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Can an SCR be used as a back emf diode?

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LAN_403

w1vlf

Wed Apr 16 2008, 02:55PM

Registered Member #1329 Joined: Mon Feb 18 2008, 07:31PM
Location: Harwinton Connecticut
Posts: 53

Hello Folks,

I aquired several large stud SCR's 800 Volts at 300 amp.
One will be used in my mass launcher as the switch.

Could I configure another as a back emf diode to protect the caps and the main SCR?
Or maybe I can use a smaller one for this purpose??
Has anyone ever done such a thing?
Thank you

Firing aluminum into the skies over Harwinton Connecticut
PauLC
W1VLF

Dr. Shark

Wed Apr 16 2008, 03:36PM

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

Paul, you can use an SCR as a diode if you just give it the same trigger pulse as the primary SCR and make sure the trigger is applied for long enough that the SCR stays on until the back EMF arrives.

You can use a considerably smaller SCR since in a typical disk launcher you will be quite close to a critically damped system, so there is not going to be much back EMF. In fact in my disk laucher

(which is quite a bit smaller though) I am using a TO247 diode which has held up fine so far.

OZZY

Wed Apr 16 2008, 05:04PM

Registered Member #511 Joined: Sat Feb 10 2007, 11:36AM
Location: Somerset UK
Posts: 55

Hi W1vlf

I agree with everything that Dr. Shark has said but I would like to add a couple of warnings.

1. The trigger pulse for the second SCR will be referenced to the high side of the coil or the high side of the cap. Normaly the primary SCR is referenced to the low side of the circuit, so the two gate drives will need to be isolated from each other.

2. It should be possible to have both SCRs on the high side so they can use a common gate drive circuit, but now the gate drive must be isolated from the low side of the circuit. If the low side is actually grounded then the gate drive circuit will be live!!

w1vlf

Wed Apr 16 2008, 05:06PM

Registered Member #1329 Joined: Mon Feb 18 2008, 07:31PM
Location: Harwinton Connecticut
Posts: 53

Mr Shark, Thank you for the information on using the SCR for the back emf diode, I have just the thing to use not that you habe explained how to do it.
Could you please recommend a design for a coil, for my next improved launcher? I have #10 wire to build it with, but would like to understand the relationship between the launching coil and the prejectile diameter before I use the wire.
PaulC
W1VLF

Dr. Shark

Wed Apr 16 2008, 07:09PM

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

Not sure I understand your post correctly, do you want a circuit diagram how to wire up the thing correctly? I'll try some ASCII art, since I don't have any schematic software on this computer:

------|>|--------
|           |    |
C           ^    L
|           |    |
-----------------

Where the ">" and "^" represent the SCRs, the "C"apacitor and "L" inductor should be obvious. This way both cathodes are connected together so they can be triggered from the same source. Since you can ground any arbitrary point in your circuit, I don't really see Ozzys second point as a problem.

w1vlf

Wed Apr 16 2008, 08:43PM

Registered Member #1329 Joined: Mon Feb 18 2008, 07:31PM
Location: Harwinton Connecticut
Posts: 53

Dr Shark, What I would like to know or learn about is, for a given diameter projectile say 2" what diameter is the best for the coil diameter? I would guess that very small and very large would be ineffecient. I am trying to optimize the current coupled between the coil and the projectile.
Thank you
PaulC
W1VLF

Dr. Shark

Wed Apr 16 2008, 09:10PM

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

Oh, now I get you. I am by now means a expert in disk launchers (fastMHZ has done some excellent experiments, check his threads and homepage), but I am pretty sure that the coil and the disk should have exactly the same diameter. If the disk is any bigger, it probably just adds dead weight, and if the coil is bigger than the disk, the extra (unused) inductance would just slow the discharge unnecessarily. Hope that helps!

Quantum Singularity

Sun Apr 20 2008, 12:56AM

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

Just wanted to add a couple things. I have not yet had a chance to test these things out personally, but its been said before that in can crushers, induction launchers, etc, the back EMF can be quite severe. Dr. Shark I am quite surprised with your results, I do not doubt them but its surprising to me. I know some folks who have blown up some pretty huge caps before (several kJ maxwells even) and was attributed to back EMF. I've never taken any measurements of the current pulses or voltages in any of my launchers though, dont have a working scope.

And as for the ideal diameter, again I have not tested it primarily because I only have one size projectile I've been using (other than various sized hard drive disks, but those dont really work well). Ideally I would think you would want to vary the projectile diameter with respect to the coil first, since changing the coil diameter effects many other variables. But what I have been told is that the ideal ratio is for the projectile to be slightly smaller than the coil. This is one area that needs some more experimentation.

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