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Registered Member #4659
Joined: Sun Apr 29 2012, 06:14PM
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Posts: 158
I finally learned how an SCR works (yay circuit simulator) , but now I'm curious. Why do people use these to switch on the current for coilguns? It seems for a single-stage coilgun (no sophisticated timing, and the caps would be depleted quickly so no suckback) just a manual switch would work. Also, it seems that a transistor with sufficiently high voltage/amperage ratings would do the trick. Why SCRs?
Registered Member #3950
Joined: Wed Jun 15 2011, 12:45AM
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Mechanical switches will weld together because of arcing, and SCRs can handle much higher peak currents than any type of transistor that costs the same amount of money.
Registered Member #5258
Joined: Sun Jun 10 2012, 10:15PM
Location: Missouri - USA
Posts: 119
It depends on the switch. If you want to use a switch the best I've found for this set up is the ball bearing type. Basically it's just a ball bearing mounted over two heavy contacts and the ball is held up with a spring. Push down on the ball to short the contacts. These can take a licking and keep on, well shorting out caps.
Just make sure to keep one hand behind your back since your thumb will be come part of the circuit if the ball makes contact with one side of the terminal before the other. ;)
Registered Member #90
Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301
Yanom wrote ...
I finally learned how an SCR works (yay circuit simulator) , but now I'm curious. Why do people use these to switch on the current for coilguns? It seems for a single-stage coilgun (no sophisticated timing, and the caps would be depleted quickly so no suckback) just a manual switch would work. Also, it seems that a transistor with sufficiently high voltage/amperage ratings would do the trick. Why SCRs?
A couple properties of SCRs are very attractive.
SCRs automatically go into saturation with no special drive circuits; transistors require careful attention to design sufficient drive with correct timing.
Large SCRs are cheap and plentiful.
SCRs conduct one single half-cycle of current and then turn themselves off; you would have to provide the timing circuit for a transistor.
Yanom wrote ...
It seems for a single-stage coilgun (no sophisticated timing, and the caps would be depleted quickly so no suckback) just a manual switch would work.
You over-simplify. A good coilgun will have low resistance and therefore a long ringdown, causing the DSE (dreaded suckback effect) unless you also provide a timing circuit. A mechanical switch might work, but if it does then it won't yield optimum or repeatable performance, and will tend to burn or fuse its contacts.
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Why are SCR's used for coilgun switches?
