Embarassing problem

[Axeman]

So, I've just finished up my laser cutter that I've built. Been working absolutely fantastic...watching it cut. I have a multimeter keeping an eye on my laser current (connected through the ground wire back to the PSU) until my current panel comes in for a permanent installation, when my multimeter gets bumped falling off the table and the ground wire pops out momentarily arcing with a flash. My Mach 3 stops and my laser doesn't fire anymore. My heart stopped.

So I checked the fuse in the PSU, it's fine. It's getting 110v in and producing a 5v signal out the side for measurement but I don't know how to check if it's sending a voltage out since it's 20kV or if it's my precious tube. All my systems work except there is no laser fire at all.

I've taken a close look at the tube, there is no scorch marks or anything to indicate the tube is damaged in anyway at all. Though I have no way to test it (that I'm aware of).

Anyone have any idea what I can do here? Is it a dead power supply now?

[Raman Narayan]

Hi,

Not sure, if you have already sorted out the issue(good/bad ).
Anyway here are my suggestions:
i) Check and make sure the tube is not physically damaged
(any crack near the terminal posts to the tube, could occur due to
mechanical pulling). This is a possibility since the multimeter sort
of pulled the ground post with it, before separating/hitting the ground.
You cannot do anything, other than replacing the tube if this is
the situation.
ii)Check the continuity from the power supply leads to the tube posts
(it will be easier if you have not permanently sealed the posts with
Solder/RTV Silicone). Quick connects(those use a screw terminal-make
sure to replace the metal screw with a nylon one) make this easier.
iii) To check the power supply itself for presence of high voltage at
output, connect it to a zero watt neon bulb(It is the one
with a high meg resistor along with a neon gas bulb). You can also connect
few of these bulbs in series, before connecting to the supply. A brief
press of the test button will show you, if the power supply works.
Worst case, the neon bulbs will be blown out(they are cheap anyway).
Never touch or be in the close vicinity of any high voltage terminal, it
could be dangerous(know what you are doing, and treat the high voltage
with respect).

If the test button works and if you do have valid input to the power supply
TTL,Analog Input, the possibility is that you might have fried the opto
coupler input stage in the power supply(Which again would need a replacement
power supply).

Hope you find this helpful.

Regards
Raman

[Axeman]

Appreciate the very detailed response. It all makes sense now...

Yeah, I looked over the tube carefully and saw no damage of any kind at all. There is no way for mechanical damage to the tube since I have the wires secured so that they don't actually pull at it at all, but I would suspect that since there is no burning, charring or anything of that sort it was probably the opto-isolators or the transformer in the PSU that died. It was a low-budget one anyway (that I have now discovered).

So I have already ordered a much nicer looking supply from the fine folks here along with a few misc parts that would be handy. Thank you again for that, helped very much. On that note, do you happen to know what the minimum frequency is necessary for running PWM into the 40w PSU's here? Or from that, since I'm not running a DSP and Mach 3 can't run high enough speeds for PWM natively, I wonder if doing a PWM -> analog conversion would have high enough accuracy for 256 levels of power or if I should be running a PWM -> PWM generator -> PSU setup for better accuracy.

[Raman Narayan]

Glad that your precious tube is safe .
I don't know what is the minimum frequency for the specific power supplies sold by lightobject.
But in general, From my understanding most high voltage glass co2 tubes are essentially analog
tubes. The combination of TTL and the analog DAC input is the most common way to drive the
tubes. You should be able to smooth your (0-255 level) 8 bit pwm to get a fixed analog voltage(
Do not run at a much lower frequency, or to smooth too much, so that it is less responsive
to the corresponding TTL(15-20Khz)). I think it should not matter for much of the
cutting, but will surely matter for highly precise engraving performance.
It is normally not advisable to run at very low frequencie(at or below 1KHz), if you need a
very stable output from the tube.
Anyway your majority of usable power level should be up from 20-100% for
good stability/predictability, any power in the 10-20% range is not consistent, and below 10%
unusable(tube almost shuts off). The power variation is also a bit non-linear esp. near the
20% range.

Whatever you do, finetuning the PWM frequency/adjusting power supply output, always keep
an eye on your ammeter current reading, and keep your ears open(esp. buzzing from PS, tube)
and nose sniffing constantly(burning smell), in that order, to stay out of trouble.
I would also recommend you do most of your adjustments with a short duration pulse(in the 50msec
range, gradually increasing it once you get the confidence that everything is functioning fine),
apart from keeping your other hand half pressed on the emergency shutoff(time to get that
big red button ).

Wearing safety goggles(most laser viewing windows are nothing by tinted polycarbonate),
hand gloves(never wear those cut resistant kevlar ones, it produces cyanide gas, when burnt),
and keeping a Co2(not the dry powder variety) extinguisher handy is optional.

Hope this helps.

Thanks/Regards
Raman