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Registered Member #2901
Joined: Thu Jun 03 2010, 01:25PM
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Yanom wrote ...
What's the proper setup for 2 or 3 in series? I'll be discharging an (about) 100uF 3kV capacitor through a 11ohm, 45mH induction coil and I need the SCRs for switching.
Rough approximation of di/dt (I don't think damping or resonance will play much of a role at turn on). U=L*di/dt, 3000=45e-3*di/dt, di/dt = OMFGBBQ ... that's a couple of orders of magnitudes over what a common stud thyristor is made for.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Today I was repairing a high voltage pulse discharge system (25 p.p.s.) that is mainly a variac, a 10 kV 50 Hz transformer, voltage doublers and 22 x 1600V thyristors in series, I can confirm that series connected scr are quite easy to configure and drive.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Pinky's Brain wrote ...
Yanom wrote ...
What's the proper setup for 2 or 3 in series? I'll be discharging an (about) 100uF 3kV capacitor through a 11ohm, 45mH induction coil and I need the SCRs for switching.
Rough approximation of di/dt (I don't think damping or resonance will play much of a role at turn on). U=L*di/dt, 3000=45e-3*di/dt, di/dt = OMFGBBQ ... that's a couple of orders of magnitudes over what a common stud thyristor is made for.
well i didnt verify his math for period or current, but yeah i was worried it would be like a nuclear EMP... My brain is limping, i had better sleep...
1/2 x C x V^2 should equal 450 joules present, and if he pushes that at 3kv in uS ,thats a lot of current. but what will the natural freq of this particular RCL circuit be?
Registered Member #4659
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Pinky's Brain wrote ...
Rough approximation of di/dt (I don't think damping or resonance will play much of a role at turn on). U=L*di/dt, 3000=45e-3*di/dt, di/dt = OMFGBBQ ... that's a couple of orders of magnitudes over what a common stud thyristor is made for.
oh yes, I forgot about di/dt triggering. That means that once I set the first one off, the rest will turn on by themselves because of the extreme voltage they are subjected to?
Barry's RLC sim, which I am using to simulate the discharge, says that a 110uF, 3000V* capacitor discharged through my coil will peak at about 100 Amps (well within the allowable range of my SCRs), then ringdown to about -40 Amps.
Registered Member #2901
Joined: Thu Jun 03 2010, 01:25PM
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No, it means all the silicon around the gates will fry ... thyristors take some time to start conducting across their entire surface, until that happens their current carrying capacity is very small.
Registered Member #4659
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Pinky's Brain wrote ...
No, it means all the silicon around the gates will fry ... thyristors take some time to start conducting across their entire surface, until that happens their current carrying capacity is very small.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
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Yanom wrote ...
Pinky's Brain wrote ...
Rough approximation of di/dt (I don't think damping or resonance will play much of a role at turn on). U=L*di/dt, 3000=45e-3*di/dt, di/dt = OMFGBBQ ... that's a couple of orders of magnitudes over what a common stud thyristor is made for.
oh yes, I forgot about di/dt triggering. That means that once I set the first one off, the rest will turn on by themselves because of the extreme voltage they are subjected to?
Barry's RLC sim, which I am using to simulate the discharge, says that a 110uF, 3000V* capacitor discharged through my coil will peak at about 100 Amps (well within the allowable range of my SCRs), then ringdown to about -40 Amps.
what time period did you sim say for 0 to 100 amps?
also your indepentant triggers comment isnt what pinky meant, he meant your SCR's main silicon channel cant fully conduct immeaddiatly, your SCR turns on like a wire slowly increasing its diameter, first at 30 awg, then eventually after a few uS it will be wider like a 10 awg wire. but in that time youve fused the silicon--- poof ! 30 awg wire cant cunduct 100 amps without glowing.
Registered Member #4659
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Patrick wrote ...
what time period did you sim say for 0 to 100 amps?
also your indepentant triggers comment isnt what pinky meant, he meant your SCR's main silicon channel cant fully conduct immeaddiatly, your SCR turns on like a wire increasing its diameter, first at 30 awg, then eventually after a few uS it will be wider like a 10 awg wire. but in that time youve fused the silicon. 30 awg wire cant cunduct 100 amps without glowing.
0 to 100 in 4 milliseconds.
I see what you mean by the independent triggers thing. How do I get around that? or should I just buy a thyratron/trigatron?
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
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ok so it peaks in 4 mS, what is the slope of the line for the first 1 mS ? (in amps per second please.)
the standard trick in your case is to use a saturable reactor, which slows the current, till full turn on, at that time the inductor saturates and effectivly disappears from the circuit, then there are high currents rising quickly.
someone built a SSTC back in 1997 using a reactor and SCR, but i cant remember the name or site, maybe on the wayback machine?
if that site can be found, it explains what youll need to do.
Registered Member #4659
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Patrick wrote ...
ok so it peaks in 4 mS, what is the slope of the line for the first 1 mS ? (in amps per second please.)
the standard trick in your case is to use a saturable reactor, which slows the current, till full turn on, at that time the inductor saturates and effectivly disappears from the circuit, then there are high currents rising quickly.
someone built a SSTC back in 1997 using a reactor and SCR, but i cant remember the name or site, maybe on the wayback machine? if that site can be found, it explains what youll need to do.
Interestingly enough, my coil isn't a Tesla coil. It's a coilgun coil. That's right - I plan on building a high voltage coilgun for my high school science fair. I haven't found many coilgun designs over the traditional 300-400V, and I wanted to try it out. My planned circuit is this ( but with 10+ capacitors where I only drew in 5). It's essentially a manually-operated Marx generator circuit. The final switching device (the one this thread is about) must discharge from the positive of the last capacitor towards the negative of the first, a difference of several kV.
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SCR's above 2,200 volts?
