Zener Heat

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

Pooh Bear wrote:

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

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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 !

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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

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

Tam/WB2TT wrote:

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Just some slimeball POS SPAMmer.

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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.

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.

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.

--
John Popelish

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.

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:

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.

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

You don't have to use a zener string to drop all that voltage. There is another way to make a low voltage power supply from line power without using a transformer, if you only need a few dozen milliamps. Take a full bridge (four diodes). Feed it from the mains through a series capacitor. The capacitor's impedance will limit current into the bridge. You can filter the bridge's output and regulate it with a zener, since you will have limited current. The current out of the bridge will be directly proportional to the capacitance of your blocking capacitor. For 50 mA you would need something like 1 uF. So with no load your 5 volt zener conducting 50 mA would dissipate .25 watts, very doable.

You might want to have a resistor in there in addition to the capacitor, to alleviate possible inrush problems.

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

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:

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:

36 mA?

-Tim

amdx wrote: