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Registered Member #1451
Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661
I had the same/similar problem with my first coilgun... I'm pretty sure it was because I didn't use a clamping diode. The SCR went short circuit and an exact same one was fine when a diode was added. A good source for SCRs is stmicroelecronics. There are 50 amp rms at 1200v ones that you can sample three for free and they're not very expensive buying straight out once you need more. Of course, sample responsibly! Three in parallel handled my 312j bank at 610V into a relatively low inductance coil... it drew 650amps.
Registered Member #191
Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720
Yet another way of triggering an SCR. 1, clamp gate voltage with zener 2, limit peak current with inductor and/or resistor 3, charge up a small capacitor, say 2200uf 10V and dump it into the gate.
BTW, i put two SCRs up for sale. I've used the exact same SCRs in my 1kV 1.3kJ capacitor bank.
Registered Member #1643
Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039
MinorityCarrier wrote ...
without current limiting I guarantee you will blow the gate if you much exceed the gate Vf.
300 volts? You know better than that.
If you really want to prevent blown SCR gates, use a pulse transformer to fire the SCR gate.
No, I do not know better than that. I told you, I never did disc launchers. I did 400V on my coilgun using a single 40TPS08, and enough caps to total around 150J 400V, and that SCR lasted me the whole year. So of course I'm not going to consider 300V bad, 400V was perfect on a coilgun, which is different. I never used disc launchers so I wouldn't know! Any who, I'm getting 10 that are similar to arcstarters, so I can parallel 5 like he did, and not have to fear a dead SCR. The actuall gate was triggered by 2AA, or 3V. And 3V is less than its normal 4V rating. So the gate is fine. My smaller coil sends a lot more back than my larger coil. Because I could do over 15 shots with my 2 inch coil, but my 1 inch diameter, 2 shots = no SCR.
Registered Member #2123
Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312
My apology for this misunderstanding, that comment of mine wasn't directed at you.
The SCR gate maximum average power dissipation is often low, around 5 watts, for a lot of very high power SCR's, and can be lower for other SCR's. I was surprised that someone quite knowledgeable would say it's ok to use 300 volts for gate drive, even with a current limiting resistor.
If you want to turn SCR's on fast, a diode clamped pulse transformer output is the way to go.
Registered Member #191
Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720
MinorityCarrier wrote ...
My apology for this misunderstanding, that comment of mine wasn't directed at you.
The SCR gate maximum average power dissipation is often low, around 5 watts, for a lot of very high power SCR's, and can be lower for other SCR's. I was surprised that someone quite knowledgeable would say it's ok to use 300 volts for gate drive, even with a current limiting resistor.
If you want to turn SCR's on fast, a diode clamped pulse transformer output is the way to go.
It's just ohm's law, man. I don't see a problem in that at all.
Registered Member #1643
Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039
Electroholic wrote ...
It's just ohm's law, man. I don't see a problem in that at all.
Which might explain why my iggy on my old transformer was able to power a LED without exploding it, it was simply under 30ma and well, yeah. LED works.
Registered Member #2123
Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312
If you want to wire 300 volts through a resistor to a gate of a SCR driving an inductive load, and rely upon Ohm's law and gate Vf to drop the voltage, by all means. I'll stick with 3 volts.
What happens to the PNPN structure when the 300-volt sourced gate PN junction is backbiased during inductive kickback, before the snubber diode turns on, would be interesting since the GE SCR manual doesn't appear to define this condition.
Registered Member #680
Joined: Fri May 04 2007, 03:20PM
Location:
Posts: 15
I contacted the largest maker of gate drives in the US to buy a gate drive for a 600V 1200A SCR. He said they did not make one for single shots. I said I charge my caps to 200V. He said to fire it using the 200V from the capacitor bank and a resistor to limit the current to 200ma and don't worry about the volts, the current is what fires the SCR. This is common in his industry, it is called "anode firing." And it works fine.
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SCR seems locked?
