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Where's the spark ??
On 30 Dec 2003 23:09:40 -0800, (William J. Beaty) wrote:
(John Albers) wrote in message om...
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.
Nope. The breakdown for short gaps in air (Paschen's Law) is:
Volts = 30,000(cm) + 1,350
Paschen's Law only holds for uniform fields. In other words it
is valid when the electrodes have a large radius with respect
to the gap distance. But if one or both of the electrodes is
sharply pointed, or other shape which produces a nonuniform
field, ie is hollow, has complex features, etc, the field strength
is much higher at points in the gap than the simple potential
divided by distance between the electrodes would indicate.
That allows the arc to establish, then cascade, ie one electron
emitted from the cathode can knock loose more than one electron
in the gas, which then go on to knock loose more, etc. Secondary
electrons are also emitted when the stripped ions impact the
cathode. These actions cause the arc impedance to fall, so that
large currents can be carried between the electrodes at very
modest voltages.
Pressure, density, temperature, and gas species also play
important roles. For example, Argon has a 3 times lower
breakdown potential than ordinary air. That's why it is the
gas of choice for GTAW welding, ie arc starting and stability
are much better with Argon than with other shield gases.
Breakdown can occur as low as 137 V/cm in Argon at .5
torr.
As the arc begins to establish, temperature skyrockets.
This grossly lowers the needed ionization potential. That's
why the arc can sustain at much lower voltages than are
required to strike it.
But this is all pretty much moot since EDM isn't done with
arcs through a gas. It is done with arcs through a liquid.
Paschen's Law doesn't apply at all in that case.
Gary
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