Where's the spark ??

[jalbers]

I am thinking about building an EDM (electric discharge machine) which remove tiny bits of metal by sparks. I have found plans and schematics on the web and in various magazines like Home Machinsts Workshop. I could just blindly copy their designs, but I wanted to understand how they work first. Most are build around a relaxation oscillator circuit which charges a capacitor and the capacitor discarges across a spark gap. Most of the EDM machines that I have plans for work with voltages around 80-100 volts. Expreimenting helps me understand things better, so I tried the following experiment:<p>I took a couple of old HP deskjet 40V DC wall warts and wired them in series to produce 80V DC. I checked this with a DVOM and I am getting 80V DC output. According to various sources the break down voltage of air is around 20V per .001 inch. I have a block of wood (pine) with a notch in it holding two machine screws facing each other to create a spark gap. Using a piece of .001 inch brash shim stock, I adjusted the gap to be .001 inch. I am thinking that if 80V is supposed to jump a .004 inch gap, then 80V should have no problem jumping a .001 inch gap.<p>I am not getting a spark. Should I be getting a spark with this set up? <p>I tried using
flat ends --|*|--
pointed ends -->*<--
rounded ends --)*(--
with no luck.<p>Also, I have questions few about the relaxiation oscilator circuits used for EDM. Why do they need a capacitor. The voltage across the capacitor is the same voltage as the power supply. Therefore the power supply wired directly to the spark gap should create a spark. Or is there a difference in trying to create a single spark and trying to create a cont. arc?<p>Any help would be greatly appreciated. Thank You

[toejam]

the reason you need a cap. is to store up energy(joules).To further your experiment, add a cap. about 5oo mf. in parallel with the series connected ww.s it must have a voltage rating of about 1oo volts or more. You wil find the sparks get much stronger.That is because the condensor / capicator stores energy that is put into it at a slow rate and discharces that energy at a much more rapid rate, due to the low impedance of the capacitor when it is charged.

[Ron H]

The minimum breakdown voltage for air between parallel plates is around 330v. For small-radius electrodes, it is somewhat less (300v?). Do a Google search for "Paschen's curve" for more info.

[Chris Smith]

Ive always worked on the value, of 25k per inch.

[Edd]

The dielectric strength of dry air is approximately 3 kV/mm.<p>73's de Edd
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[ian]

As long as you're fiddling with this care to make a few bucks? I need a (battery operated)spark plug demo device. I need a circuit to create a spark from 6 AA batteries ideally. The bigger the spark the better but it only has to be a one time spark and the charge time isn't critical. I just need the design, but if you'd like to make them as well that's ok too.
Ian

[John Paul]

Ian,<p>If your looking for a way to make a spark from batteries the best way I can think of is to use the flash circuit from a disposable camera.<p>You use the output from the capacitor with the lead from the triger coil.<p>just make sure you don't touch the bare wires or the underside of the circuitborad