High Voltage Arc

[tayexdrums]

I am trying to make an ignition system for my spud gun. At first I began to use the flash circuitry from a disposable camera but it decided not to work. The way I have it set up is there are three screws coming to a point within the chamber of the gun. The problem I am having is that the arc will not jump the gap! I decided to ditch the flash circuitry and step up my game (literally). I got a HV microwave oven transformer and connected it up to two of the screws but once again, no arc. I am now thinking about getting a flyback transformer and using it. I just want to generate a reliable electric arc, is that too much to ask?

[pullin-gs]

Get an old car ignition coil.
a 12V battery and a capacitor will get you 1/8+" of spark every time.

[IanB]

The camera flash circuit will only generate about 300 V on the charge storage capacitor. That is not enough voltage to produce a long spark, for that you need 1000's of volts. Even using the trigger coil to induce a spark using the third electrode it is going to be difficult. You probably need a very tiny gap, a fraction of a millimetre. I have thought of trying this experiment myself but have never got round to it--I have always wondered if it could work.

I think the easiest way to get a spark is to use one of the piezo ignition sources that you find in gas and barbecue lighters. You can get those lighters for peanuts almost anywhere and you will get a spark for no effort at all.

[tayexdrums]

The spacing of the electrodes or screws in my case are most likely the problem. I have come to the conclusion that you need a high voltage put in between the other two electrodes in order for an arc to form. And yes I know I could use a BBQ grill igniter or lantern igniter but wheres the fun in that?lol

[Psi]

You could also try a totally different method of ignition, a "glow plug" bought from a model shop. They glow red hot when you put 3V through them.
They're designed to ignite flammable gas so should work fine. 3V is also a lot easier to work with

If you really want high voltage then you have to watch the material you mount the spark electrodes in. Some plastics, especially black PVC pipe, is conductive so the current may prefer to going through the plastic rather than jump the gap.
If you can insulate it with some nylon washes that may help.

Also, the 'distance vs voltage' is different with different gasses and mixes.
You may find that it jumps fine in air while you're checking it but won't jump when the tube contains lighter fluid fumes. So the gap may have to be reduced.

If you're not using high voltage wire to get the HV to the electrodes you will need to make sure the wires don't rest against each other, or current will flow through the insulation instead of spark.

You really want the smallest gap possible because it will be a hotter spark.

I once blew up a coke bottle full of pressurized air + acetylene with some screw terminals in the bottle and a car ignition transformer.
(Hint, don't do that, the bang is louder than dry ice and scares the crap out of people including yourself.)
The point is a car ignition transformer does have enough energy do ignite flammable gas if you really want to ignite using high voltage.

Here's the correct way to wire up a ignition coil, the capacitor is important (try 0.1uf 300V mains filter cap).
The spark happens when you "let go" of the button when the backemf starts to ring at ~300V into the capacitor and back into the primary.

[sonicj]

+1 for the piezo

ps, don't look down the barrel to confirm your spark.

[G7PSK]

All the ones I have seen in the past use a pizeo igniter from a gas lighter or stove always seem to work and are far more portable than coils and batteries etc. Or you could use easy start which is ether in an aerosol can for starting diesel engines you then need a cycle tyre pump, a sharp push on the pump is enough to get that to work. When I was a kid I used it in an air rifle (pellet gun) fill the back of the pellet with ether and light oil mix and when you fire the gun on occasions enough force was created to re cock the gun, and when firing at night a flame would come out of the muzzle two feet long, rather buggered up the seals in the end but great fun at the time.