September 28th, 2010

Steampunk Geiger Counter, Part 4

SparkGapLashup.jpg

Sparks. Let's talk about sparks. Last night I finally got things lashed up sufficiently to see whether I could translate three volts--a pair of C cells--to the neighborhood of 600 volts, using an old 25,000 ohm : 3.2 ohm output transformer and a spark gap. Got sparks. Didn't get 600V. (Got about 350 at best) Drained the batteries pretty quickly.

Nonetheless, it was a fascinating experiment, in a technological backwater I've never really messed with before. In summary: Put a pulse of current through the low-impedence winding of an output transformer, and a pulse of high voltage (compared to the input voltage) will appear across the transformer's high-impedence winding. Rectify the pulses, and you can accumulate voltage in a good, high-value low-leakage capacitor.

One way to rectify the pulses is to send them through a spark gap. The air gap breaks down against sufficiently high voltage and current passes one way across the gap. Put a cap in series with the spark gap and it will store a certain amount of charge each time the spark jumps.

At least, that's how it works in theory. In practice, with a very high resistance voltmeter across the capacitor, I saw two phenomena I wasn't expecting:

  1. About half the time, I get sparks on both a make pulse and a break pulse. (Ordinarily you only expect a spark on the break pulse.) If both make and break generate a spark, a pulse jumps the gap in the opposite direction as the pulse that preceded it. This means that the charge placed across the capacitor is then of the opposite polarity, which drains the cap by about as much energy as the previous pulse placed in it. Tinkering with the gap spacing didn't help, though the effect happened more often with a higher voltage (>6VDC) into the transformer.
  2. Eventually, the spark refuses to jump. It looks to me like accumulating a certain voltage on the cap bucks the spark gap and makes it harder to jump with the same pulse from the output transformer. And of course, once the spark ceases to jump, voltage on the capacitor ceases to rise.

With my lashup, once voltage got to about 320, there were no more sparks, with about .003" across the gap. Putting a stronger current source across the input didn't help. I was eventually pulsing 12.6VDC from my 30 amp linear bench supply, which heated up the poor transformer pretty badly but didn't give me any more voltage across the cap. Now, 320V may be enough to get conduction through a Geiger tube (I'll find out shortly) but the articles I've read suggest 600-900V, and seem to think that this can be had from a couple of C cells and a spark gap.

I did better placing a husky 1000 PIV 1N5408 silicon rectifier diode across the spark gap. The charge went up and only up (because current reliably passes only one way through a rectifier diode) but it still topped out at about 350V. I suspect that that limit may be inherent in the relatively small output transformer I'm using, and when time allows I'm going to troll the collection for the largest one I have and swap it in.

Now, a steampunk mad scientist never runs out of #40 copper wire and thinks nothing of winding his own transformers, so if that's the secret, a steampunk Geiger counter remains a possibility. However, I'm beginning to wonder how well I can achieve the steampunk ideal (no active devices) with only what I have lying around. Winding my own step-up transformer is just on the other side of what I'm willing to do.

The next step is making sure my two Geiger tubes are good by lashing them up to my 525V DC supply and exposing them to a (mildly) radioactive gas rectifier tube. Don't know yet when I'll be able to do that (real work has been piling up this week with the two of us trying to recuperate) but I'll continue the series here as time permits.