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SCR as a coil switch

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LAN_403

Troy

Thu Sept 23 2010, 04:35AM

Registered Member #3194 Joined: Tue Sept 14 2010, 04:16AM
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Posts: 13

So, I'm starting another coil gun project, three stages optically triggered. The idea is that each coil will run just over 1300V. To switch that kind of load I understand that an SCR would be prefered over a standard relay. I've breadboarded a working optical sensor that correctly triggers a relay, but having some difficulty in figuring out how to wire an SCR in place of the relay to switch on 1300+V. Any ideas or suggestions? sample schematics would be helpful as well to visualize if at all possible... Thanks ahead of time all! The attached schematic I'm working off I found on

, great website!

1285216430 3194 FT0 Ir Detector Relay Driver

Troy

Thu Sept 23 2010, 05:52AM

Registered Member #3194 Joined: Tue Sept 14 2010, 04:16AM
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Posts: 13

Think I might have figured it out after much reading and googling. Attached is a schematic that I think is a possible solution. Inputs always welcome!

1285221140 3194 FT97149 Hv Drive With Ir Sensor And Scr

GhostNull

Thu Sept 23 2010, 06:38AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

Yep, that looks good. Just make sure that you meet/do not exceed the gate drive current/voltage; Consult the datasheet of your SCR.
And with your capacitors, if you are putting them in series to get the voltage then make sure you add some balancing resistors.

Otherwise that looks fine. Good job. =) It's nice to see that someone is doing their own rescearch

Cheers
-Ken

Troy

Thu Sept 23 2010, 06:28PM

Registered Member #3194 Joined: Tue Sept 14 2010, 04:16AM
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Posts: 13

Thanks for reply, Ghost. When looking for SCRs, I'm looking to match the gate drive voltage requirement with the operating power requirement for the LM393 (the current will be provided, theoretically, but the 3906 - breadboarding will determine if that is even needed).

But then shopping at, say, mouser.com, aside from the gate trigger voltage, what is parameter do I need to focus on for a forward biased SCR? Rapid Repetative Off Voltage or Rapid repetative forward blocking voltage? Thinking maybe the last one would be my primary objective now that I think a little more about it....

GhostNull

Fri Sept 24 2010, 01:10PM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

Oops, I just found a small problem with you circuit, the SCR-K diodes should be the other way around.

There should be a gate current rating (Igt) on the datasheet or a gate power rating (Pg...). Get the output power to be at least the gate current but don't exceed the gate power rating.

It would be a good idea to test the circuit before puting in on your coilgun. Put the SCR in series with a 12v lamp or some indicator and a 12v source. Then you can test it to make sure it works first

Troy

Fri Sept 24 2010, 09:42PM

Registered Member #3194 Joined: Tue Sept 14 2010, 04:16AM
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Posts: 13

Good catch! I've got some Neon lamps I could test in place of the coil for the time being. As much of a pain in the butt it is to wind a coil and custome etch PCBs, I'm all about the breadboard and testing.

GhostNull

Sat Sept 25 2010, 07:15AM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

Great, I hope to see some results soon =D

Troy

Wed Oct 06 2010, 03:32AM

Registered Member #3194 Joined: Tue Sept 14 2010, 04:16AM
Location:
Posts: 13

The SCRs finally came in from Mouser, so I wired it all up on my breadboard, flipped the switch on the power supply, loaded a round in the face of the barrel, broke the IR beam aaaaannnndd....

nuthin'!

A few observations:

1.I noticed when I powered on both boards - the HV board and IR sensor board - there is about 1.5VDC constant output from the collector of the 3906. this jumps to around 4.5/5VDC when the beam is broken. With power going to just the IR sensor board the collector output with a closed beam is 0VDC. Somehow, somewhere there is a stray voltage complicating things

2. With both boards powered on (and getting the stray voltage) when I break the IR beam, the capacitors apparently discharge slowly because the NE lamp fads out over about 1-2 seconds and the round doesn't move (obviously).

I think the stray voltage may have something to do with a common ground since I'm using a converted ATX power supply. Here is an updated schematic.

For testing purposes I'm using lower voltages, especially on the HV converted. The data sheet for the SCR said that the max Gate trigger voltage was 2.5VDC but with the 1.5V stray and the lowest 3906 output I can produce with my power supply being 4VDC I'm thinking there might be an issue in there somewhere.

GhostNull

Wed Oct 06 2010, 12:17PM

Registered Member #2648 Joined: Sun Jan 24 2010, 12:45PM
Location: Australia
Posts: 291

Um, what? I can't really understand what you're saying. Could you rephrase that?

Troy

Wed Oct 06 2010, 05:23PM

Registered Member #3194 Joined: Tue Sept 14 2010, 04:16AM
Location:
Posts: 13

Well, after a few hours of troubleshooting I just decided to with a standard high current relay instead of using the SCR. Granted, these will wear out faster than an SCR, but in bench testing it seemed to work just fine so thats what I'm sticking with for now.

Its always satisfying to see the fruits of your labor start to pay off!

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