EDM Build

[EDMGuru]

Hi aeroHAWK,

Yes that’s basically it only it’s the positive side of the 100v output that’s pulsed and the negative side goes straight to the work piece. Also your voltmeter is connected wrong; the left side of the wiring as shown in your drawing should go to the negative side and not the positive side of the 100v power source.

 

[SmithDoor]

Has any one use a welder for power supply?
Mig welders start around 40 volts DC.

Dave

Hi aeroHAWK,

Yes that’s basically it only it’s the positive side of the 100v output that’s pulsed and the negative side goes straight to the work piece. Also your voltmeter is connected wrong; the left side of the wiring as shown in your drawing should go to the negative side and not the positive side of the 100v power source.

 

[aeroHAWK]

Hi aeroHAWK,

Yes that’s basically it only it’s the positive side of the 100v output that’s pulsed and the negative side goes straight to the work piece. Also your voltmeter is connected wrong; the left side of the wiring as shown in your drawing should go to the negative side and not the positive side of the 100v power source.

Thanks EDMGuru! You caught a dumb mistake for me. I just put the meter in as an afterthought and paid no attention to the polarity.

I really appreciate you providing this information. You've really taken the mystery out of it for me. It makes sense now.

Cris

 

[EDMGuru]

Hi SmithDoor,
Unfortunately I believe welders use an auto transformer which is unsuitable for EDM as their output when rectified is a plus and minus voltage in relation to ground (0v) so +20v DC to -20v DC if this makes sense, although you could use one in series with an isolation transformer which should make the output voltage a more useable 0v to 40v DC.

 

[SmithDoor]

Welders are not auto transformer type. They like the microwave type.

At higher amperage the voltage is between 18 and 24 volts

Dave

Hi SmithDoor,
Unfortunately I believe welders use an auto transformer which is unsuitable for EDM as their output when rectified is a plus and minus voltage in relation to ground (0v) so +20v DC to -20v DC if this makes sense, although you could use one in series with an isolation transformer which should make the output voltage a more useable 0v to 40v DC.

 

[aeroHAWK]

Hi aeroHAWK,

Yes that’s basically it only it’s the positive side of the 100v output that’s pulsed and the negative side goes straight to the work piece. Also your voltmeter is connected wrong; the left side of the wiring as shown in your drawing should go to the negative side and not the positive side of the 100v power source.

I updated my schematic to reflect your corrections:

 

[EDMGuru]

Hi aeroHAWK,

Unfortunately your voltmeter is still connected incorrectly. It needs to be connected either somewhere between R1 and the mosfet or direct to the electrode and the other side connect to the negative side. Also don’t forget that the output pulse from your mosfet will be inverse to the input pulse so a 90% duty cycle input pulse will result in a 10% duty cycle output pulse which is no good.

You’ll need to add a pre amp stage before the mosfet to reverse this probably a small transistor will do for your design but I’m not sure as I find mosfets temperamental when ever I use them so I prefer to use transistors instead. Also check that the mosfets source and drain is connected the right way round for this circuit as I’m not sure the drawings right but I’m no expert.

 

[aeroHAWK]

Hi aeroHAWK,

Unfortunately your voltmeter is still connected incorrectly. It needs to be connected either somewhere between R1 and the mosfet or direct to the electrode and the other side connect to the negative side. Also don’t forget that the output pulse from your mosfet will be inverse to the input pulse so a 90% duty cycle input pulse will result in a 10% duty cycle output pulse which is no good.

You’ll need to add a pre amp stage before the mosfet to reverse this probably a small transistor will do for your design but I’m not sure as I find mosfets temperamental when ever I use them so I prefer to use transistors instead. Also check that the mosfets source and drain is connected the right way round for this circuit as I’m not sure the drawings right but I’m no expert.

Hi EDMGuru,

I now understand the voltmeter connection. I was suspicious of my thought process before.

I double checked the MOSFET connections with other circuits I've built and it matches them. Also it shouldn't be inverted. When the PWM signal to the gate is high, the MOSFET conducts so it looks correct to me. Since it is an N-channel MOSFET it requires a driver to provide the proper voltage to the gate (N-Channel instead of P-Channel may be why you commented on the connections of the source and drain). I have used power MOSFETS many times. It is common to use N-Channel MOSFETS for the high side switch of an H-bridge or Half-Bridge. I prefer MOSFETS for high power switching and I happen to have a bunch of them.

Here is a schematic for a Half-Bridge (typically used for switching one of the phases of a BLDC motor) and the MOSFET Driver. The high side MOSFET is analogous to the the way the MOSFET is used in the circuit I show.

Thank you for your expertise.

Cris

 

[aeroHAWK]

Hi aeroHAWK,

Unfortunately your voltmeter is still connected incorrectly. It needs to be connected either somewhere between R1 and the mosfet or direct to the electrode and the other side connect to the negative side. Also don’t forget that the output pulse from your mosfet will be inverse to the input pulse so a 90% duty cycle input pulse will result in a 10% duty cycle output pulse which is no good.

You’ll need to add a pre amp stage before the mosfet to reverse this probably a small transistor will do for your design but I’m not sure as I find mosfets temperamental when ever I use them so I prefer to use transistors instead. Also check that the mosfets source and drain is connected the right way round for this circuit as I’m not sure the drawings right but I’m no expert.

Here is an updated schematic: