Need help on a capacitor circuit

Hey guys...I want to build a simple capacitor circuit that will use a 9v transistor battery to charge a capacitor that, when a switch is closed, will deliver enough current through a short segment (~ 0.5 inches long) of resistance wire to cause it to glow, or even melt within about a second or two. I need to know what type/size capacitor will do the trick. I'm a mechanical engineer who hasn't done such a calc since I was forced to learn some electronic stuff in college close to 30 years ago, and those brain cells are long gone. I figure this will be child's play for most people on this forum! Any assistance will be greatly appreciated!

Thanks,
Jim

The figure of merit that will determine if you can fuse the piece of wire will be proportional to the Capacitance, and inversely proportional to the effective series resistance of the capacitor (dont overlook the contact resistance of the switch and the wire conductors between the capacitor and the fuse). WAG: ~100,000uF 10WV Computer Grade Electrolytic Capacitor.

Thanks for the suggestion, Mike. Is that something I may be able pick up at Radio Shack, or where? Price guestimate? I figure the resistance of the conductor (only a couple feet long) to the fuse, and presume that of the switch will be negligible compared to the fuse itself.

I'm not married to the 9v battery, but assumed that would be as good and cheaper than say 6 D batteries in series. There's no need for the capacitor to charge in a short time (several minutes will be fine), if I correctly understand that to be the main difference with using a 9v transistor battery. Would any other voltage supply make the required capacitor cheaper or more readily available?

Look up wire fusing currents from a wire table. To be sure, multiply the number by five. It's the gauge that matters, not so much the length which only contribute to more heat dissipation.

The calculus is I = C dV/dt. I in amps, C in Farads, dV the voltage on cap versus delta time.

Don't forget there is some resistance within cap and connecting wires. Adding this to resistance of fuse wire gives an RC discharge profile. Discharge will be exp(-t/RC) from initial current of Vcharge/Rtotal. Copper has a positive temp coefficient of about 0.004 per deg C so fuse wire resistance will go up as it gets hotter.

Doubt if you will find a cap large enough at Radio Shack but depends on your fuse wire gauge. I'd go with 12v charge but must not exceed electrolytic cap voltage rating. You'd probably drain a 9V alkaline battery just to charge the cap.

You might be able to scroung a cap from an old computer power supply.

Source for large caps: https://www.surplussales.com/Capacitors/Electrolytics/10000uF-300000uF.html

If you can start with a higher voltage, that would be much better. Energy stored in a capacitor is proportional to V^2. In the limit, it would likely be best to use a switching step-up inverter like the flash unit in your camera. I would use heavy gauge wire between the capacitor and the fuse, and only reduce the wire diameter to create the fuse (weakest link).

The large capacitors it would take to do this are available from DigiKey or Mouser.

RC and Mike...Let me clarify something to make sure we are on the same page. You both refer to the fuse wire gauge as being key. Are you thinking of using regular copper wire, but just a much smaller gauge as a way to increase its resistance for the fuse? I had in mind to use a short piece of resistance wire that would take much less current to get hot. I currently have some on hand from a prior project that I can get very hot with a 9v battery if I apply the voltage to a length about a half inch long. But I want it to get orange hot or melt over that length, thus I figured the capacitor could give the short current boost I need to make the resistance wire I currently have glow or melt. Does this make sense, and would that make a smaller capacitor work, or are you guys already factoring in me using resistance wire?

I was thinking you want to vaporize a short piece (< 1") of small gauge wire, such as would be used to ignite a model rocket motor. A 9V battery has a high-internal resistance, so charging the cap, and then dumping the energy in the cap into the wire in a very short time requires that the resistance be low. How much resistance would 0.5" of your resistance wire be?

I guess it would be similar to a model rocket engine ignitor, but I don't need to "vaporize" it exactly...just turn it orange-hot within maybe a second or so of the switch closing. And I'm not concerned with how long it takes to charge the capacitor--10-15 minutes would be sufficient. All the lead wire between battery, capacitor, switch and "fuse" wire would be standard copper wire...just the fuse itself I thought needed to have the resistance to get hot (just like a model rocket launch circuit to the ignitor). I don't remember off-hand what the resistance of my wire is...I'll have to measure it and let you know. But like I noted above, it would get quite hot (would make paper smolder) when connecting leads from a 9v battery to a half-inch long piece of resistance wire. The shorter the section of wire I apply the voltage to, the hotter it gets. I guess I'm basically short circuiting the battery terminals, and the shorter the length of resistance wire, the higher the current that goes through it, thus the hotter it gets? Sorry if my mechanical inclination prevents me from clearly explaining myself in electrical terms!