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LAN_403

voyagerscout

Sun Sept 21 2008, 11:18AM

Registered Member #1696 Joined: Sat Sept 13 2008, 06:01PM
Location:
Posts: 21

I'm building a coil gun for a school project, but our budget is 20 Euros max.
and because a SCR is kinda expensive, I was trying to find an other way of triggering my coil gun.
normal trigger put 2 metal plates together, right?
but just before the make contact, they spark, and therefore they lose a lot of energy
what if you put those 2 metal plates into a non conductive liquid? It is just an idea, but, I think there wouldn't be a spark, right? therefore keeping a lot of the energy, and making my coil gun more powerful
so, my question is, is this doable ?

NiG

Sun Sept 21 2008, 11:38AM

Registered Member #1372 Joined: Tue Mar 04 2008, 12:16AM
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Posts: 6

Try this: BT152

Pretty cheap and extremely useful. You can also use them in a parallel configuration, if you need high currents.

Dave Marshall

Sun Sept 21 2008, 01:21PM

Registered Member #16 Joined: Thu Feb 02 2006, 02:22PM
Location: New Wilmington, PA
Posts: 554

Actually, you can't parallel SCRs, atleast not easily. Two SCRs are highly unlikely to turn on at precisely the same moment, even if current is applied at exactly the same time to both.

This causes a single device to take all of the current briefly, which will almost certainly result in the death of both devices, as one dying open then forces the other to take all of the current.

The BT152 has a max pulsed current of 200A. For a low power system, it would be okish, but unless you carefully design the system to keep current low they won't last long.

If you're operating under about 300J with a single stage, you can safely use a regular wall light switch. They don't spark too much, and won't weld together, though the contacts will need cleaned fairly often.

Dave

voyagerscout

Sun Sept 21 2008, 05:23PM

Registered Member #1696 Joined: Sat Sept 13 2008, 06:01PM
Location:
Posts: 21

my coilgun will be about 400J
I have used a tv switch , but that one welded together
I'll try a wall light switch
but is my solution , with the non conductive liquid, an option?

Killa-X

Sun Sept 21 2008, 05:59PM

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

Poor you, Mine has power to shoot good, but I got my SCR for $6, and its like 800V 50Amp rated, but I only use 400, So it's rated for 500V surge. Works great for me and since its just a quick trigger it never even gets warm, always cold

But I'm talking enough power to shoot into a coffee can at about 150 joules.

guitardude012

Sun Sept 21 2008, 06:01PM

Registered Member #968 Joined: Fri Aug 24 2007, 04:54PM
Location: Norman, Oklahoma
Posts: 104

Dave Marshall wrote ...

Actually, you can't parallel SCRs, atleast not easily. Two SCRs are highly unlikely to turn on at precisely the same moment, even if current is applied at exactly the same time to both.

This causes a single device to take all of the current briefly, which will almost certainly result in the death of both devices, as one dying open then forces the other to take all of the current.
Dave

This actually doesn't matter unless you have an extremely unbalanced circuit. Typical gate turn on time for SCRs is measured in microseconds, 2us in my case. If you put 2 of them in parallel, even if one triggered in 1.5us and the other in 2.5us this is still not an issue unless you require a huge bank of SCRs because your SCR max current rating is a very small fraction of your peak current through the coil.

The reason this is true, is that current starts out at zero and you won't hit max current for a considerable amount of time(by properties of the RLC equations). If you have 2 SCRs rated for 400 amps, because you expect 600 peak, and the first triggers at 1.5us while the second triggers at 2.5us, current through the SCR may not be equal to the 400 amp max of the first SCR until something like 500us. Plenty of time for the second SCR to turn on.

Check the RLC simulation:

It will confirm that triggering time in parallel SCRs won't be an issue, unless you have something like a 4us total pulse(not going to happen).

voyagerscout wrote ...

my coilgun will be about 400J
I have used a tv switch , but that one welded together
I'll try a wall light switch
but is my solution , with the non conductive liquid, an option?

Try mounting the light switch on a project box and flood the box with mineral oil.

My guess is flooding the switch will greatly reduce the contact area (increasing resistance and losses) This will also cause certain contact points to heat faster. On the other hand you will probably prevent most arcing like you wanted.

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