Measuring Plasma Cutter Voltage - Page 1
Forum section: electronics / beginners

author: tjb1
author_rank: Regular Contributor
timestamp: June 22, 2012, 02:52:50 am
content: 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.

author: tjb1
author_rank: Regular Contributor
timestamp: June 22, 2012, 04:25:04 am
content: 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.

author: Psi
author_rank: Super Contributor
timestamp: June 22, 2012, 07:06:25 am
content: 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

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 22, 2012, 11:21:29 am
content: Are you sure you can't just connect pin 5 or 6 to ground (13)?

author: tjb1
author_rank: Regular Contributor
timestamp: June 22, 2012, 06:24:18 pm
content: Quote from: Psi on June 22, 2012, 07:06:25 amI did figure out it needed another 2k resistor up there, thanks for all the input.

author: tjb1
author_rank: Regular Contributor
timestamp: June 22, 2012, 06:27:11 pm
content: Quote from: PA0PBZ on June 22, 2012, 11:21:29 amHonestly not sure but I dont want to do anything to the plasma cutter that would destroy it without knowing for sure, kinda expensiveI can post the schematic here. Here is the schematic.

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 23, 2012, 01:22:45 pm
content: 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.

author: tjb1
author_rank: Regular Contributor
timestamp: June 24, 2012, 01:53:25 am
content: 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.

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 24, 2012, 08:48:37 am
content: 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.

author: G7PSK
author_rank: Super Contributor
timestamp: June 24, 2012, 08:31:32 pm
content: 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.

author: tjb1
author_rank: Regular Contributor
timestamp: June 24, 2012, 09:46:50 pm
content: Its a Hypertherm, they already step the voltage down from around 350 to 7 at those 2 pins.

author: tjb1
author_rank: Regular Contributor
timestamp: June 28, 2012, 03:27:08 am
content: 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.

author: PA0PBZ
author_rank: Super Contributor
timestamp: June 28, 2012, 09:06:54 am
content: Can you post the final schematic you used? The ones you posted before have some errors, and I wonder if you corrected them.