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Registered Member #4659
Joined: Sun Apr 29 2012, 06:14PM
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Posts: 158
So I'm building a coilgun and I've got an SCR that i'm going to use to discharge a 550uF 600v capacitor through the coil. But I'm having trouble triggering it. Connecting the positive terminal of the cap to the gate of the SCR is infeasible, because it will probably blow out the SCR (max gate power 10W) unless I use a resistor of such high value that it reduces the gate current below the required 50mA or so. So I must use a battery circuit. I built this circuit, but there is an arc across the fire switch as I close it. Does anyone know why? How would you reccomend triggering the SCR?
Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
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Posts: 615
connect battery negative directly to SCR anode. In your current circuit as soon as the SCR triggers the anode voltage rises to within a few voltage of cathode - well above the 9V trigger. This is due to the inductor on the anode side.
Registered Member #4659
Joined: Sun Apr 29 2012, 06:14PM
Location:
Posts: 158
2Spoons wrote ...
connect battery negative directly to SCR anode. In your current circuit as soon as the SCR triggers the anode voltage rises to within a few voltage of cathode - well above the 9V trigger. This is due to the inductor on the anode side.
are you sure you dont mean cathode side? inductor is on cathode side. also its supposed to be 30mH not 30H
Registered Member #4659
Joined: Sun Apr 29 2012, 06:14PM
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Posts: 158
so, I should put the coil before the SCR in the circuit? Will do. Also, it appears that I hook up the neg. terminal of the 9v battery to the (cathode, emitter, output, whateverYouCallIt) terminal of the SCR.
What if I had to trigger multiple SCR's on different circuits (circuits I don't want touching each other) with the throw of a single switch? How would I make that work?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
You've got it ass-backwards- the trigger voltage should be applied between gate and cathode of the SCR. (Edit: I see you realised this in your above post. )
To trigger multiple isolated SCRs, pulse transformers are often used. Suitable transformers are available for a few bucks.
Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
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Posts: 615
Sorry, not thinking straight - got anode cathode reversed. Re that circuit: "current flows in loops". That simple statement is absolutely fundamental to all electronic design. Now ask yourself how could current flow into the gate when there is no return path back to the battery?
Registered Member #4659
Joined: Sun Apr 29 2012, 06:14PM
Location:
Posts: 158
2Spoons wrote ...
Sorry, not thinking straight - got anode cathode reversed. Re that circuit: "current flows in loops". That simple statement is absolutely fundamental to all electronic design. Now ask yourself how could current flow into the gate when there is no return path back to the battery?
...I'm an idiot. :/
I guess this means that an SCR trigger circuit can never be isolated from the power flowing through the SCR. Which is bad, because I may need to trigger many SCR's at different voltages with the throw of a single switch. The fact that there is a current path from gate to cathode also makes this problematic.
I'm thinking about trying to build what's essentially a marx generator, but at lowish voltages (100-400v) with SCRs instead of spark gaps.
Registered Member #2939
Joined: Fri Jun 25 2010, 04:25AM
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Posts: 615
Steve Conner has already told you the answer to this : pulse transformers. This will provide the isolation you are looking for. Opto couplers are another possiblity.
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Need Help Triggering SCR
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