(English) Adding a digital amperemeter to your lasercutter

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DISCLAIMER: ONLY CONTINUE READING WHEN YOU KNOW YOUR WAY AROUND HIGH VOLTAGE!!! EVERYTHING YOU DO TO YOUR LASER IS AT YOUR OWN RISK!

Lasercutters are great. Especially since they got relatively cheap (you can buy a „40W“ K40 Laser on ebay for sub 400€). Since I am a newly certified laser safety officer in my hospital I felt I need such a machine. However I decided against a K40 and opted for a more powerful, more big „50W“ 300x500mm Lasercutter (one of the „China blue“ variety – but it is actually green).

Long story short I am quite impressed, delivery of the ~100kg palette was really quick (as in faster than the safety-googles). The machine is still quite well tuned, the beam seems to be aligned quite well. All in all the machine is not the death-trap I thought I was buying.

However as it was advertised being a „50W“ laser, it only has a tube capable of outputting 40W continuously (50W peak). Laserpower is mainly defined by tube current, which the machine has no way of measuring. When you search ebay you can relatively cheaply buy current meters which measure from 0 to 999.9mA in .1mA steps (search for „dc digital ammeter 0 999.9“).

Nice. BUT: Do you really want to add some cheap chinese current meter in a quite dangerous high-voltage path, potentially (should the tube arc) delivering ~15kV @ 20mA? Let’s see here: The positive lead of the Tube is connected to the HV output of the supply, but the negative connection is made to N a.k.a. the case a.k.a. ground. So in theory we can add the current meter between the tube and GND, while adding a power Shottky-diode (in pass config) in parallel, limiting the voltage across the current meter to somewhere around 0.3V (relative to GND; making it more „safeish“).

So I cut the tube GND wire and inserted the current meter. So far it works great and measures the tube current nicely (hasnt blown up yet).