what fuse to use or gauge wire to act as a fuse?

im making a battery powered coil gun.. each coil draws about 180-200 amps of a pulsed current. I want to know what fuse ratingi should use to protect the circuit.? or if there aint such a fuse.. what gauge wire should i use as a fuse..

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I don't think a fuse is wanted, or needed, for the output side of a pulsed power setup. For pulsed power, the load is protected, in a sense, by the fact that there's only a finite amount of energy available for the pulse.

icey.hood (author)  Jack A Lopez2 years ago

thats exactly what i thought, however in case of a short..?

Hey, uh, I just read the new comments for this topic. When I answered this question yesterday, I did not realize you were using batteries to drive the coil directly. I assumed you were using a discharging capacitor. That's what I meant by there being a finite amount of energy, just the energy stored in the capacitor.

Now I am wondering what you are using for a switch. (IGBT? GTO?) I mean what do you have that can turn on to let 200 A flow through it, then turn off to bring the current back to zero?

If you have several of this big switch, maybe you could put one more in series with the big battery. Then command it to be on, only during the time when the coils are firing, and command it off the rest of the time. That would introduce additional resistance, but then so would a fuse.

Actually, to make a long story short, I don't know what to use for a fuse for something like this.

icey.hood (author)  Jack A Lopez2 years ago

im using a mosfet rated at 110 a and can pulse 180 amps for few seconds and i will have a transistor bank for each coil if need be.

OK. Big mosfet(s). I was just curious about that. I've never actually built anything like this. Always wanted to, of course. So I am curious to hear/read about these things, and I thank you for sharing. Also I wish you luck and success with your build. Happy making!

I also thought I should mention "flyback". Switching the current to a coil off suddenly causes big voltage spike, and the usual trick for dealing with this is a diode placed in parallel with the coil.

http://en.wikipedia.org/wiki/Flyback_diode


icey.hood (author)  Jack A Lopez2 years ago

i have a voltage suppression circuit in place, i have a diode anti-parallel to the coil with a resistor to help with power dissipation..

-max-2 years ago

It is probably just going to be a hassle to have a proper fuse on the pulsed current. It takes time for a fuse to 'warm up' and burn out. It is sort of like inertia thing. 200 amps up current will easily fry many loads if it was RMS (average) or steady state condition, and fuses are designed for such standard loads. Because it is a pulse, the pulse lasts for such a short period of time, most things do not even have enough time to warm up and burnout, that includes the filament of a fuse. If anything will cause the coil to fail beyond the capacitor bank output, it would be a pulse containing too much energy, and taking longer to discharge into the coil.

Have the fuse of the input of the inverter that charges the capacitor, that is where the sensitive electronics you need to protect are found. Better yet, use a PTC resettable fuse. It will be a low resistance device until too much current causes it to get too warm and causes the resistance to rise and a voltage drop to develop across it, causing it to warm up even more until it has reached a new equilibrium where little current flows, and it stays very hot to the touch. Once you let it cool down, everything is all good again!

icey.hood (author)  -max-2 years ago

sorry, i should have mentioned.. there are no capacitors in this particular CG. its all battery power.. either 77.7v @ 2.2 ah or 33.3v @ 4.4 ah.. the coil i have to test atm will draw about 180 amp. coils are opticaly triggered..

-max- icey.hood2 years ago
What type of batteries are you planning to use? SLA? car batteries?
icey.hood (author)  -max-2 years ago

lithium polymer

Look The I squared t rating of the fuse. Knowing your pulse width, and the current, you can look at fuse rating tables and see how fast a certain fuse will blow at a given current. You'll be looking at fuses a lot lower than 200amp. You could experiment with a car battery and various wires until you get an acceptable performance. I hope you are using very heavy wire throughout your circuit, because at 200 a, the losses will kill your

icey.hood (author) 2 years ago

multistaged coilgun..

Itake it you're driving this from a capacitor bank?

icey.hood (author)  steveastrouk2 years ago

no.. i have 7 lipo battery all at 11.1v 2200 mAH, I may have them all in series for 77.7v @ 2.2 ah or pairs in parrell for 33.3 @ 4.4 ah.

rickharris2 years ago
icey.hood (author)  rickharris2 years ago

google is a best friend, so is youtube.. lol

do them rating apply to a pulse current..

I don't know - In general a wired fuse is a fairly rough protection and will withstand some abuse before blowing - i.e. it need a sustained overload to blow it.

Slo blow fuses are available but only as far as I know as cartridge fuses.