Demonstration Cell - Variable Spacing Plate Cell

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I've been doing some math, trying to figure out how I'm going to get this optocoupler set up.

The resistor that Stan uses... 100 Ohm Clarostat VP25K [$8.33] http://www.newark.com/jsp/search/productdetail.jsp?SKU=10M5288&CMP=AFC-TL10000001

appears to be too low a value and too low a wattage rating.

Dynodon demonstrated the setup running up around 80 volts, which will put 800 mA through the H11D1 optocoupler, which is rated for 100 mA, so say 50% duty cycle, it's still getting 400 mA. Also that resistor, as large as it is at 25 watts, would be getting 64 watts.

Re-doing the math for what I would expect it to be, this should be a 1000 Ohm resistor, possibly missing a zero on the posted value?

This resistor, http://www.newark.com/ohmite/l25j1k0/power-resistor/dp/02F2291?matchedProduct=VP25K+1000&whydiditmatch=rel_default
listed as "Alternative for VP25K 1000" (compared to VP25K 100), is probably what is required.

This will give 100 mA to the opto at 100 volts, with 10 watts (now within the 25 watt rating), and 10 mA to the opto at 10 volts, the collector needs about 4 mA to work, so this would probably work fine from 10 volts to 100 volts on the variac.

Or... considering a 50% duty cycle all the time, we could drop the resistor value by half...

a 500 ohm resistor: http://www.newark.com/ohmite/l25j500/power-resistor/dp/02F2288

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Did you notice the 100 Ohm Clarostat VP25K is a wirewound resistor compared to the other two you linked to are ceramic. Could the wirewound aspect make a difference in the circuit? Where is this resistor in the circuit? Between the op-amp and the optocoupler?

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can someone explain what a longitudinal pulse would be?

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The animation below depicts a longitudinal pulse in a medium.

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Donald I don't think the opto is carrying the amps you see on the amp gauge.That power goes through the SCR.The opto is only triggering the SCR.The resistor just drops voltage down to within the range of the opto.Stan stated in a video I saw, in Stans collection, where he says it take 30 volts for the gas to start.
The part number for the resistor came off of it,so it's right.
Thats how I see it.
Don

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Ah yes, I see my mistake, I was assuming the full voltage would be across the resistor, but it's not, because it's not grounded anywhere there, it's only the voltage across the SCR which is much less.

(http://img.photobucket.com/albums/v81/bigbuba/Picture2-3.png)

So I can probably try my 204 Ohm 20 Watt resistor without too much trouble.

Thank you!

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I'm planning on using a ~10 A transistor out of an old power supply or computer, forget just where I got it.

Will this work fine compared to the SCR? What might the difference be?

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(http://img.photobucket.com/albums/v81/bigbuba/Picture15-5.png)

Just adding this to the collection of info on this set up. Here it shows the 30 volts required to start the EPP that Dynodon just mentioned.