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I need to monitor a plasma cutter's DC voltage. I selected the AD629 from Analog Devices and have a schematic and layout that works on a test bench.

Admittedly, my talents are in software and what I know about EE just makes me dangerous. :)

My concern is that welders and plasma cutters emit a high-frequency AC signal in order to start the arc. Not knowing any better, it seems to me that signal would fry my board. Although I've seen torch-height controllers and similar and see no obvious technique for "shielding" the PCB from this.

I am assuming that a 5K volt AC signal would happily create arcs anywhere the POS and NEG lines are very close -- such as on a PCB.

How is this working?

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  • \$\begingroup\$ You need EMI filters on the input line. What frequency is the start-frequency? \$\endgroup\$ Jul 31, 2019 at 14:37
  • \$\begingroup\$ It can vary with different makers, but the usual number I've found is "5,000 VAC at 2 MHz". \$\endgroup\$
    – GaryLa
    Jul 31, 2019 at 14:44
  • \$\begingroup\$ If I understand you correctly, you're saying that components that would form a lowpass filter would do the job? Something that blocks signals above 1MHz, for example. \$\endgroup\$
    – GaryLa
    Jul 31, 2019 at 15:30
  • \$\begingroup\$ That's the idea, but it's going to be tough to create a LPF which works to 5kVAC. \$\endgroup\$
    – rdtsc
    Aug 9, 2019 at 19:40
  • \$\begingroup\$ I found 2 resistors and a cap rated for 10kVAC. Limited choices, but they work. \$\endgroup\$
    – GaryLa
    Aug 12, 2019 at 4:46

2 Answers 2

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Probably something like a low pass filter with a high-voltage rated resistor to the input. The HF start is in the ~1MHz range so filtering that out is not difficult.

Looking at the block diagram of my AC/DC TIG welder with HF start, they monitor the voltage from the other side of the HF coupling transformer. Image from here.

enter image description here

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  • \$\begingroup\$ I studied those PDFs. When you say "they monitor the voltage from the other side of the HF coupling transformer" -- how are you concluding this? Also, why would it matter where in the circuit this is monitored, doesn't this AC signal appear everywhere along both cables? \$\endgroup\$
    – GaryLa
    Jul 31, 2019 at 15:10
  • \$\begingroup\$ Nothing is connected to the output terminal side of CC1 except the output '-' terminal. The CC1 secondary is in series with the output voltage. \$\endgroup\$ Jul 31, 2019 at 15:11
  • \$\begingroup\$ Meaning that the injected AC signal only appears on one side of CC1 ? \$\endgroup\$
    – GaryLa
    Jul 31, 2019 at 15:12
  • \$\begingroup\$ Yes, because the other side is more closely coupled to ground/chassis potential (by parts that are not explicitly shown). \$\endgroup\$ Jul 31, 2019 at 15:14
  • \$\begingroup\$ I'm wondering why the arc doesn't jump between IC pins or between 0.100" headers and the like. After all, it jumps through air a fairly good distance. \$\endgroup\$
    – GaryLa
    Jul 31, 2019 at 15:33
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Here is an example of what Spehro was saying would be needed for a high-side monitor, click for full-size:

Monitoring DC level with high AC pulses present

It would have to be built on a circuit board, with adequate spacing around the HV components to avoid any corona, arc-over, or radiated energy from getting into anything else.

Note that the DC level will likely be very low, as plasma is inherently very conductive. I'd call this all a guess, with lots of experimentation required. You may find that C1 removes too many fast transients and C1/D1 leakage current requires correction. Inject 10.00vDC into it and adjust the gain of the op-amps for 10v out.

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  • \$\begingroup\$ Thanks for the input. I found some 10kV resistors and caps. I used two 10M -- one on each lead from the welder. Paranoia, I guess. Or ignorance. I have been able to block the AC and get only the DC component into the opamp. So far, it's working. Nothing's caught on fire. I used a separate breadboard for the HV stuff and was very careful to keep it away from everything. The post-filter DC run is about 6 inches. \$\endgroup\$
    – GaryLa
    Aug 14, 2019 at 12:20
  • \$\begingroup\$ Curious, how low is the DC component? Even at 50 or 100A, plasma isn't very resistive. \$\endgroup\$
    – rdtsc
    Aug 14, 2019 at 14:23
  • \$\begingroup\$ By "low", I assume you mean volts and/or amps. The voltage ranges between 5 and 70 VDC. It depends on the length of the plasma. I haven't been monitoring current, I assume the machine is set up to allow the max that the user has entered. \$\endgroup\$
    – GaryLa
    Aug 16, 2019 at 16:04

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