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I am building a multi-stage coilgun and need to be able to electrically switch 450V and over 100A somehow (virtually shorting large capacitor bank)... I've tried using IGBTs but fried them. I also need to be able to turn it off so I can't use thyristors either. It also needs to be able to turn on and off relatively quickly since it is multi-stage. What is the best way to do this?

The logic runs on 5V so I currently (no pun intended) use a MOSFET to control the gate with 20V (I have a variable boost converter which can go up to 40V)

Thanks!!

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  • \$\begingroup\$ So what is the question? \$\endgroup\$ – venny Aug 25 '14 at 19:35
  • \$\begingroup\$ How I can switch the capacitor bank on and off... It is pretty clear in what I wrote. \$\endgroup\$ – jjmirks Aug 25 '14 at 19:37
  • \$\begingroup\$ What do you consider "relatively quickly"? \$\endgroup\$ – Jarrod Christman Aug 25 '14 at 19:40
  • \$\begingroup\$ under 10ms would be nice :) \$\endgroup\$ – jjmirks Aug 25 '14 at 19:46
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    \$\begingroup\$ Try the mighty thyratron: www2.l-3com.com/edd/products_thyratrons.htm \$\endgroup\$ – EM Fields Aug 25 '14 at 19:53
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450V 100A? Wow. That sounds like you're using a capacitor to power each coil? Also, with that kind of energy, I believe it would put holes in brick walls.

Keep in mind that the larger the ratio of number-of-coils/energy-per-coil the more efficient your coilgun will be. Using capacitors may not be the best idea if you are looking to make an efficient gun. Most hobbyist coilguns have an efficiency of 1-2%. Depending on the mass you're accelerating, it may be a better idea to use more coils or more turns per coil and a constant current source rather than a capacitor. Keep in mind that with a capacitor, you are going to have to seriously think about heat dissipation.

That said, I don't know about anything up to 450V. My experience with large amounts of electricity stop at about the same level as wall current. I would suggest looking here though. He seems to have some stuff on capacitor driven coilguns.

This might help also.

Good luck, and don't blow a hole in a wall. ;)

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  • \$\begingroup\$ Haha, it is a lot of energy, I'm using 6 450V 2200uF caps (they're the size of pop-cans :D). I've seen pretty much every other coilgun on the internet and no-one has built a multistage high powered capacitor driven gun, although I have seen some really nice battery powered multi-staged guns. \$\endgroup\$ – jjmirks Aug 28 '14 at 20:05
  • \$\begingroup\$ I at some point want to make a dead-simple disposable camera multi-stager. I haven't done it though because I'm already working on something (not a coilgun). Good luck with yours. ;) \$\endgroup\$ – CoilKid Aug 28 '14 at 20:25
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I'm just getting into the coilgun game myself, I'm planning to use less power than you are, and I'm FAR from any sort of expert here, so please consult people other than just me, but I have some electronics knowledge and I think you're on the right track. I'm planning to use MOSFETs too, but keep a few things in mind:

1--MOSFETs have an "on" resistance, which can be found on their datasheet. The lower the on resistance, the better. Generally speaking, higher voltage, lower current MOSFETs have higher resistances, while lower voltage/higher current MOSFETs have lower resistances. I found what may be a good one for what you are doing, but, well, it ain't cheap:

TK100L60W,VQ on Digikey it runs $31.20, each BUT it's rated for 600V, 100A with an astounding on resistance (by MOSFET standards in this voltage range) of 0.018 Ohms! Bear in mind you don't have to use this, and depending on your capacitor's uF (microfarads) and your coil resistance, you may not need a resistance this low.

You can put them in parallel to lower the resistance, but if you do, leave some margin of error because the distribution won't be completely even. Bear in mind that during the brief time you are discharging that cap, you are creating some intense heat: Power dissipated (Pd) = I^2*R so Pd = 100amps^2*.018ohms = 180 W In fact, given you are intending to run 100A of current through it, I would recommend using at least two of the ones I mentioned.

2--Mind your gate voltage and, importantly, reach your target voltage quickly. MOSFET gates are like tiny capacitors and must be charged before the MOSFET is completely in the on state. Between on and off, it is acting like a resistor which means during that time it is soaking up power from your circuit by turning it partially on and, worse, generating more heat, which can damage it. Don't let this scare you off. Simply make sure you are charging it with a reasonably large amount of current (an amp or more, depending on how fast you want it to charge) and you'll probably be fine. By turning on the MOSFET quickly, you can significantly reduce the amount of heat it generates and ensure that more of the cap's energy goes to your coil. Don't forget a bleed resistor to ensure the gate discharges after the power to it is turned off.

3--Keep in mind the total resistance of your circuit relative to the on resistance of your MOSFET. Your circuit's resistance is the total of the following, IF everything is in series:

A--Wire B--Capacitor ESR (Equivalent Series Resistance, look for it on the cap's datasheet) C--MOSFET on resistance D--Coil Resistance--If you don't have the means to measure it, estimate the footage of wire used to make it and multiply that by the resistance per foot of wire for the gauge of the wire you used. There are plenty of charts/calculators for this on the web.

If you notice that total on resistance of your MOSFET(s) is a large part of the total resistance, you may want to rethink your design a bit. Basically, whatever percentage of the total resistance is coming from your MOSFETs is going to be the percentage of the cap's power that they'll be dissipating. That's less power for blowing holes in things! If your circuit resistance is too low for your MOSFET you can make your coil bigger to increase its resistance and the extra number of turns should make up for the lost current, within reason. The bad part about increasing coil resistance is you decrease overall circuit current (although a larger percentage of what's left is now going to your coil) and increasing your cap discharge time, which may or may not be bad depending on your present discharge time. You can add more MOSFETs in parallel to lower their total resistance, but if you then make sure they're all still getting sufficient gate current when turning them on.

I hope this helps you out some. Sorry to ramble on, it's late here and I'm only semi-conscious at this point :P Best of luck, let us know how it works out!

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