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Registered Member #1329
Joined: Mon Feb 18 2008, 07:31PM
Location: Harwinton Connecticut
Posts: 53
Hello Again,
Now I have a new problem.. I bought some very larhe SCR's These are rate at 800 Volts and 300 amps with a 60 Hz 1/2 cycle rating of over 5000 amps. This is a big Powerex SCR with a stud the size of my thumb!!
On my disc launcher, As I raise the voltage of the capacitor bank with a variac, the SCR fires everytime I hit 130 volts. Here is what I have tried so far. A 1K ohm resistor in the gate to ground, no change. The gate shorted to ground. No change. The SCr in the + side of the circuit with the coil to ground. I and also with the + side of the circuit to coil and the SCR to ground. The SCR will fire evry timeas long as the voltage keep below 130 volts.
I fired the SCR without putting a freewheeling diode in a couple times but this was at lower voltage. This was not a cheap SCR and I do not want to kill another one.
The coils are 2 2" diameter pancake flat coils each made of #12 wire and in paralell. Each has about 12 micro HY of inductance. Please help me with this... What a bummer, am I doing something wrong or am I just a killer of SCR's
Oh Yes... when it self triggers... ther is a nice clean 200 usec long pulse with almost no overshoot
Registered Member #1329
Joined: Mon Feb 18 2008, 07:31PM
Location: Harwinton Connecticut
Posts: 53
Hello Dr. Spork, Thank you for replying. Yes I did try that but no difference in the device. The 130 Volts is measured at the capacitor bank.
I made a few discoveries in the mean time.
On the storage scope there is indeed a very large negative going current spike. I was so intent in looking at the turn on pulse that the scope was expanded to the point where it was off the screen.
I added some fast recovery diodes across the coil and the offending spike was reduced considerably.
Also I noticed that "dry firing" the launcher produced a very large spike offending spike.
After a number of shots below 130 volts with a projectile I had a nice clean current pulse with little if any reverse pulse.
So I changed the SCR. Charged up the cap bank and let it sit at 500 volts... NO PROBLEM.
A couple of test shots and I was up to 500 volts, still watching the current pulse. The energy imparted on the projectile is incredible. The way I test the launcher is with a very thick cardboard box on top og the coil. The box is about 1 foot tall and the surface the projectile hits is 10 thichnesses of cardboard. On top of that there is a 4 bs weight to hold the box in place. At 600 volts it launched that into the cieling and another SCR died. Using barry's calculator I find that I exceeded the fusing current on a single SCR. At least I know what is going on!!
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SCr keeps self firing AHHHH I think I killed it.
