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SCR Trigger for Coil Gun

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fatboyslim

Fri Nov 23 2012, 09:56AM

Registered Member #1779 Joined: Sat Oct 25 2008, 11:05AM
Location:
Posts: 32

Hi There

I am planning a coil gun project, however while sourcing parts the SCR has been the most difficult to find. What I am wondering is whether anyone has any experience using the SCR which are so accessible on ebay.

This is what i'm referring to: SCR Thyristor 1600v 100A

My capacitor bank is 450v @ 5000uf. Barry's coilgun calculator puts my peak amps at around 800-900 amps. What i'm wondering is if anyone has previously used these SCR's and whether they have a peak amp rating high enough.
The one i have linked have a average rating of 100A and i've seen OTHER SCR with a average rating of 70A which can handle 1200A peak, so was wondering about this one.

Messaging the seller yielded no information or datasheets :(

Help would be much appreciated

Thanks :D

Shrad

Fri Nov 23 2012, 11:31AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
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Posts: 780

such rated block thyristor have a pulsed current rating of at least 1000 amps, for what I know

anyway, for their price, its worth testing them

Yandersen

Sat Nov 24 2012, 06:32AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

Don't!
At first, there are no datasheet. Second - this huge monster is obviously far more than you really need. Third - for $20 you can buy a few couples of normal thyristors so to have a backup.

fatboyslim

Sat Nov 24 2012, 12:04PM

Registered Member #1779 Joined: Sat Oct 25 2008, 11:05AM
Location:
Posts: 32

I've increased my capacitor bank to 10000uf. Unfortunately i cannot find any Thyristors which can handle switching roughly around 1000-1100 amps. I'm limited to what I can find on Ebay as i live in New Zealand. Would it be safe to assume that since its so beefy that it should be able to handle this peak current?

klugesmith

Sat Nov 24 2012, 07:18PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

Are you inexperienced at searching the Internet? Google helped me to find KP100 data in less than a minute.

This one

points to a nice-looking data sheet without the spec you want.

This one

points to a fuzzy scanned datasheet with the spec you want. KP100 without the A suffix, but IMHO close enough.
Note the pulse rating I_TSM of 1.8 kA @ 50 Hz, without further detail. A conservative interpretation would be that that's the peak of a single half-sine pulse 10 ms wide.

Good luck with your coilgun. Keep these things in mind:

1) SCR gate, unlike MOSFET, is low impedance & depends on current, but not too much! A fresh 9V battery could probably source enough current to damage the gate.

2) While waiting for your SCR, you could trigger the coilgun by just touching wires together. This is relatively gentle with a coil in the circuit, compared to a short circuit on the capacitor bank. Wear eye protection & mind your 450 volt insulation. If LCR network is underdamped, the metallic switch (unlike SCR) does not turn off at the reverse-voltage peak. (click the "show voltage" box in Barry's simulator.)

Yandersen

Sun Nov 25 2012, 09:52AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

I believe, DigiKey ships everywhere, check them out:

Here, directly to SCRs:

fatboyslim, to find thyristor which will fit your circuit you need to find out the approximate pulse length (T=Pi*sqrt(L*R)), peak current (I=U*sqrt(C/L)) and see I*I*t rating for fusing for the thyristor. Probably, your pulse length will be around 1ms, so you may just triple amperage given for 50Hz operation mode of a thyristor - it should handle the estimated current.

fatboyslim

Mon Nov 26 2012, 01:08AM

Registered Member #1779 Joined: Sat Oct 25 2008, 11:05AM
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Posts: 32

Thanks so much! Just as a back up I went out and bought SKKH57/12E Thyristor. I know its has a much higher pulse current rating than I need but it was only 15 dollars. It gives me a bit of leeway in case I up the cap bank.
This may sound dumb but do you have to wind the coil in one direction?
For example when starting from the left do you wind from (left ---- right, then start back at the left and go right) or is it ok to go (left ---- right then go back from right ---- left)

PSCG

Mon Nov 26 2012, 08:29AM

Registered Member #3792 Joined: Sun Mar 27 2011, 06:07PM
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Posts: 136

Only make sure that ALL the turns of the coil are facing the same direction while winding, i.e. clockwise or counter - clockwise (i hope it makes sense). Winding it as left-->right-->left is the same as left-->right-->back to left-->right.

Yandersen

Mon Nov 26 2012, 09:55AM

Registered Member #6944 Joined: Fri Sept 28 2012, 04:54PM
Location: Canada
Posts: 340

And if projectile is symmetrical, make sure to mark one end as a front and do not insert it in different direction, otherwise efficiency will drop a little each time you mess with the direction (remagnetisation losses).

"only 15 dollars" - you are pretty rich, huh? :) Keep in mind, that no matter how much current your fancy SCR can handle, it will be fried more likely due to some shortcut somewhere. So far I fried few, but at no case it was overcurrented by the coil. BTW, double check insulations all around the coil - if shortcut will happen inside, the coil will blow like a grenade, and SCR together with it.

fatboyslim

Mon Nov 26 2012, 12:47PM

Registered Member #1779 Joined: Sat Oct 25 2008, 11:05AM
Location:
Posts: 32

Oh Wow, thanks for the tip, i'll definitely double check my insulations. Sorry i'm just confused about the I*I*t rating. Is that a peak rating before the thyristor fuses?
My simulation right now (after some changes) peaks at over 1600 amps @ 5.4ms (91.9hz). Ascertaining from what you said, since the frequency is higher, the thyristor should be able to handle it because its for a shorter duration of time? or have i completely lost the plot here :P

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