Time 'on' limiter?

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Time 'on' limiter?

Postby ogivemeahome » Wed Jan 18, 2012 9:39 pm

Hey all. Progress is slow on the RainBird thanks to my obsessive nature - here's the problem.
I would like to cut down on the consumable nature of deployment charges. Thus, I intend to have a deployment canister with a non-consumable resistive element. The output 'on' time for the stratologger is one second which is long enough to cause excessive heat. I need something that will limit the current to a reasonable level and also limit the 'on' time to <0.5s (adjustable?). Is there something like this out there already?
I've sketched a simple circuit with a MOSFET, R1, R2, and a capacitor C1. R1/R2 provides voltage bias to the gate of the MOSFET and C1 is in parallel with R1. The load is grounded via the source. If my homework was done right, the capacitor charges, placing the gate at a neg. potential relative to the source thereby limiting the 'on' time.
Now the tricky bit. I think, if I put a resistor R3 in series with C1, this will provide the correct bias to limit current immediately. I've made a spreadsheet to calculate values without R3 but with R3 is beyond my ability. Plus I'm finding the array of MOSFETs a bit confusing.
Ideally there would be no need of a circuitboard - I would use paint on insulation and encapsulate the whole thing in JB Weld.
I can post a sketch and/or e-mail the spreadsheet if needed.
Any help appreciated. Thanks.
A smooth sea never made a skilled mariner. (English Proverb)

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