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SCR seems locked?

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Killa-X

Wed Jun 03 2009, 03:55AM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

I wanted to get more than arcstarters opinion. So let me explain.

I have a 40TPS08 with a camera capacitor bank of 330v 96J. I made a 2 inch diameter coil with 16 awg, 12 coils. This was for a disc launcher. It fired a heatsink plate peace 3ft. So I decided to make a 1 inch diameter coil, 16 awg, 12 layers. Charged to 300v, it hits my ceiling kinda hard. ~6ft. Put a soda can on it next. 4 inches.

Uh-Oh! It won't charge! Meter stays 000.00mv. My SCR was locked. Constant cap drain. Why? My new coil shot a plate 6ft at 300v, and a can 4 inches at 250v. It died after the can. No unusual sounds. I was using 8.45v (9v battery) and 10ohm to the gate. I was told by arcstarter to use at LEAST 100ohm. The thing is, why would it last 15+ shots and just happens to die when I try my new coil on a can, 2nd test? What's your opnion as to what went wrong!

*typed on iPod

blame any random words on autocorrect! Thanks!

Arcstarter

Wed Jun 03 2009, 04:01AM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

May i just say i would still not suggest a 9v. I misread the datasheet, and i thought the max gate voltage was 10 volts for normal operation. It is actually 4 volts. maybe 3 AAA's or other battery with the same voltage and a current limiting resistor of something like 50 ohms? Im not sure, you can calculate the resistor by using Ohm's Law.

I actually do not use a resistor, i turn them on as hard as i can, i am running them right on the edge and linear switching is completely unacceptable. this might kill the gate at some point, but that ould be better than having the SCRs die quickly, and dying by exploding and embedding SCR shrapnel into my skin (always wear eye protecting BTW, with hundreds or thousands of amps flowing through something so small, it could explode fiercely).

Killa-X

Wed Jun 03 2009, 04:34AM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

So your saying my problem is due to me using 8.45v. And over time (15+ fires) it killed it self due to over-powering my gate. I wanted to know because I only have 1 SCR left and it may be awhile until I can get more! Eventually I will when I move on to more power such as 400J at 400v

big5824

Wed Jun 03 2009, 06:58AM

Registered Member #1687 Joined: Tue Sept 09 2008, 08:47PM
Location: UK, Darlington
Posts: 240

You could simply be putting too much current through it, its not exactly a big SCR. Have you run barrys inductor sim for your coil parameters? (if you can measure them that is)

Electroholic

Wed Jun 03 2009, 12:16PM

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

What are you coil parameters?
12turn pancake?
Your SCR is too small for induction launchers.
But you do have free wheeling diodes right?

Killa-X

Wed Jun 03 2009, 07:20PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

I just did as arcstarter instructed me, 12 coils around tube and smash it down. It worked over 15 times until it randomly decided to die, which is why im more geared towards the 9V over-power issue. I know a coilgun is very different, but that was able to go 400J on my single SCR with no issues.

Electroholic

Wed Jun 03 2009, 07:28PM

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

A 16awg 12turn coil will have much lower inductance than a reluctance type coilgun coil. peak power/current would be a lot higher, and that's what I think killed your SCR.
Also, 9V trigger is not ok either.

Arcstarter

Wed Jun 03 2009, 08:13PM

Registered Member #1225 Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253

Killa-X wrote ...

I just did as arcstarter instructed me, 12 coils around tube and smash it down. It worked over 15 times until it randomly decided to die, which is why im more geared towards the 9V over-power issue. I know a coilgun is very different, but that was able to go 400J on my single SCR with no issues.

When did i say 12 turns 0_o. Haha.

Anyway, you will need to mess with turns. With my 624 joule bank i used only 9 turns of 12 gauge. Any other wire i have will explode or melt after one shot. For your cap bank, I'm sure lower turns would be better (when all 10 are hooked in parallel). I don't know much about what kind of coils work best and all that mess though. But it seems to me the higher the voltage the more turns it would require for optimum performance. Could someone lend me a hand and clear this up for me? I know that discharge time is what governs it all, but say the discharge time for two different banks where the same, one was 300 joules at 400 volts, the other 300 joules at 800 volts. Which would like the most turns for a disk launcher with a, for instance, 5 inch aluminum platter? This would clear a lot up for me.

Considering the disk is in a way similar to a shorted secondary on a transformer (eddy currents aren't exactly the same, but as long as you understand where i am coming from), it would seem the higher voltage, the more coils to insure you have a very high current induced into the disk, because such low resistance means it will easily conduct high current at lower voltage. I'm not sure.

Sulaiman

Wed Jun 03 2009, 10:06PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

I believe that this kind of launcher relies on eddy-currents (Lorentz forces)
the force will be proportional to the rate of change of magnetix flux
which is mainly equivalent to rate of change of current in the coil. dI/dT

Unfortunately one of the important/costly specifications for SCRs is dI/dT
(Voltage, current, speed, power etc. are all important)

A FAST diode in parallel with the scr is required also, to pass the current resonating in the coil and capacitor, around the scr, which would be reverse biased on negative voltage swings.
I haven't looked at the specs. for your scr, but in general the turn-off time is significant compared to the resonant frequency of the coil/capacitor.

If you consider the energy that started in the capacitor, and the energy actually delivered to the projectile, where does the rest go ? (one of my favourite musings)
You may consider the launcher as a resonant circuit that loses some energy to the projectile.

EDIT: regarding gate voltage..it's not the gate voltage that turns on the scr, it is the gate current. (True, voltage and current are related) So you can use any gate supply 'voltage' higher than the gate turn-on voltage, get the required turn-on current from the datasheet, and use a resistor R <= (Vgatesupply - Vscrgate) / (turn on current). You could use 6V, 9V, 300V....

Killa-X

Wed Jun 03 2009, 10:13PM

Registered Member #1643 Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039

Yup.. it's my stupid SCR. I manage to get 2 shots of 7ft and my SCR once again, got locked and shut my caps down to solid 000.00V This was using only 2AA as a gate trigger. So, My smaller coil is too much current. My larger coil i did many times and it never died, my new coil, apparently 2X = DEAD SCR.

Where can I manage to get some lower-cost SCRs that might work for such a device. I mean it's only 330V 96J..I still got my caps that total 370J 400V >_> I know arcstarter used 5 SK065K in parallel, and he managed to fire his 2 large caps, even short them through the SCR, and the SCRs still live.

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