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I have very little experience in this field so bear with me Very Happy

I want to make a small second stage rocket igniter with just capacitors + transistors. The first capacitor would be the "timer" capacitor which slowly drains while the first stage burns. When it sufficiently depleted, it should cause a second capacitor to discharge across the igniter.

I can see how to make something like this with relays, but want to use transistors. Thanks for any help!
Only having transistors and capacitors makes this annoying, but it's doable (..if you have resistors too). The main annoyance is that you don't have a reference voltage to control the triggering so you need to take advantage of the diode drop across transistors.

A sample circuit to get you started:


V1 starts at 3V and switches off at simulation start; the blue plot is voltage on the high side of C1, and the green plot is current through R2.
Thanks tari! Is v2 supposed to be where I put the second capacitor + igniter?
I'd look at V2 as a charged capacitor (approximated as a voltage source) and R2 as the igniter.

Storing enough energy to reliably trigger an igniter might be tricky. I don't know what kind of current is typical, but one source suggests 1.5A is a reasonable amount of current draw for nichrome. Maintaining that kind of current for long enough to actually ignite an engine may require a pretty large capacitor.

If you want something properly reliable I'd probably just go for a digital timer and switching the load with a FET, but *shrug*.
  
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