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Registered Member #680
Joined: Fri May 04 2007, 03:20PM
Location:
Posts: 15
Which would be better to trigger the gate of a 600V 2000A SCR, a 9V 1A regulated wall wart, 4 AA alkaline cells in series, or one 9V alkaline battery?
Secondly, would an old tyme mercury wall switch be better than a simple push button switch in avoiding any contact bounce when triggering the gate of the SCR?
I will be using the SCR to discharge a capacitor bank.
Registered Member #51
Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263
Depending on the SCR, you probably need between 50ma and 200ma of current to trigger the gate. Do you have a data sheet? one would be very helpful in finding the gate current. I would vote for the 4 AA's in series just because 9v's don't last very long. I would stay away from a walwart because they only offer say 3kv isolation from mains voltage. I would not worry about contact bounce because the SCR will latch on anyway, and you will not notice if the contacts bounce.
Registered Member #205
Joined: Sat Feb 18 2006, 11:59AM
Location: Skørping, Denmark
Posts: 741
cjk2 wrote ...
I would not worry about contact bounce because the SCR will latch on anyway, and you will not notice if the contacts bounce.
I would worry, and you should!
With a 200mA gate trigger signal requirement, you have a rather big SCR on your hands.
With incomplete triggering, you will open a channel in the SCR that doesn`t allow current to pass in the whole cross section of the die, a condition referred to as tunnelling. This may cause the SCR to latch. (Permanently! )
The right way to avoid this is to create a proper gate trigger signal.
1st. you should debounce the contact, with a schmitt trigger, then use this signal to activate the enable pin of a gate driver chip, like a UCC37xxx type.
You want to dump a, say, 220uF cap into the gate as fast as you can.
Connect a Zener diode across the gate to protect against over voltage, and an inductor to drain leakage charge, so that the big scr gate doesn`t slowly charge up during a standby condition.
There is also a good schematic on which I have used to good effect, triggering a 3 inch dia. SCR to controll 20kA in a disk shooter.
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SCR gate triggering
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