"William J. Beaty" wrote in message
om...
(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
...in other words, you get no spark at all for voltages below 1.35KV,
and then the voltage rises like this:
Gap (millimeters) Voltage
0 1400
0.2 1900
0.4 2600
0.6 3200
0.8 3800
1.0 4400
That's all very interesting, but tap busters (primitive EDM machines that
use air dielectric, and that are used for removing broken taps and drill
bits in workpieces) typically have an open-circuit voltage ranging from 90V
to 120V. They'll spark without contact, although, given the servo mechanisms
on those things (often a hand crank g), it's hard to tell when you've
actually contacted the work.
FWIW, more sophisticated EDM machines, which use oil for dielectric
(kerosene with flash suppressants) have an open-circuit voltage that ranges
from around 100V to 350V. Those aren't the power-delivering cricuits. The
power supplies are cascade arrangements with high-impedance, high-voltage
circuits to polarize the gap; medium-impedence, medium-voltage (~90V)
circuits to ionize the channel and to initiate discharge; and low-impedence,
low-voltage (~15V) circuits to deliver the amperage that does the real work.
In those machines, the electrode never contacts the work. They have a servo
mechanism that maintains a gap on the order of 0.0002 - 0.003 in.
Ed Huntress