Laser current measuring question (20KV) damaged laser tube
W9GFO
Posts: 4,010
My laser machine came with a milli-ameter (like this one) which is wired in series with the laser on the ground side. Max current is about 25 mA and the voltage is 19,000V. Someone I know ordered an ammeter to use in their laser as well but mistakenly ordered a digital ammeter that measures up to 50 amps. To make it work he replaced the shunt with a 1.5 ohm resistor. Now his laser tube is damaged.
The question is, could modifying the ammeter have anything to do with it or is it just a coincidence? I would not have done what he did but I do not know if there was anything wrong with his fix.
The question is, could modifying the ammeter have anything to do with it or is it just a coincidence? I would not have done what he did but I do not know if there was anything wrong with his fix.
Comments
How do you mean "damaged"?
The laser doesn't operate? Or something physical occurred with the "tube"?
1.5 Ohms is darn near a short ,, so I doubt that the presense or lack of a 1.5 Ohm VS 0 Ohm is gonna blow it ..
this is a Hard sealed CO2 tube right ?\
the other issue is . how is it damaged .... a CO2 LASER is a VERY hearty laser and besides lack of cooling cant realy just burnt in a day .
Peter
The electrons began taking a different path, either exploiting or creating a pinhole in an interior tube which heats up the area surrounding the pinhole enough to glow.
Yes, it is a sealed CO2 laser.
As far as a laser not being able to be burnt in a day - what can I say? The problem occurred in the space of a couple minutes - so clearly they can be "burnt" in a day - if something is wrong.
Was the problem remedied by replacing the laser tube and mounting a milliammeter of the same internal construction as the original one or are you describing an "as is" situation of the problem?
A moving coil milliammeter (D'Arsonval meter) behave as an inductor in AC circuits, so impedance and frequency response are in play when they are present. Have you double checked the driving circuit of the tube to be sure that the problem was not there?
Yanomani
It also ocured to me to ask if the resistor used in place of the milliammeter was constructed for 19 kV use or not. Such kind of resistors tend to be very large so they do not represent a short circuit (spark like) to high voltages. Also I'm not sure about the impedance of the original meter that was in that circuit originaly.
Yanomani
Of course, if you don't have a 19,000 volts across the resistor, then the device becomes much smaller.
100 volts at 25 ma and the resistor suddenly becomes a rather small device, less than 5 watts maybe.
-- Gordon
I wanted to rule out the possibility that the resistor had contributed to the damage. I do not think there is anything special about the ammeter. I believe it is a coincidence that the damage occurred after installing the modded ammeter due to a defect in the tube.
Your post didn't make it clear if your friend's laser tube was ever used before. If brand new, then I suppose there could have been a manufacturing defect that manifested into damage upon application of full current. Hopefully he can return it under warranty.
-- Gordon
I'm a bit familiar with YaG-based lasers, not CO2. There's no form of electrostatic focusing in a CO2-based laser, right?
From the info in the previous posts, I'd guess it was a defective tube - and the damage not caused by the current meter mod.
David James . On a CO2 there is no focus besides the optics used to Cut in the end , Esp on a hard sealed Tube ,,,
the HF and OC are all one mess once the tube is made ....
I had a chat with my Proff here at OIT on optics. He said the meter is not the cause...
As Peter mentions, the tube may not have gone through an adequate QC burn-in period. From your description of the circuit, I can't see how the low resistance of the meter would have made a difference, so it certainly seems reasonable it was a manufacturing defect. All the more likely if it was an imported tube, where QC may have been minimal.
I sense that you're concerned that if the circuit modification did the damage you'd feel it's unfair to return the tube under warranty and ask for a free replacement. I'd have the same concern, but I think in this case the meter made no difference, and the tube was defective to begin with.
-- Gordon
Actually, it is common to have a beam expander/collimator immediately after the tube.
This poor guy has been the recipient of all the bad luck that the rest of us that have imported lasers have avoided. This was his fourth tube, hopefully the fifth will be the charm. The second and third replacement tubes were shipped DHL and both arrived damaged - so it is really only his second defective tube. The latest tube was shipped by boat so the defect can't be blamed on rough handling.
I only use the amp meter to set the limits on the controller and then I remove the amp meter.
It sounds like he had the polarity switched on the tube. The low end should be near the output mirror.
I would like to buy USA made stuff, but it is hard to beat a $250 tube and $350 power supply.
Yag lasers are my main business for 30 years.
The resistor failed open and so did the meter. This caused a loss of feedback to the power supply. The power supply went to full current and the high voltage found a new path into the water circuit.
Has he tested the resistor and amp meter to see if they are still good?
edited
The problem I have with this scenario is that the electrical power would be too small to take out most resistors.
Volts = amps * resistance.
0.025 amps * 1.5 ohms = 0.0375 volts
Watts = amps * volts.
0.0375 volts * 0.025 amps = 0.0009375 watts.
I'm back to the tube being hookup backwards.