Laser Current Meter
Posted: Wed Oct 05, 2016 10:53 am
It's handy to monitor a laser tube current. This gives a good indication of the tubes power and if it's being over driven. Yes, mine is being over driven. I think I have a 50W tube in a '60W' system, as it's only 1030mm long and at 100% power, its being driven at 26mA, whereas it should only be driven at around 20mA max. Over driving a laser tube will quickly shorten it's life.
The Chinese/eBay laser unit doesn't come with an tube amp meter, so I made my own. After assessing the few odd meter I have stored away, I selected a 1mA Full Scale Deflection (FSD) meter. This meant I had to come up with a current shunt to shunt most of the current away from the meter. The closest I could come up with using the resistors on hand was a 1.8 ohm shunt. This was made using 2 x 1.2 ohm resistors in parallel and then adding this in series with another 1.2 ohm resistor. Here is my quick test arrangement:
Once I had them worked out, I tested the meter out to find it's new FSD, which worked out to be 32mA. I then imported a photo of the old meter front plate into Inkscape and then used a new layer to draw up a new front plate and used my laser cutter to etch and cut out the paper template.
I used KiCAD to design up a simple circuit and PCB, which was laser etched into a paint covered PCB and etched using Hydrochloric acid and peroxide solution:
and the components where transferred from the meter onto the PCB (ok, the ex-TV 1968 capacitor was upgraded to a newer style):
The last thing to do was quickly design up a housing for it all and laser cut it out of 3mm MDF:
and the final product perched on top of the laser cutter:
I might redo the front paper to include power levels for my tube. It turns out that safe max power (20mA) is delivered at around 65% tube power. Anything beyond that is pushing the tube.
The Chinese/eBay laser unit doesn't come with an tube amp meter, so I made my own. After assessing the few odd meter I have stored away, I selected a 1mA Full Scale Deflection (FSD) meter. This meant I had to come up with a current shunt to shunt most of the current away from the meter. The closest I could come up with using the resistors on hand was a 1.8 ohm shunt. This was made using 2 x 1.2 ohm resistors in parallel and then adding this in series with another 1.2 ohm resistor. Here is my quick test arrangement:
Once I had them worked out, I tested the meter out to find it's new FSD, which worked out to be 32mA. I then imported a photo of the old meter front plate into Inkscape and then used a new layer to draw up a new front plate and used my laser cutter to etch and cut out the paper template.
I used KiCAD to design up a simple circuit and PCB, which was laser etched into a paint covered PCB and etched using Hydrochloric acid and peroxide solution:
and the components where transferred from the meter onto the PCB (ok, the ex-TV 1968 capacitor was upgraded to a newer style):
The last thing to do was quickly design up a housing for it all and laser cut it out of 3mm MDF:
and the final product perched on top of the laser cutter:
I might redo the front paper to include power levels for my tube. It turns out that safe max power (20mA) is delivered at around 65% tube power. Anything beyond that is pushing the tube.