Measuring Plasma Cutter Voltage

[tjb1]

Im trying to make my own torch height control and I have most of the code for the arduino figured out but I cant figure out how to measure the voltage with the arduino.  The plasma cutter itself outputs a 50:1 voltage of the actual arc voltage and this is a maximum of 7 volts dc.  In the picture, using a DMM between pins 5 and 6 you get the arc voltage but neither of these is common to the ground pin(13).  Can anyone guide me on how to measure this voltage with an arduino and I do realize I need to use a voltage divider to get this to 5v before putting it in the arduino.

[tjb1]

Well here is what a few people on an irc channel came up with that should work, any comments on this besides the crappy non-standard drawing.  This current setup should give me 3.5v into the arduino but I can change that and probably will to get at least 4.5v for a better resolution.

[Psi]

The 2k input section isn't right, the isolated amp input has Vin+ and Vin- shorted.

You should also put a 0.1uF bypass cap between ground and 5v.
The datasheet for the dc/dc supply also recommends a 22uH inductor + 1uF cap for extra filtering. You probably do what that since your trying to monitor a voltage and don't want to pickup noise.

You might want to put a cap on the input to the arduino. Just to smooth the input signal a little. 0.1uF or something

[PA0PBZ]

Are you sure you can't just connect pin 5 or 6 to ground (13)?

[tjb1]

The 2k input section isn't right, the isolated amp input has Vin+ and Vin- shorted.

You should also put a 0.1uF bypass cap between ground and 5v.
The datasheet for the dc/dc supply also recommends a 22uH inductor + 1uF cap for extra filtering. You probably do what that since your trying to monitor a voltage and don't want to pickup noise.

You might want to put a cap on the input to the arduino. Just to smooth the input signal a little. 0.1uF or something

I did figure out it needed another 2k resistor up there, thanks for all the input.

[tjb1]

Are you sure you can't just connect pin 5 or 6 to ground (13)?

Honestly not sure but I dont want to do anything to the plasma cutter that would destroy it without knowing for sure, kinda expensive   I can post the schematic here.  Here is the schematic.

[PA0PBZ]

It looks like pin 6 is connected to the work clip, so I would be surprised if that pin could not be connected to earth also. I imagine that when you operate it with CNC the table will be earthed anyway. Unfortunately that way you will end up with a negative voltage on pin 5, looks like the nozzle is on the negative side.

[tjb1]

I did a continuity check between the ground and pin 5 which is negative(electrode) and while the torch was operating it was going beep....beep...beep....beep....beep.  The work clamp is positive in a plasma cutter so I think I am going to continue with this isolation setup.

[PA0PBZ]

Or ground pin 6, and use an inverting opamp to invert and scale pin 5 to use as input for your A/D. That way you will only need one opamp but you will need positive and negative supply.

[G7PSK]

What make of plasma cutter is that, there could be very high voltages on some of those pins if that is the torch socket. My Lincoln plasma cutter has an open circuit voltage of 300 volts and a switchable 30 or 50 amps, so although the transformer is wound for a drooping voltage such that once the arc is struck and stable it is only around 24 volts it would still be lethal to come into contact with the circuit also the HF gets every where when the trigger is first pulled and before the arc is established and that voltage is in the thousands, so you will most likely need inductance in the circuit to stop the HF from travelling into you circuit.

[tjb1]

Its a Hypertherm, they already step the voltage down from around 350 to 7 at those 2 pins.

[tjb1]

Alright, hooked that circuit up and its not working.  Im getting 3.66 volts between the VDD2 and Vout+ and thats causing it to output voltage between vout+ and vout- with no input.  Any solutions??  Ive isolated the iso amp and it does this with nothing else hooked up.

[PA0PBZ]

Can you post the final schematic you used? The ones you posted before have some errors, and I wonder if you corrected them.