I'm an EE student, trying to design a coilgun project. I have had luck with a basic prototype similar in construction to the one on this site, however with smaller flash capacitors in parallel (total of 600uF). The problem is that, despite including a trigger circuit, I must charge the caps with the coil/scr sub circuit disconnected, then disconnect the charging circuit and plug in the "firing" circuit. Any idea what could be causing this?

Also, I'm curious about the firing circuit in the schematic. What prevents it from frying when the SCR gate is triggered? It seems like a ton of current will be dumped back into that side of the circuit, frying the switch and damaging the 9 V battery at the very least.

Note: I'm using a different SCR to that used by that schematic - I'm using a 2N6509G in a standard to-220 package. He is using a silicon power cube, with 4 of the 5 contacts connected. Perhaps this difference is what protects his trigger circuit? Any help appreciated!


1 Answer 1


A SCR when triggered it conducts until the current through decreases to the value of less than holding current Ih. Bigger SCRs have also higher Ih, smaller SCRs have low Ih.

Now what happens is that you trigger the SCR and when the caps are drained, the charging circuit still delivers the current which is higher that the Ih of your SCR. You can increse the resistor value of R1 to limit that current.

R3 limits the gate current, so you might increase the value according to used SCR.

  • \$\begingroup\$ Ah, alright, I suppose i'll have to try it. R3 is what was bugging me. I just feel like since he's using 24mF total in his caps, even 4.7k ohms would still allow a ton of current to blow through the charging circuit and fry the contacts on the switch. I unfortunately only have relatively small momentary switches at my disposal, and have had this happen on more than one occasion with smaller designs. The idea is to get this working then swap out the caps for a much bigger bank I made, so i'll need to get the trigger isolated a bit more first. Would an optoisolator work? \$\endgroup\$
    – W. MacTurk
    Commented Feb 13, 2019 at 20:23

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