Whilst mere hobbyists like myself can easily get away with making up a desired zener value from putting two smaller value ones in series to add together their rated voltages, can such a dodge be employed by professional designers without losing face? I mean, in the case where there is no commercially available zener with precisely the desired breakdown voltage? Would a pro designer be open to ridicule and risking his rep by pulling such a stunt?
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capable of operation to 204 degrees Centigrade / 400 degrees Fahrenheit and can program them from 50V to 2500V in 50V steps. A ten volt step version is in prototype now. Been sold for over ten years, and in many countries.
But, is it really good practice to use only a stack of zeners? Should there not be a hi impedance bleeder path in parallel to each of the diode(s) stack so they all start to conduct and zener properly. Phil I'm surprised you did not mention this.
ere not be a hi impedance bleeder path in parallel to each of the diode(s) stack so they all start to conduct and zener properly. Phil I'm surprised you did not mention this.
About the only problem with using a stack of zeners is running them at high enough current to prevent the avalanche process from turning itself off.
The isn't a problem with low voltage zener diodes which rely on the Zener m echanism, but anything above about six volts is an avalanche diode.
The volume in which the avalanche multiplication occurs is tiny, and if you operate at a current low enough that there's only one electron going throu gh that region at a time there's a risk that it will get through without ge nerating another pair of charge carriers, and the voltage across the diode will rise rapidly until another charge carrier gets generated by thermal no ise.
People exploit this effect by running zener diodes at low current to serve as noise sources.
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If you are very unlucky, a zener diode operating at low current can look as if it is oscillating at a few hundred MHz. We had a thread about this here many years ago.
It's perfectly fine. Adding a new part to the stockroom is a big deal, only worth it if you expect to need a lot of them. Two zeners in series may be good economics. It spreads the power dissipation, too.
I sometimes put resistors or caps in series or parallel, to avoid adding another line to the BOM of a board, even if all the right values are in stock. Saves loading up a reel on the pick-and-place to slam down one cheap part.
I even -gasp- sometimes use only half of a dual diode!
But ridicule? Reputation? Who would know?
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John Larkin Highland Technology, Inc
lunatic fringe electronics
I was then working for a Scientologist - Allen Wright.
Long story.
It first of all involved knowing the max possible average current flow into the screens of four EL34s inside Marshall. The answer is about 80 mA with the tubes driven into hard distortion from a sine generator.
I had used 5 x 18V, 5W zeners in series at fist - worked fine on the bench for 15 minutes. Case temp was hot but not sizzling and I knew power zeners were tough buggers.
Only 1.44 watts each dissipation, mounted well spaced on a large sized tag board that soaked up much of the heat. Must be OK.
But with the chassis back in the wood case, no through ventilation whatever, lots of heat dripping from the power tubes, a hot night and with a loony guitarist who did not take a break for hours - one of the solder joints got a few degrees too hot.
Sound stopped, musicians wept.
A change to 7 x 12 volt zeners was the fix.
All came about because tube factories in Europe and the UK closed down, leaving one US plant going - Philips /Sylvania.
They made an odd version of the EL34 /6CA7 tube that hated high screen voltages and would quickly burn out.
My "zener mod" made it possible to use them safely, I know of no other tech who came up with the same or any solution.
The very first customer got to do some Beta testing for me.
John Larkin does expect rather more praise than he deserves, and does seem to be correspondingly sensitive about his reputation.
Ridicule doesn't work on him. He writes it off as an expression of personal animosity and ignores it, which does mean that he posts the same nonsense time after time, which does make him ridiculous, but not in a way that he can detect.
There are programmable zeners, like the TL431 for example. That said, I have some zener with a red and green LED in series in my PIC programmer....
In the old days temperature compensation and stuff was done by putting things in series. But I'd use the TL431 for small currents. I got 50 of those on ebay for 1$84 free shipping in 2015...
Zeners around 5 V have nearly zero temperature coefficient, those above have a positive TC and those below a negative TC. Thus making a
10 V zener from two 5 V zeners would have zero TC. Using less than 4 V and over 6 V in series may also reduce the TC. Alternative put some forward biased diode(s) (TC -2.2 mV/K) in series to compensate for the positive TC of more than 5 V zeners.
You get a stable 6.2V reference voltage at precisely 7.5mA. How precisely it's worth hitting 7.5mA depends on the price of the diode - the 1N829 sells for quite a bit more than the 1N821.
Alternatively you can buy one of the cheaper parts and find the exact current gives the lowest temperature coefficient, but it requires a fairly expensive test rig to cover the temperature range.
There are integrated circuits around that can do better.
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