Re: [CAD_CAM_EDM_DRO] Another EDM Approach

Ian Wright wrote:

> Hi,
>
> I wouldn't really want to get an EDM anywhere near a PC I was bothered
> about. Don't forget that the power line to the electrode which you're using
> to sense the current has pretty high power RF spikes flying about on it. An
> early circuit I was playing with tried to sense the current by feeding it
> directly onto a transistor - the transistors lasted milliseconds!!!
> Its a problem I have been musing about for some time and, whilst it would be
> appropriate to use a PC for the motion part of a wire EDM, I would want to
> be very sure that there was very effective isolation between it and the
> power parts of the circuit.
> I had wondered whether it would be possible to pick up enough current for
> sensing by wrapping a few turns of wire around the power wire to the
> electrode and limiting the voltage on this by using a fast zener diode or
> something.

No, that will just pick up a lot of noise. There are two ways to do it. One
is to put
in series a current shunt that drops enough voltage to light the LED in an
opto-isolator. You would need to drop about 2 volts, and have a series
resistor
to prevent excessive LED current.

The second way is to use a current transformer. Find a toroid core with
several
hundred turns of wire already wound on it. pass one of the EDM wires through
the center of the core. This is a 1:several hundred current transformer. This

signal could be prepared to trigger digital circuits rather easily. It could
trigger
a 555 timer by just using a low resistance across the secondary winding as
a 'burden', and a series resistor to limit current into the 555. You'd need to

make the polarity correct. if the pulses came out negative, then reverse the
way the power wire passes through the core.

As for a PC, well my CNC machine is controlled by a PC, and it has wires that
run just inches away from the EDM setup, and needs to work correctly for
positioning the electrode. I've had no problems at all. Don't put the EDM
power source 20 feet away. The wires then will radiate LOTS of EMI.

Jon