Ammeter Problem

I’ve got an old Full Spectrum 4th Gen Hobby Laser I’ve upgraded with a Cohesion3D board and it works, and I cut things, but I noticed that the ammeter never went above 2 milliamps. I knew the tube was old, but that still seemed really low, and did not change when I adjusted the potentiometer.

I replaced the ammeter thinking it was faulty, but got the same results with a new one…

Finally I replaced the tube (it was many, many years old) and when doing so noticed the black low voltage return line, which I believe should go directly to the ammeter, was also connected directly to the frame of the machine, so in effect, to ground.

Would this be the reason the ammeter never goes up much? (I think it goes to 3 or 4 now with the new tube.)

My only assumption is that the low voltage line connected to the frame just as it comes out of the tube is negating the amount of current running to the ammeter… Does this sound correct? If so, should I remove that ground wire?

That is correct. If there is less resistance on the wire to ground than there is across the internal shunt in the ammeter, you will be stealing current from the ammeter.

I’m not sure what the schematic of your machine is and how the original power supply is connected but I can make a couple statements providing that you understand that I will not be held responsible for any modifications you make to the machine which could result in personal harm or harm to others. Now that that is out of the way

  1. The reason your cabinet is grounded is so if you get any type of equipment failure which could cause arcing to the cabinet, the least path of resistance will be through the cabinet to ground rather than through you.

I would make sure your cabinet is tied into a good ground lug and verify your cabinet ground is finding it’s way back to earth ground

  1. Once verified, I would remove the wire that is directly from the cathode to machine ground. I would think internally your power supply laser power ground would by tied to earth ground somehow independent of the existing ground wire that you may remove.


Thanks, Dave. That was my suspicion, but I really wanted someone else to review my idea. I disconnected the ground wire that connected the low voltage laser return line to the chassis, and the ammeter is working properly now!

(And yes, the chassis, power supply, and machine itself are all properly grounded.)