Hi all

I got a problem with a Laser CNC machine. Problem is as soon as i fire the laser on, there is sparking/arching on the higvoltage side of the laser tube terminal. I have heard of this caused by a bad laser tube but i don't see how a bad laser tube can cause an external spark. the spark is hitting the CNC body which in my experience is because there is an insulation problem and the current is finding a shorter distance for it to travel back instead of traveling trough the laser tube. What do you guys think ???


Thanks in advance

 

Do you have a photo of the setup? Insulation is possible but dirt/skin oil/carbonized spider web/... contamination and even internal damage could be the cause.

 

If the laser tube developed a higher operating voltage due to a change in the internal gas mixture and/or pressure then I could see how that may induce external arcing. Do you know what the normal tube operating voltage is? How far is the spark jumping?

 

Do you have a photo of the setup?

Unfortunately not in the mean time, i will have to take a picture of it tomorrow

 

If the laser tube developed a higher operating voltage due to a change in the internal gas mixture and/or pressure then I could see how that may induce external arcing. Do you know what the normal tube operating voltage is? How far is the spark jumping?

The maximum voltage it can reach is around 30KV. The spark is jumping 8-11 cm around the positive terminal on the laser tube hitting the gantry body, but if there is no insulation problem where does the current in the spark end, i haven't noticed any grounding point on the high voltage side of the power supply.

PS

Even with the insulation cap on the positive terminal it sparks externally to the gantry body

 

According to this chart on spark gap versus voltage it takes about 70kV to jump 8.8cm in air with needle electrodes, more for spherical electrodes. So that's well above the 30kV max. you stated for the normal tube operation. This does indicate a faulty tube.

The power supply is likely grounded to the chassis. Where does the other end of the laser tube connect?

 

According to this chart on spark gap versus voltage it takes about 70kV to jump 8.8cm in air with needle electrodes, more for spherical electrodes. So that's well above the 30kV max. you stated for the normal tube operation. This does indicate a faulty tube.

The power supply is likely grounded to the chassis. Where does the other end of the laser tube connect?

Other end goes directly back to the power supply, I dont think its the tube that is the problem, because the tube is brand new and i got another new one that i have also tried with the same result, if the tube is not working like you said why is the spark happening at the input terminal of the tube ? its not happening inside the tube its all externally.

 

what brand is it? I have worked on coherent brand and a few others. some have high voltage + on the ends and - in the middle. most have a much higher starting voltage than operating voltage. cleanlieness is very important, has it been cleaned lately? a gas mixture/pressure problem could cause this also, low vacuum (air leak into tubes) can cause the tubes to not fire, which increases the voltage a lot.
a surface arc takes less voltage than through air.

 

what brand is it? I have worked on coherent brand and a few others. some have high voltage + on the ends and - in the middle. most have a much higher starting voltage than operating voltage. cleanlieness is very important, has it been cleaned lately? a gas mixture/pressure problem could cause this also, low vacuum (air leak into tubes) can cause the tubes to not fire, which increases the voltage a lot.
a surface arc takes less voltage than through air.

the machine is a Chinese brand, its very clean. the arch is happening trough air, from + terminal to chassis

 

High humidity could cause the spark to jump early. Also, bad caps could cause an initial surge in voltage at startup. Once the first spark jumps, it only requires a holding voltage (much lower) to maintain the arc.

Pollen, humidity, dust can all contribute.

I would look at the start up voltage surges on the power supply - especially if happening with a new tube.

 

High humidity could cause the spark to jump early. Also, bad caps could cause an initial surge in voltage at startup. Once the first spark jumps, it only requires a holding voltage (much lower) to maintain the arc.

Pollen, humidity, dust can all contribute.

I would look at the start up voltage surges on the power supply - especially if happening with a new tube.

hmm.... i am thinking of trying to megger the supply lines that goes to the tube to make sure there is nothing shorted to the chassis, because the spark is hitting the chassis wouldn't that mean that the current has found a shorter path ?

 

http://www.cnczone.com/forums/gener...n/138755-2-inch-arc-laser-tube-chassis-3.html

 

hmm.... i am thinking of trying to megger the supply lines that goes to the tube to make sure there is nothing shorted to the chassis, because the spark is hitting the chassis wouldn't that mean that the current has found a shorter path ?

Unless the spark is coming through the insulation, then there would appear to be no problem with that. It just appears that the voltage is sufficiently high to jump the air gap between the tube contact and ground. As I noted, that voltage appears to be much higher than the normal tube operating voltage.

 

Maybe a silly question, but... did you possibly forget to re-install some insulation when you replaced the tube?

 

the tube operating voltage is less than the starting voltage, just like a large neon lamp, they take more to fire than to conduct.

 

As mentioned before everything must be spotless, even the tube brackets. We had some Coherent General Super 48 lasers that were terrible about carbon tracking from the anodes to ground. We had to scrub them with acetone to get rid of the carbon. Once one was so bad we had to use a die grinder with a small round burr in it to disrupt the carbon tracking.

 

You might try adding a thick layer of silicone rubber (grout) all around any exposed connects/wire to increase the insulation resistance and track length of any arcing to the chassis.

 

i suggest you ask the question here http://www.lightobject.info/ they are very helpful with chinese machines

 

Unless the spark is coming through the insulation, then there would appear to be no problem with that. It just appears that the voltage is sufficiently high to jump the air gap between the tube contact and ground. As I noted, that voltage appears to be much higher than the normal tube operating voltage.

I got a new power supply yesterday, installed it and there goes the spark again. After doing some searching on google and going trough the link posted in this thread (http://www.cnczone.com/forums/gener...n/138755-2-inch-arc-laser-tube-chassis-3.html) i read that similar problems like mine was solved by changing the Laser tube, so i bought a third one and installed it using the old power supply, i run a test and this time there was not spark but the laser didn't work (no laser beam) no matter if i run it on the old power supply or the new one.

What is confusing me is why does the spark occur on the old tubes, even if they where bad laser tubes what does it have to do with allowing a spark to occur ???!!!

Maybe a silly question, but... did you possibly forget to re-install some insulation when you replaced the tube?

All insulation is correctly installed, i even insulated the terminals with some extra insulation but ended up with the same result : (

You might try adding a thick layer of silicone rubber (grout) all around any exposed connects/wire to increase the insulation resistance and track length of any arcing to the chassis.

Sorry for my last replay regarding the arch length, i mistakenly mentioned that the spark length was 8-11 cm. after observing the arching i noticed a 2-4 cm spark length.



Thanks in advance
John

 

The power supply model is as following: MYJG80R

http://en.jnmydy.com/products_detail/&productId=4f7a1603-e335-44dd-ac94-711a4a6bdde7.html