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Registered Member #485
Joined: Sun Jul 09 2006, 01:14AM
Location: San Francisco
Posts: 36
I currently use scrs to trigger my coilgun i just made a multistage coilgun and use two thin pieces of wire on either side of the barrel so when the projectile gets to that point it will rigger the next coil. my first question is if scrs are the best option iv seen things about igbts but im not sure about the pros and cons. seccond i tried to make an optical trigger instead of the two thin wires but it wont work are there any easy to make optical triggers u guys know of?
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
SCRs can't be turned off until the current flowing through them sinks below a certain level, usually less than an amp. This means your first coil might be on longer than it should, causing suckback. IGBTs can be turned off again, but have lower pulse current ratings for the equivalently priced SCR. (they do here at any rate) An optical trigger would be a good idea. Are you sure you hooked it up right? All it needs to be is 2 resistors, 1 LED and 1 phototransistor, and there's your photogate. + some control circuitry, but it doesn't need to be too complicated. Try it again and check out how other people did it.
Registered Member #546
Joined: Fri Feb 23 2007, 11:43PM
Location: Albuquerque, NM
Posts: 239
Here is a really nice resource for photogate triggers:
It's geared for high speed photo, but the gate and delay can be used for anything really.
... now if I may kinda threadjack ya for a second would some little Fairchild FQP 9N50 (1A 500V) MOSFETs be of any use in a coil gun? I have several left over from another project.
Registered Member #485
Joined: Sun Jul 09 2006, 01:14AM
Location: San Francisco
Posts: 36
GreySoul i actual used that exact one cuz i also do some high speed photography it just didnt seem to work for me it is possible that i built it wrong because i am not too sure how to follow schematics sometimes i get it sometimes i dont
Registered Member #546
Joined: Fri Feb 23 2007, 11:43PM
Location: Albuquerque, NM
Posts: 239
Probably the projectile is too small to fully break the beam.
when I used it to photograph shooting pellets through cards I used little blinders to constrict the IR beam spread.
Also, the faster and more sensitive your transistor is the better your trigger will work. a 2N2222 from radioshack isn't the best component in the world. I'm no good with part numbers, but maybe someone here can suggest a better NPN tranny.
edit: Did you try building the Schmitt photogate trigger? It's a little trickier to fine tune, but you can tweak it so that a hair blowing by will trigger it. There is a little more delay in the Schmitt trigger, but it is far more sensitive than the simple NPN gate.
Registered Member #485
Joined: Sun Jul 09 2006, 01:14AM
Location: San Francisco
Posts: 36
for some reason i cant get the photogate to work this is the scr i am trying to trigger would it be possible for someone to show me an easy to follow schem
Registered Member #95
Joined: Thu Feb 09 2006, 04:57PM
Location: Norway
Posts: 1308
Is it supplying enough current? Try this circuit. The transistor can be any small signal transistor which can supply 0.2A. Adjust resistor values as needed. Make sure the circuit is triggering before putting a SCR on the output.
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