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Can SCR's be used in parallel?

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LAN_403

aarpcard

Wed Jun 30 2010, 04:38AM

Registered Member #2848 Joined: Tue May 04 2010, 05:19AM
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Posts: 44

The peak current coming from my capacitor bank for my coil gun is about 700 amps at 360 volts. If I have several SCR's rated at 40amps and 400 volts can I put a bunch of them in parallel, so that way they will be able to handle the peak current?

I think I've seen people do it before with coilguns but I'm curious because I've read contradiction claims in which putting SCR's in parallel has and has not worked.

Turkey9

Wed Jun 30 2010, 05:20AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

You can to a point. The most I've seen in parallel is 5. You have to have a 100r or so resistor in series with EACH gate so that they share current equally. But you have to downrate the current when you do this. Add all the surge currents together and then multiply by .8 and that is the top current rating.

Electroholic

Wed Jun 30 2010, 08:22AM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

Putting SCRs in parallel definitely works for coilguns.

Is your SCR rated at 40A continuous?
How long is your pulse?

A 40A SCR will probably have a pulse rating close to 1000A. But more importantly, you have to look at the action integral.

Pinky's Brain

Wed Jun 30 2010, 09:04AM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Everything you ever wanted to know about thyristors :

If it's 40A continuous and the pulse is short enough it might indeed already be able to handle it ... apart from the integral also watch out for max di/dt.

Slid

Wed Jun 30 2010, 05:27PM

Registered Member #853 Joined: Thu Jun 21 2007, 03:08PM
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Yes, In my multistage I used 1200volt/40 amp thyristors since it was cheap to buy a lot of them. It worked good, I just had to add some more thyristors at each stage so they could share the load (Max 6 at last stage). As Turkey9 sayes, you need however to add resistors. When I tied all the thyristorgates together, not all thyristors would turn on and one would be fried from the overload. The thyristor is current triggered, so there is nothing preventing the thyristor from stealing more trigger current, even when it has triggered. For this reason, I would recommend to use a resistor AND a small capacitor in series to each gate. This way all the thyristors will get the triggercurrent they need, and they will not blow your gatedriver by overheating it.

Z28Fistergod

Wed Jun 30 2010, 07:50PM

Registered Member #2040 Joined: Fri Mar 20 2009, 10:13PM
Location: Fairfax VA
Posts: 180

What's the part number?
Assuming you have more than one energy storage capacitor you can configure your bank so that a single capacitor, or group of capacitors, only has one discharge path through one SCR. This will ensure equal load sharing, and as a plus you can change the effective capacitance by choosing how many legs you want to trigger.

aarpcard

Wed Jun 30 2010, 08:06PM

Registered Member #2848 Joined: Tue May 04 2010, 05:19AM
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Posts: 44

Thanks for all the replies. Ok I have a couple questions.

So I can put the SCR's in parallel to have them share the load, but I would need to include some 100 ohm resistors in series with the SCR's. How badly would these resistors effect the current traveling to the coil?

Yes, the SCR's are rated at 40 amps continuous, 800 amps pulse. However I tried using one of them alone by itself and it fried. My pulse is about 12ms, but I'm working on bringing that down. My guess is the pulse of 12ms was too long for the SCR causing it to fry. I just don't want to put the rest of the SCR's I have in parallel, and end up frying them as well.

Z28Fistergod

Wed Jun 30 2010, 08:18PM

Registered Member #2040 Joined: Fri Mar 20 2009, 10:13PM
Location: Fairfax VA
Posts: 180

Turkey said to put the resistors in series with the triggering leads.
What was the circuit setup for your test? Usually the SCR surge ratings are based on one half of a 60Hz sinewave, 8.3ms, so you may be just over the absolute max.

aarpcard

Wed Jun 30 2010, 09:39PM

Registered Member #2848 Joined: Tue May 04 2010, 05:19AM
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Posts: 44

I just realized that the rest of my SCR's are not working correctly. Would these be a decent replacement for my voltage and current ratings?

Pinky's Brain

Wed Jun 30 2010, 11:12PM

Registered Member #2901 Joined: Thu Jun 03 2010, 01:25PM
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Posts: 837

It is more likely you exceeded di/dt during turn on.

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