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Registered Member #2123
Joined: Sat May 16 2009, 03:10AM
Location: Bend, Oregon
Posts: 312
You are correct with the current, you can go a bit lower if you are concerned about blowing the gate junction.
I don't know if the capacitance you are referring to is the gate-cathode PN junction capacitance, but its usually a good idea to have a resistor between gate and cathode to prevent DV/DT charging of the gate, so your drive circuitry would be looking at a parallel RC load.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I didn't know what "compliance voltage" of a SCR was, and I defer to MinorityCarrier because he actually makes semiconductors for a living, and I mostly just wreck them for a living.
Besides I'm too busy b-slapping Patrick for posting on here while drunk.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
No. I don't even remember what a Shockley diode is.
SCRs aren't guaranteed to work in parallel, no matter how you drive the gates. If one triggers early, then it can rob the anode voltage from the others and they may never trigger at all. I've seen railgunners parallel up SCRs, but it's one of these "At your own risk" things. Just because it worked for some guy once, doesn't mean it'll work for you. Maybe a fast risetime for the gate pulse will help, but a diac or tunnel diode isn't the only way to achieve that.
I assume here we're talking about the situation where the SCRs are in "parallel" as far as their gate circuits are concerned, but as far as the anode/cathode circuit is concerned, they're in series or independent, as you often see in a controlled rectifier. That works fine, and all you need are the ballast resistors to ensure that one gate doesn't hog all the drive.
Some power electronics textbooks say that you should steer gate pulses away from any SCRs that have reverse voltage on them, but in the controlled rectifiers I've built, I just drive all the SCRs all the time and have got away with it.
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Min pulse width to trigger SCR
