<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class=""><br class=""><div><blockquote type="cite" class=""><div class="">On Jun 30, 2020, at 9:30 PM, David G Dixon <<a href="mailto:dixon@mail.ubc.ca" class="">dixon@mail.ubc.ca</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div class="">I'm going to go out and measure the voltages vs current by using a bunch of<br class="">different resistors, draw a curve, and report back with my findings. (I'm<br class="">predicting that the voltage won't change that much as long as the current is<br class="">higher than about 1 mA.)<br class=""></div></div></blockquote></div><div class=""><br class=""></div>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.<div class=""><br class=""></div><div class="">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.</div><div class=""><br class=""></div><div class="">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.</div><div class=""><div class=""><br class=""></div><div class=""><br class=""><div class="">
<div style="orphans: auto; text-align: start; text-indent: 0px; widows: auto; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><div style="color: rgb(0, 0, 0); letter-spacing: normal; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; orphans: auto; text-align: start; text-indent: 0px; widows: auto; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">Terry Bowman, KA4HJH<br class="">"The Mac Doctor"<br class=""><br class=""></div><div style="orphans: auto; text-align: start; text-indent: 0px; widows: auto; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><a href="https://www.astarcloseup.com/" class="">https://www.astarcloseup.com/</a></div><div style="orphans: auto; text-align: start; text-indent: 0px; widows: auto; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class=""><br class=""></div><div style="color: rgb(0, 0, 0); letter-spacing: normal; text-transform: none; white-space: normal; word-spacing: 0px; -webkit-text-stroke-width: 0px; orphans: auto; text-align: start; text-indent: 0px; widows: auto; word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space;" class="">“The book said something astonishing, a very big thought.<br class="">It said that the stars were suns, only very far away.<br class="">The Sun was a star, but close up.”—Carl Sagan,<span class="Apple-converted-space"> </span><i class="">Cosmos</i>, 1980<br class=""><br class=""></div></div>
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