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SCR Triggering

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LAN_403

Quantum Singularity

Sat May 20 2006, 12:44AM

Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282

I am getting close to building my first electrlytic/scr powered launcher (induction). So far all my launchers have used maxwell caps and spark gaps. My problem is I dont have the spec sheet on my SCR's, and no one here was able to find any info on them either (atleast a while ago when I posted the part #). I am not sure how much the triggering requirements very from one SCR to another. Where would be a good starting point? Are SCR's voltage or current triggered? And do they require a resistor on the gate? Any help here would be appreciated. I plan on making a prototype launcher in the next few weeks. However I am putting in long hours at work lately and also starting building a quarter shrinker so it might take a while....

Wilson

Sat May 20 2006, 02:27AM

Registered Member #78 Joined: Thu Feb 09 2006, 11:27AM
Location: Sydney, Australia
Posts: 133

SCRs are current triggered. No including a gate resistor leads to the gate passing excessive current and dying. The large usually require 200-300mA gate trigger current
If you are using multiple SCRs in parallel, i would recommed that each should have its own gate resistor, as to aid current sharing somewhat.

Good luck,
Wilson

Quantum Singularity

Sat May 20 2006, 04:32AM

Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282

Sounds good. So 2 AA batteries and a 10 ohm resistor gives me 300mA without any other circuit resistance. But is there internal resistance in the gate? Or a voltage drop across it when on? I assume the proper R value would be influenced by these?

Another thing I was thinking... would it be better to put the launcher coil in series with the scr between the cap + and scr anode rather than between the scr cathode and ground? Since I assume the gate trigger curcuit must be grounded then the cathode should probably be direct to ground too - or the gate may float up to a high V when the circuit fires? Does this sound right?

Dr. Shark

Sat May 20 2006, 09:57AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

You want to check this thread on the old board:

Look for the circuit Texasrailgunner has posted, you can't get better than that.

Steve Conner

Sat May 20 2006, 12:34PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Yes, there is an internal resistance and a voltage drop, both in series, on the gate. Circuits I've seen tend to use a 12-24V supply with a 10-100 ohm series resistor for the triggering. So rather like a current source.

Dr. Shark

Sat May 20 2006, 04:03PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

I usually dont argue with Steve, but here I feel I have to. While it is perfectly acceptable to trigger an SCR like that in "normal" duty, with a pulse rise time in the order of 10s of ms, for an induction coil gun you could hardly do worse. You need to hit the gate with a pulse in the 100s of amperes, to kick the device into conduction as quick as possible.

Electroholic

Sat May 20 2006, 06:52PM

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

in my CG, i discharge a capacitor which was charged to 9v, with zener to clamp the voltage within 5.1v, no current limiting resistor. scr was a SKKT132 12E
130a cont.
4.7ka 10ms
at 1.2kv

Quantum Singularity

Sun May 21 2006, 04:51AM

Registered Member #158 Joined: Sun Feb 12 2006, 09:53PM
Location: Central Ohio
Posts: 282

I checked out that link to the old forum and I didnt realize how complex triggering an scr can be! I was expecting to use some batteries, a switch, and a resistor. Think I'll try the simple approach first and if I have troubles might try some of the more complex circuits.

FastMHz

Wed May 24 2006, 05:27PM

Registered Member #179 Joined: Thu Feb 16 2006, 02:08AM
Location: Hagerstown, Maryland - Close to Prime Outlets
Posts: 287

I think some of these suggestions, while electronically sound, may be overcomplicating the whole thing a bit. In my 3.5kj induction launchers, with 6 SCRs in parallel, I'm using 2 AA batteries with a 10ohm resistor to each gate. It fires reliably and powerfully every time.

Here's my project page:

Scroll down to see my spreadsheet. I'm pretty sure my SCRs are working properly with the efficiencies I'm getting. I also know that just 3 in parallel are enough to fire my launcher, since I was using just 3 for a while when I decided to add 3 more for good measure.

Now things are most likely a bit different for different SCRs and their intended purpose. The SKKH SemiPacks I have seem to be a great match for induction launchers.

Steve Conner

Wed May 24 2006, 05:49PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

I don't have a problem with what joe is saying. I've seen circuits for pulsed power applications that charge a fairly big capacitor to a few hundred volts, and use an IGBT to dump it into the SCR gate, through maybe a 1 ohm non-inductive resistor. >_< You might think you could use a second smaller SCR, but in fact you need an IGBT to stand the high di/dt. So it all gets complex.

I guess I don't really use SCRs in pulse applications: I've only used them for phase control of 50Hz AC power in the 10s of amps. I always use a small inductor in series with the AC line, to limit the di/dt that the SCRs can ever see at turn-on.

To trigger my SCRs, I used a ferrite cored pulse transformer whose primary was pinged by a MOSFET and whose secondary was connected to the gate through a resistor. I put an antiparallel diode across the gate to protect it from kickback when the transformer core reset. I only needed the transformer to isolate everything from the AC line for safety.

For one 90A SCR brick I used, the datasheet said to trigger it with 20V through a 20 ohm resistor, or something, so I wound the pulse transformer to give about 20V. It was a SKKT... brick similar to what a few of you are using, so ironically, I guess I'm actually driving the gates harder than FastMHz does, even though it's not for pulse duty. He only has 3V which is not really enough imo. I did trigger those same SCRs with a 9V battery, 10 ohm resistor, and a microswitch, with anode currents up to 100A peak, but again there was an inductor in circuit to limit di/dt.

For some smaller TO-220 SCRs I tried, it was enough to use 12V and a 100 ohm resistor or thereabouts.

I guess the only thing I would worry about in pulse applications is triggering the SCR with a mechanical switch whose contacts could bounce and cause a weak gate pulse. Or, hitting the gate too hard and trashing it >_< Again the SCR datasheet is your friend, it should always tell what voltage and series resistance to use.

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