat nicely
so that the temperature is within spec, but I agree that this design is
a bit flaky for continuous use.
I will give the doubler-capacitor and capacitor-rectifier tricks a try.
Will have to see what I have in the parts bin.
|
Once you have everything except the supply to your satisfaction, you
might look into an isolated low voltage logic supply (wall wart based)
and a small step up switcher to power the neon. Everything cool,
isolated and efficient.
--
John Popelish |
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Tim M
Guest
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| Posted:
Sun Feb 13, 2005 6:11 am Post subject:
Re: Zener Heat |
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I've noticed, and I haven't fallen into the deathtrap yet. ;)
Adding copper fins to the leads of the zeners does dissipate heat nicely
so that the temperature is within spec, but I agree that this design is
a bit flaky for continuous use.
I will give the doubler-capacitor and capacitor-rectifier tricks a try.
Will have to see what I have in the parts bin.
-Tim
Rich Grise wrote:
| Quote: | On Sat, 12 Feb 2005 05:37:42 -0600, me wrote:
I'm designing a power supply that involves putting a bunch of zeners
in series, specifically 6 1N4742As (12V, 1W) and 12 1N4735As (6.2V,
1W). Yes, I know that's a lot of zeners, but current drain of my
circuit is only 40-50 mA. However, I notice the zeners getting very
hot, even though I'm well below the max power dissipation per diode.
Did anyone mention that that is a really stupid design, and why is he
running 40-50 ma through them all??????
I didn't specifically say, "stupid," but in another post I said that
rather than try series dropping zeners from a nixie supply, since it's a
death trap already, just feed a half-wave doubler with a capacitor and
zener regulate it, with a current path, of course.
But that was just a few minutes ago, so by tomorrow maybe he'll have
noticed. ;-)
Cheers!
Rich
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Rich Grise
Guest
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| Posted:
Sun Feb 13, 2005 6:11 am Post subject:
Re: Zener Heat |
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On Sat, 12 Feb 2005 05:37:42 -0600, me wrote:
| Quote: | I'm designing a power supply that involves putting a bunch of zeners
in series, specifically 6 1N4742As (12V, 1W) and 12 1N4735As (6.2V,
1W). Yes, I know that's a lot of zeners, but current drain of my
circuit is only 40-50 mA. However, I notice the zeners getting very
hot, even though I'm well below the max power dissipation per diode.
Did anyone mention that that is a really stupid design, and why is he
running 40-50 ma through them all??????
|
I didn't specifically say, "stupid," but in another post I said that
rather than try series dropping zeners from a nixie supply, since it's a
death trap already, just feed a half-wave doubler with a capacitor and
zener regulate it, with a current path, of course.
But that was just a few minutes ago, so by tomorrow maybe he'll have
noticed. ;-)
Cheers!
Rich |
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