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The manufacturer of the CO2 Laser tube instructs to put an amp meter on the low side of the laser tube for calibration. While I understand the amp meter will only see a few milli volts above ground; the tube will be at 19kV @ 26mA.

Will any amp meter work for this?Is it safe to leave the current meter in the system after calibration for monitoring?

Should I put a zener diode or some sort of protect around my amp meter? If I used a hall effect current sensor and attached the output to a controller board would I need optical isolation?

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    \$\begingroup\$ Most of the cheaper Chineese laser engravers have an analogue or digital mA meter between the tube and the negative HV PSU reference point (which is often chassis ground). While there is risk of floating HV if the link wire gets disconnected I have not head/read that it is a common failure mode. The meters are usually left in circuit as taking it out carries a much larger risk of leaving wires unconnected. Isolation is nice but seldom used as it would require an isolated PSU. \$\endgroup\$ – KalleMP Nov 2 '15 at 6:53
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    \$\begingroup\$ The hot end of the tube is at 19 KV. The end that the ammeter connects to is a few millivolts above ground. I'd go looking for a 50 mA analog movement and mount it behind plastic, (ie, a plexiglas window in the laser case) permanently connected. \$\endgroup\$ – Ecnerwal Nov 2 '15 at 14:31
  • \$\begingroup\$ Thanks guys. I put my multi-meter in the ground line and did not get electrocuted :-). I did however have a close call because if the line was on volts instead of amps it would have toasted the meter. Here is the video if you are interested. youtube.com/watch?v=xOA96iQiC1c \$\endgroup\$ – ericnutsch Nov 3 '15 at 8:12
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When High Voltage is involved, safe is a matter of perspective. I would consider it safe to place a shunt resistor or other current measurement system system on the low side of the laser tube as long as no humans are in contact with the device and it is isolated from everything else. For example, an old analog amp meter gauge that is plastic and mounted inside of an enclosure securely, I would consider safe because it is not very conductive and can be easily viewed without the need to hold it.

In a commercial solution, there would be an electrical isolation set up. Some sensor on the high voltage line, then a physical/electrical barrier that the signal is routed across via light or magnetics (a non conductive medium rated at-least 1.5x the working voltage of the laser most likely) to a device that is low voltage and safe to handle or interface to other instrumentation.

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