Burned ammeter om my Co2 laser

Hi, On my 500w Co2 laser, I have installed 3 Amp meters, one for each tube.
They were working great for the last 3 days (Installed them 3 days ago)and then, two of them just started to burn with a buzzing sound. I was getting 30mA to each meter at 80% power.

What could have caused that, they are cheap meters from AliExpress. One was still working but, I disconnected it anyway. They were 100mA meters.

Thanks, Jeff

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If the connection from the Negative (-) terminal of the meter disconnects from the high voltage power supply for that tube, then the voltage on the Positive (+) terminal will rise until something in the circuit arcs to the machine frame.

It looks like those arcs happened inside the meters …

Also, contrary to popular assumption, the cathode lead from the tube does not connect to frame ground. The high voltage power supply connects that lead to a current sensing resistor allowing it to regulate the tube current. If the lead shorted to ground between the meter and the power supply, then the supply will attempt to drive an absurdly high current through the tube, which will vaporize the fine wires inside meter (thus disconnecting the cathode lead), then cause the arc torching the meter case.

Basically, your machine just had a near-death experience.

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Could that happen because of the wires I used from the negative side of the tube
to the positive side of the meter, should I need high voltage wires, I used regular 16 gage silicone wire. Since black and red wire are side by side, could they have arced and destroy the meters. Where they were connected to the meter, they were not close to each others.

Thanks a lot for the reply, I really appreciate, Jeff
wire

The cathode lead does not need high voltage insulation because it (should) operate only a few volts above frame ground. In a fault condition when it disconnects from the power supply cathode lead, the meter will likely break down before the wire insulation.

The negative and positive meter terminals go to two separate places, so the wires should not be running together as do the red + black wires in the cable.

Before you turn the machine on again, you must verify all the wiring is correct, continuous, and solidly connected. This is not the place for casual twisted joints with tape insulation and bare wire ends with strands sticking out from the terminal nuts. :grin:

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I know, I’ve been working with electronic for a long time and have every kind of connector and crimping tools but, I’m not really used to work in high voltage.

Since you pointed out the meter itself, I remember seeing a really little tiny arcing inside the meter but, they were working normally. I knew I could have some small voltage from the negative lead but, many times, we are talking about millivolts.

The three positive leads are on the left side of the laser and the three negative leads are on the right side of the machine and goes back to the power supply.

Some parts of the negative leads are inside a silicone tube and other parts are not.

Thanks again for your time, Jeff




This something really weird, interior of each ammeter has no damage at all, shunt resistor still good. It looks like the arcing happened between the thin aluminum plate where the printed graduations are and the hole I cut on the laser frame. There is a picture on how I made all connections

I do have some hv experience.

HV will find any metal grounded or not . It may not have happened if you meter was not mounted to metal. Use perspex .
I dont know your circuit so that is all I can offer.
Though it does seem wrong to have the current meter directly on the hv line .

Hi, thanks for the reply.

This is the ground lead that return to the power supply ground, there is no High voltage in those wires, maybe 1 or 2 volts

What you show is surely hv arcing of course a few volts will not do that .

A circuit sketch might help to be of more help.

Not familiar with CO2 lasers but what kind of power is running to the tubes? Is it pure DC or is it PWM (Pulse Width Modulated). If it is PWM, maybe the meters couldn’t handle the high frequency. Even if that’s it I would think it would burn out the meter and not arc to ground. But like someone said a high frequency could induce a voltage into a nearby conductor, like the metal metal scale plate. Typically a mA meter would be designed for DC or pure 60 cycle AC current and would not read correctly with a PWM current

Having had some time to think this over …

I have been assuming this is a large-but-standard DC-excited glass-tube laser, with three tubes about 180 W each. Is that correct?

If so, those meters may not have been rugged enough for the application, because the peak current through a 180 W tube is much higher than its average current. This has nothing to do with PWM: it’s the normal behavior of a negative-resistance gas-discharge tube.

I measured brief spikes beyond 500 mA in my 60 W tube with low average currents, so I would expect much higher pulse currents on your laser.

Judging from the movements inside those meters, a pulse broke down the coil insulation, arced to the faceplate, then to the metal frame.

Given that moving-coil meters have a fairly high inductance, you may need a meter with a low-value external shunt resistor to ensure the meter movement carries much less than the total tube current and is not subject to pulse overloads.

Yes, if the meter went open circuit then it would have hv on it which would seek a ground via the shortest path.
You need a 50ua meter with a shunt resistor across it .

Thanks everyone for your time.

This it how the meter was connected, I also watched a lot of videos on how to do it. There is a shunt resistor between positive and negative terminals of the meter.
The tube operate at 20Khz.
I measured about 3mm between the thin graduated aluminum sheet and the body of the laser. Everything inside the meter looks OK.

As mentioned, there is a shunt resistor(I think a 2.5 Ohm) between both connectors, I don’t know the value, but I will cut the wires inside the meter and measure it. Those are the blue color 1% resistor, and my eyes are too old to look at the color bands on the resistor. :grin:

This is the tubes assembly,

There is one power supply for each tube

This is a message I received from the seller:

‘‘Our pipes (tubes) are not equipped with ammeters. After adding them, the ammeters will be damaged due to strong voltage and current’’.

Could it be possible that a return path to ground could have high voltage and current.

The ground wires are high voltage wires, the same as the high voltage ones, except they are white instead of red.

Thanks again. Jeff

That is a flare-lit tip that the tubes are not the “large-but-standard DC-excited glass-tube laser” hardware we’ve been assuming all along.

What you have is different than the usual setup, so the generic advice for typical lasers is wrong for your machine.

Yes.

Given the new information, the return lead puts the ammeters at a very high voltage relative to the machine frame, which explains the severe arc damage to the meters. I expect the tubes use a symmetric supply, with the anode end at (maybe) +15 kV and the cathode leads at -15 kV. A symmetric supply reduces the voltage-to-ground enough to not require truly exotic HV hardware, but means the cathode lead is nowhere near frame ground potential.

You must carefully insulate (or, better, replace) the white wires we have been incorrectly assuming are the near-ground-potential cathode leads with high-voltage material. Although we don’t know the exact voltages involved, ordinary electrical tape / silicone tape / whatever will not be adequate.

In particular, do not use silicone caulk as insulation. All common varieties use a moisture-cure chemistry that releases conductive ions and are not rated for electrical applications. Bonus: most release acetic acid, which will corrode nearby metallic objects.

The power supplies may have sustained internal damage from severe overcurrent, but there’s no way for us to diagnose that apart from seeing the tubes not operate correctly after you repair / replace the wiring.

Thanks for the reply, I will give up with the ammeters and fix the wires with high voltage heat shrink tubing and silicone tube.

What I don’t understand is this wiring diagram comes from my power supply manual. I just assumed I could do it, but I was wrong.

Thanks for your help, it was greatly appreciated, Jeff

And it applies to the standard = cheap laser hardware the rest of us have, where the cathode lead is very very close to frame ground = 0 V and no fancy wiring is required.

Verily it is written: Hell hath no fury like that of an unjustified assumption. :frowning_face:

Been there, done similar horrors to similarly expensive hardware: I share your pain.