CO2 laser PSU cathode voltage relative to ground?

audiomath
Posts: 20
Joined: Wed May 11, 2016 1:01 pm

CO2 laser PSU cathode voltage relative to ground?

Postby audiomath » Tue Sep 19, 2017 3:37 pm

I just ordered a new milliammeter to be permanently installed in my 60W laser cathode circuit. I plan on cutting out the sheet metal and mounting the meter directly in the cabinet.

My question is does anyone know the cathode voltage (not current) of (say) a 60W tube, relative to ground?
My reason for asking is that if I was designing a high-power tube-type RF amplifier, the cathode return would be "clamped" at some voltage higher than the normal operating voltage for safety reasons. For example, if the meter movement opens, a wire breaks etc.

I don't know enough about the physics of CO2 glass laser tubes and power supplies to know how that works. If it's just a volt or two, I can use two back-to-back strings of high power diodes (like 6A10's) to make sure the voltage of the cathode circuit at the meter never exceeds the sum of the forward voltage drops of the diodes against ground. If it's less than 50V or so, I have 50W zener diodes in my parts box as well. (used to set the cathode bias of triode power tubes, in case anyone is interested. :-) )

I haven't seen this done in laser engravers, but it's standard practice in radio transmitter designs to protect the operator from high voltage appearing on the panel meters in the case of a circuit fault.

Thanks,
--jim

audiomath
Posts: 20
Joined: Wed May 11, 2016 1:01 pm

Re: CO2 laser PSU cathode voltage relative to ground?

Postby audiomath » Wed Sep 20, 2017 1:44 pm

Since this doesn't appear to be common information, I'll measure it at close to full current once the new tube and PSU are installed.
I'll post the cathode voltage value and what safety measures (if any) I used once everything is up and running.

Thanks,
--jim

daniel.macke
Posts: 29
Joined: Mon Jul 28, 2014 10:30 am

Re: CO2 laser PSU cathode voltage relative to ground?

Postby daniel.macke » Thu Sep 21, 2017 4:19 am

Hello,
The cathode voltage should be very close to the ground voltage, the only difference is caused by the internal shunt in the current meter. On my system (80W RECI tube, 80W PSU "Jupiter" from lightobject) I use bidirectionnal transil diodes (reference 1,5KE6,8CA voltage 6,8V) wired between the cathode and also each PSU input on one side, and directly to the ground on their other side, to prevent any high voltage return (in case of arcing for example). I have had no problem so far.
Regards.
Daniel

audiomath
Posts: 20
Joined: Wed May 11, 2016 1:01 pm

Re: CO2 laser PSU cathode voltage relative to ground?

Postby audiomath » Thu Sep 21, 2017 10:48 am

daniel.macke wrote:Hello,
The cathode voltage should be very close to the ground voltage, the only difference is caused by the internal shunt in the current meter. On my system (80W RECI tube, 80W PSU "Jupiter" from lightobject) I use bidirectionnal transil diodes (reference 1,5KE6,8CA voltage 6,8V) wired between the cathode and also each PSU input on one side, and directly to the ground on their other side, to prevent any high voltage return (in case of arcing for example). I have had no problem so far.
Regards.
Daniel


Exactly what I was looking for, thanks!
I suspected the cathode would be very close to ground, but I can't measure it until I get the machine repaired.

--jim

audiomath
Posts: 20
Joined: Wed May 11, 2016 1:01 pm

Re: CO2 laser PSU cathode voltage relative to ground?

Postby audiomath » Sun Sep 24, 2017 1:27 am

Daniel,

You were correct. I got the machine back together tonight, and the cathode voltage relative to ground is essentially zero.
Now to decide how to mount the meter. It's currently in a plastic box that attaches to the laser with a cable. I bought a new raw meter from LO to replace the flaky one I'm using; I need to decide whether to mount it in a plastic box and attach the box to the laser, or take the plunge and cut into the sheet metal.

Thanks,
--jim

audiomath
Posts: 20
Joined: Wed May 11, 2016 1:01 pm

Re: CO2 laser PSU cathode voltage relative to ground?

Postby audiomath » Mon Sep 25, 2017 8:48 am

Problem solved -
A search of the LO site turned up a 30mA panel meter with a rectangular mounting bezel.
I ordered two, and will install one and the protection diodes in the sheet metal in the front of the machine.

--jim

audiomath
Posts: 20
Joined: Wed May 11, 2016 1:01 pm

Re: CO2 laser PSU cathode voltage relative to ground?

Postby audiomath » Sat Sep 30, 2017 11:41 pm

There is this man named Murphy...

After installing the new 60W SPT tube and power supply, I fired the machine up and the chiller came on, but it wouldn't shut off!
It turns out the 10K thermistor in the bottom of the coolant tank quit working - for some reason?? (I flushed the coolant twice before installing the new tube, so who knows...) To fix it, I potted a new industrial-quality stainless 10K unit into a plastic tubing tee, and installed the tee into the output line from the pump in the chiller. (see attached images) I mixed some titanium dioxide into the epoxy to strengthen it, which is a trick I was taught as a green kid in aerospace.

I also installed one of the new 30mA meters I got from Marco. MUCH better quality than the flaky meter I had bought earlier through Amazon.
I really like the bezel mounting system too. I used a rotary tool with a cutting disk to cut the mild steel in the front of the cabinet, then installed and wired the meter. I used 2 sets of 2 10A10 diodes back-to-back to "clamp" the cathode circuit close to chassis ground in case the meter movement opens or the cathode wire breaks. With 2 diodes in each string, the cathode cannot go more than 1.4V above or below chassis ground. It may not be explicitly necessary, but it makes me feel better.

After all the new parts were installed and the tube and mirrors aligned, it likes to cut 3mm clear cast acrylic at 10mm/second with 55% power. What's interesting to me is that 55% PWM reads 14mA with the coolant at 17.2C, and 13mA with the coolant at 20.8C. I have the chiller set at 20C, with 2C differential and 0.8C hysteresis. The temperature of the tube definitely makes a difference. I thought about reducing the differential temperature to 1C, but that would make the chiller cycle on/off quite a bit more often.

In the end, the machine is working fine and I've added a permanent cathode current meter that replaces the plug-in meter I built a year or so ago. So far, so good. I'm impressed with the quality of all the parts I ordered from LO.

--jim
Attachments
spt_60w_tube.jpg
Meter 4.jpg
Meter 2.jpg
Meter 1.jpg
Coolant Tee 4.jpg
Coolant Tee 3.jpg
Coolant Tee 2.jpg
Coolant Tee 1.jpg


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