Laser Power Calibration

I am running an OMtech 130w laser with a Ruida 6442G controller, Yongli H6 laser tube and ZYE MYJG150W-1 laser power supply. The tube is rated for 30mA max. It is recommended by the manufacturer, for maximum tube life to stay under 80% power level which is then 24mA. When running I hit the 24 mA at 55% power as set in LightBurn. 30 mA is at 65-70%. It will go higher still, and when doing the initial power tests I aborted at 75% and 34 mA which was already exceeding the maximum for the tube. The the laser came with a 150w power supply. It would be nice to have the scaling in LightBurn set so that 100% is 30mA so I do not inadvertently damage the tube.

There is apparently no maximum current trim pot on the laser supply (nothing noted in the manual and nothing visible). I did not feel like ripping the supply out of the laser and removing the cover to see if there are labeled hidden control pots. I am not inclined to fiddle with unlabeled pots as I do not have a high voltage divider / probe available to recover from inadvertently mis-adjusting the high voltage level (~34KV).

In the Lightburn ->Machine Settings → Vendor Settings, if you scroll down to the Laser Settings block there is a line for Laser 1 maximum power which is currently set at 98% (from the factory). Unfortunately, there is no documentation from either the manufacturers of the hardware, or the Lightburn software as to what exactly does this parameter do. Is it a multiplier (e.g. lower setting lowers the max mA), a limit/ clamp function whether in the LB menu (simple number not to exceed in the layer parameters) or applied in the controller, or is it really a divisor (e.g. currently measured value is a percentage of max and could go above 100%)? It would have been nice to know the meaning.

I also had questions about the accuracy of the digital mA meter that is supplied and connected to the HV supply and its accuracy. So, this meant the starting point for the exercise would be to check the calibration of the existing mA meter with known good instruments. This requires cutting into the return line from the laser tube and inserting a mA meter. For this, I have both my trusty 45+ year old Triplett 310 analog VOM, and a modern Klein Tools MM1000 true RMS reading digital VOM. Note that the laser power supply is Pulse Width Modulated (PWM) and many inexpensive non true RMS reading digital meters will give erroneous results. Please note: You must be experienced and comfortable with high voltage electrical circuits when doing this. Setting up to do these measurements improperly and leaving this return lead open, will expose you to potentially lethal high voltage and current as well as exposing the equipment to likely damaging high voltage arcs. Early in my medical imaging career, I was an X-ray system field service engineer both for diagnostic (up to 150 KV) and therapeutic (up to 20 MEV) systems.

To run the tests, I used a “test card” available from the Facebook OMtech user group that has a series of ~5″ long lines drawn at increasing power levels from 10-85%. I modified this a bit to make the lines longer (and allow the meters to settle) and reduce the power of the text engraving (as it would exceed the max mA of my laser). The digital readout on the laser is reasonably accurate, matching the VOMs at -1.5 to+0 mA. . The displayed values are quite close. The values shown are truncated rather than rounded e.g. both 26.1 and 26.95 show as 26mA on the display. I now know what the accuracy of the display is (-1.5 to +0mA of actual). Repeatability was within about 5% but I did not explore this deeply.

Next, I ran a series of tests to see if I could use the min and max percentage levels to: 1. Safely limit the max current. 2. Re-scale the % power settings so that in LB 100% would be 30mA. Te next step is then tackling the power / mA linearity and limiting. I had hopes that the max and min values could be used as gain and offset values for calibrating the percentage vs actual mA . However, it turns out that the max mA percentage instead acts as a “clamp” or limiter.

Rather than list the whole table of measured values, I will use some example values In my case for example 40% is 17mA, 50% is 22mA, 60% is 26mA, 70% is 31mA and 75% is 33mA (after which I stopped due to being over the tube rating). If I set the max % to 70 then the max current (regardless of % form 71-100% ) is limited to 31mA and lower values are unchanged, if I set the max % to 50% then the current is limited to 22mA and all values above 50% are also 22mA. I settled on setting the max to 65% – 29mA.

So, at least the first goal was achieved in that I cannot accidentally exceed the max rating. This also means that my materials library in LightBurn is unaffected and I don’t need to repeat the growing pile of test cards for cutting and engraving.

I also explored the bottom end (low power % / low mA settings). The tube will start to weakly lase at 8% drawing 0.5mA with 0 indicated on the display 9% is 1mA, 10% is 3mA. It is clearly non linear on the very bottom end and at these very low levels the plasma glow in the tube is not stable and is seemingly flickering. I did not bother to adjust the min % and will just continue using 10% in my LB min settings. However, it is interesting to know that with a complex / dense line engraving I could go lower (but may have inconsistent results).

With theses tests and setting the max power percentage limit, I now know that the tube current display is reasonably accurate and I now have a limiter that should prevent inadvertent damage due to over-driving the laser tube.