Best way to switch capacitor for coilgun - Page 1
Forum section: electronics / projects

author: smashedProton
author_rank: Frequent Contributor
timestamp: November 15, 2013, 09:36:40 pm
content: Hello all. From the title it's pretty much self explanitory. What is the most proper modern way to switch a giant capacitor into a big inductor. I know to have a free wheeling diode to protect against reverse emf if I'm using a transistor. Is an scr the best way to do it? SCRs are awefully expensive... What about a capacitor supressed relay? Would the contacts still flash over and weld together? That wouldnt be good... Thanks in advance

author: c4757p
author_rank: Super Contributor
timestamp: November 15, 2013, 09:58:00 pm
content: Quote from: smashedProton on November 15, 2013, 09:36:40 pmMust be those solid gold ones...

author: Zero999
author_rank: Super Contributor
timestamp: November 15, 2013, 10:04:43 pm
content: An SCR with a high enough current rating might be expensive - I haven't checked though.

author: nctnico
author_rank: Super Contributor
timestamp: November 15, 2013, 10:44:02 pm
content: Did you look into a spark-gap?edit: I looked a bit further. If the voltage is high enough then a triggerable spark gap is definitely the way to go. It is much more robust than a thryristor and costs very little.

author: gotvolts
author_rank: Contributor
timestamp: November 15, 2013, 10:59:58 pm
content: What voltage and how much capacitance?

author: fcb
author_rank: Super Contributor
timestamp: November 15, 2013, 11:04:02 pm
content: An SCR would be the ideal way to do it. I would suggest an IGBT or two, but it wouldn't be any cheaper.What is your peak capacitor, capacitor capacity, and estimated peak current?

author: smashedProton
author_rank: Frequent Contributor
timestamp: November 16, 2013, 04:33:09 am
content: It's going to be multi-stage, that is why I need the electronic switching. Each capacitor is going to be at 350 volts and 3700 uF. 227 joules per capacitor. Very very fatal if I get shocked by it... I understand the safety, and have played with it before.

author: SeanB
author_rank: Super Contributor
timestamp: November 16, 2013, 07:22:28 am
content: Look for old forklift SCR packs and use the hockey puck ones as the SCR's. With a low repetition rate you will not need the 20kG of heatsink they typically have bolted to them, just a pair of polished steel plates clamped across the ends with a thick copper wire bolted to each end. Those should handle the current with little problem provided you have enough gate current to switch it on very fast.

author: staze
author_rank: Frequent Contributor
timestamp: November 16, 2013, 08:16:59 am
content: IGBT's are what modern strobe camera flashes use... they probably could handle the current, and might be easier to control (not positive on that). Though, they're probably more expensive.

author: G7PSK
author_rank: Super Contributor
timestamp: November 16, 2013, 10:02:43 am
content: If you have really high currents and highish voltage ignitrons are very robust which is why they were the choice for spot welders before solid state came in. I used to have a Sciaky spot welder which had two in tha could switch several hundred amps at 415 volts over a fraction of a cycle that was on the input side the conductors were about 40mm by 15 mm solid copper bar. The output was 2 copper plates 20 mm thick with a hole in the middle and a slot.

author: Psi
author_rank: Super Contributor
timestamp: November 16, 2013, 10:20:11 am
content: Quote from: smashedProton on November 16, 2013, 04:33:09 am*Psitakes one step backExactly how many of those are you going to have in series?

author: smashedProton
author_rank: Frequent Contributor
timestamp: November 17, 2013, 05:18:06 am
content: Two in parallel for the first stage, then one for each subsequent stages. A total of 6 capacitors. The coilgun would be mounted on a tripod, so the capacitors wouldn't be anywhere near my face

author: Marco
author_rank: Super Contributor
timestamp: November 17, 2013, 07:34:10 am
content: Quote from: smashedProton on November 16, 2013, 04:33:09 amAt those kind of voltages the currents for a discharge in the 10 msec range are actually not that high, less than a kA, which is pretty low for SCRs. You don't even need hockey puck or even stud devices since the repetition rate is low. A SCR in the 5 buck range can handle that.Since you are talking about a 100 buck capacitor I don't see why you are calling the SCRs expensive ...

author: smashedProton
author_rank: Frequent Contributor
timestamp: November 17, 2013, 08:26:15 am
content: I got the caps for free. I was also wrong about the price of the caps.

author: Marco
author_rank: Super Contributor
timestamp: November 17, 2013, 09:35:44 am
content: Still, do you consider say the VS-70TPS12PBF expensive?