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Zener Heat

T

Tim M

Jan 1, 1970
0
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.

What sort of heat sinking considerations should I be taking. The
1N4742As are glass, the others metal. I have them laid out next to each
other lengthwise (see below), which probably wasn't the best choice for
heat dissipation.

| | |
--- --- ---
| | | | | |
--- --- ---
| | |

Thanks,

Tim
 
P

Pooh Bear

Jan 1, 1970
0
Tim said:
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.

What sort of heat sinking considerations should I be taking. The
1N4742As are glass, the others metal. I have them laid out next to each
other lengthwise (see below), which probably wasn't the best choice for
heat dissipation.

Indeed.

Heat dissipating components need to be separated by a respectable distance
on a pcb.

You may improve heat dissipation by using more copper foil around each
component leg.

I had a guy lay out a pcb for me with a 1W3 zener run at about 70%
dissipation. Despite my requests, he didn't allow much space for the pads.

Luckily it was simply a prottoype. The leads of the zener virtually *welded*
themsleves to the pcb ! I'm not kidding either !

Component ratings are entirely dependent on mounting /orientation /airflow -
etc etc.......


Graham
 
T

Tim M

Jan 1, 1970
0
I was also thinking of making a (long) heat sink out of aluminum and
having it contact the package of each zener, probably with some heat
conductive paste in between. With the devices so closely spaced, there
isn't a lot of working room at the leads.

-Tim
 
M

me

Jan 1, 1970
0
I'm designing a power supply that involves putting a bunch of zeners

Did anyone mention that that is a really stupid design, and why is he
running 40-50 ma through them all??????
 
M

Mook Johnson

Jan 1, 1970
0
Some quick math shows that your zeners will drop about 145 volts
collectively and pass 50mA which is 7.3 watts.

Even a power transistor mounted to a standard PCB will show significant
temperature rise.

Do you realize that you 12 volt zeners are disipating twice the power as
your 6.2 volt zeners (do you run out of 12s and started throwing in 6s to
fill in?).

Remember that power rating is making assumptions as to the temperature and
haet dissipating ability of what it is soldered too. look at the derating
curve for your 12V diodes and measure the lead temperture at the PCB and
I'll bet you're outside the recommended operating range.
 
R

R.Lewis

Jan 1, 1970
0
Tim M said:
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.

What sort of heat sinking considerations should I be taking. The
1N4742As are glass, the others metal. I have them laid out next to each
other lengthwise (see below), which probably wasn't the best choice for
heat dissipation.

| | |
--- --- ---
| | | | | |
--- --- ---
| | |

Thanks,

To get any credable answer you will have to tell us more about why you are
putting so many zeners in series and what you are trying to achieve with
this.
 
T

Tam/WB2TT

Jan 1, 1970
0
Tim M said:
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.

What sort of heat sinking considerations should I be taking. The 1N4742As
are glass, the others metal. I have them laid out next to each other
lengthwise (see below), which probably wasn't the best choice for heat
dissipation.

| | |
--- --- ---
| | | | | |
--- --- ---
| | |

Thanks,

Tim

Why don't you make up a compound circuit using a TO3 power transistor and
fewer zeners. This way you can keep the zener current down to 1 ma, and bolt
the TO3 down to some heavy metal. What are your input and output voltages?

Tam
 
T

Tim M

Jan 1, 1970
0
I am using multiple diodes because I would like a reliable voltage drop
across a variety of currents, with the maximum being 50 mA and the
nominal current being much less, probably around 25 mA. I also need to
pull off multiple different supply voltages for different parts of my
circuit. I am powering multiple ICs (older TTL which draw more current)
some of which require eclcectic operating voltages.

My maximum current estimate is drawn from the summed maximum current
rating from all devices in the circuit.

With limited board space as well, I need to crowd the diodes together.

-Tim
 
F

Fred Bloggs

Jan 1, 1970
0
I am using multiple diodes because I would like a reliable voltage drop
across a variety of currents, with the maximum being 50 mA and the
nominal current being much less, probably around 25 mA. I also need to
pull off multiple different supply voltages for different parts of my
circuit. I am powering multiple ICs (older TTL which draw more current)
some of which require eclcectic operating voltages.

My maximum current estimate is drawn from the summed maximum current
rating from all devices in the circuit.

That's not how zeners are used. Why don't you tell us what the
"eclectic" voltages are, referenced to common, and what is the loading
on each, what is the input voltage referenced to common.
 
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F

Fred Bloggs

Jan 1, 1970
0
i have used their milia treatment for milia seed on my eye brown area
and below cheek. after three applications, i noticed a reduction in the
milia size, quite impressed as i thought only laser surgery can solve
my problem.
--------------------------------
[email protected]
i keep seeing reviews and raves about this
http://www.naturalisproducts.com and http://www.organiconline.com.sg .
many people are discussing in beauty forums and magazines have positive
reviews on this . but this thing ain't new, its been around for many
years! anyone tried can feedback to me on exactly how good it is?

----------------------------------------
can anyone help me please, am looking for the local distributor or any
shop selling the naturalis range of skin and body care products, from
this company http://www.naturalisproducts.com . looking for this
urgently. for those who have not come across it, its some foodbased
anti-aging products. i googled for this and received result showing its
available at http://www.organiconline.com.sg. i need this
urgently but shipping from singapore will take some time, if anyone is
distributing this please contact me at [email protected]
urgently. i have a group of us looking to buy this. thanks!

That's makes about as much sense as anything else in this thread....
 
A

amdx

Jan 1, 1970
0
Tim M said:
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
 
J

John Fields

Jan 1, 1970
0
I am using multiple diodes because I would like a reliable voltage drop
across a variety of currents, with the maximum being 50 mA and the
nominal current being much less, probably around 25 mA. I also need to
pull off multiple different supply voltages for different parts of my
circuit. I am powering multiple ICs (older TTL which draw more current)
some of which require eclcectic operating voltages.

---
If you're expecting a reliable series of voltage drops with different
currents going through the Zeners, you may well wind up being nastily
surprised, since the Zener voltages will only be guaranteed to be
within spec at a particular current (Izt, the Zener test current) and
all the errors will add up in the string and show up at the taps.
 
J

John Popelish

Jan 1, 1970
0
Tim said:
I was also thinking of making a (long) heat sink out of aluminum and
having it contact the package of each zener, probably with some heat
conductive paste in between. With the devices so closely spaced, there
isn't a lot of working room at the leads.

You will probably do better if you solder a small copper or brass shim
stock wing on each lead of the diodes right against the body. The
leads are the main conduit of heat out of the die. A pair if 1/4'
wide by 1" high wings can dump a lot of heat, compared to the diode
and ordinary circuit board pads.
 
T

Tim M

Jan 1, 1970
0
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
 
A

amdx

Jan 1, 1970
0
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
 
R

R.Lewis

Jan 1, 1970
0
Tim M said:
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.
 
T

Tim M

Jan 1, 1970
0
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
 
T

Tim M

Jan 1, 1970
0
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
 
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