[sdiy] really inaccurate zeners

[KA4HJH]

> On Jun 30, 2020, at 9:30 PM, David G Dixon <dixon at mail.ubc.ca> wrote:
> 
> I'm going to go out and measure the voltages vs current by using a bunch of
> different resistors, draw a curve, and report back with my findings.  (I'm
> predicting that the voltage won't change that much as long as the current is
> higher than about 1 mA.)


I'm a member of several groups for hobbyists building and repairing geiger counters and related gadgets. The ubiquitous CDV-700 Geiger Counter that was issued to Civil Defense authorities during the Cold War (and later) was manufactured by a number of companies under government contract. All of them had to conform to a laundry list of specifications but beyond that it was "make it as cheaply as possible". Every manufacturer had a uniquely different circuit design(s) from the others but almost all of them used the Corortron, a cold cathode device, as the -900VDC regulator for the GM tube power supply. Naturally many of these have gone bad over the decades and they're routinely replaced with several high voltage zeners in series.

You can guess where this is headed. The current used by a GM tube even at high avalanche rates is very small so each "regulator" has to be tested under load (such as it is). Fortunately, good GM tubes have a large enough plateau that they can tolerate -10V here and +15V there. The only really critical element is that you need a ~1kV DVM with an input impedance on the order of 1GΩ. 100MΩ just doesn't cut it and even a decent Fluke DVM is only something like 50MΩ. The ones they're giving away at Harbor Freight are 10~20MΩ.

Fortunately, one of the bright lights in the GeigerCounters group sells both "calibrated" (he marks them with a Sharpie) regulators and custom-made DVMs to members at cost. He's a prince.


Terry Bowman, KA4HJH
"The Mac Doctor"

https://www.astarcloseup.com/

“The book said something astonishing, a very big thought.
It said that the stars were suns, only very far away.
The Sun was a star, but close up.”—Carl Sagan, Cosmos, 1980


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