"John Albers" wrote in message
m...
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:

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.

I am not getting a spark. Should I be getting a spark with this set
up?

I tried using
flat ends --|*|--
pointed ends --*--
rounded ends --)*(--
with no luck.

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?

Any help would be greatly appreciated. Thank You

John

If your set-up reaches a high enough field strength to ionise the air, the
current will almost certainly flow through the wood, it's not a good
insulator. Substitute plastic, preferably not black (some black plastics are
loaded with carbon) for the wood. Add a capacitor because the transformers
have way too much resistance and inductance to get a healthy spark.
Martin
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martindot herewhybrowat herentlworlddot herecom