Puzzling Power Issue - Output Current Not Achieving Tube's 80% Current Max at 99% Max/Min Power

Hi Everyone,

I just converted a Full Spectrum Hobby 40W CO2 laser over to a Ruida RDC6442S controller. It was a fairly large project, but I got it fully operational. One last issue I am having is with the Laser’s max output power levels. I installed a new 40 watt CO2 laser power supply, which for the most part is working well with a new 40 watt tube for engraving and low-power cuts.

The issue I am having is with the laser’s maximum output power/current. At between 80-99%, the laser is only putting out 8 milliamps with a new tube that is rated for 16 milliamps. To make sure the current meter on the laser was working correctly, I measured the current with my multimeter and it also measured 8 milliamps at 99% power. To make sure the PWM signal from the Ruida controller going to the power supply was working, I used an oscilloscope to measure the duty cycle with the laser’s min and max power set at 90% and the PWM duty cycle was registering 90% on the oscilloscope.

So, with the current meter and PWM signal working properly, I took a chance that the 40 watt power supply was not capable of achieving the power needed for my tube, so I replaced it with a new 50 watt power supply. Unfortunately the 50 watt power supply did not increase the laser output current, which was still hovering around 8-9 milliamps at 80-99%.

So I am stuck. The only thing left that I can think of is that the tube is bad, but it is new and the core has a nice healthy pink color to it. Does anyone have any ideas on why the laser is maxing out at 9 mA?

Also, can someone explain what the Laser pre-ignition frequency of 5000 Hz and Laser Frequency of 20,000Hz in the Ruida’s configuration? I tried adjusting the default ignition and operational frequencies but I saw no discernable difference in laser output power levels.

Thank you very much in advance for any assistance you can offer me.


Laser frequency is the PWM frequency, 20KHz is fine. pre-ignition is not used for CO2 Lasers so don’t worry about it, some RF Lasers need it.

There is max mA and working mA ( meaning the mA will be less than max mA to get full rated power ), though 8mA seems a bit low when asking for full power. The only way to know for sure is a Laser power meter

Seeing as your scope shows 90% when the controller is set to 90% makes me think your max % is probably set OK.

What was the amplitude of the PWM, should be near 5V?

What are you doing to test the output? Are you pushing pulse on the controller or cutting something from LB?

The PWM voltage is 5V. I am running my tests with both Lightburn and with the pulse button on the Ruida controller head, both showing the same current to power setting ratios.

It seems like everything is pointing to the tube, but it is new. Can CO2 tubes of similar output power ratings from different manufactures have different voltage requirements? With V=IR, “V” being the voltage of the supply and “I” being the laser’s current, the only variable is R, the resistance the supply sees from the tube. Do I maybe need a power supply with a higher voltage, or am I looking at this incorrectly.

Thank you.

Sounds like a bad Tube, is the old tube working? Try the old.

Have you checked if your power supply has a maximum current limiting potentiometer somewhere? At least all the power supplies I know for the 80W - 150W laser tubes have such a potentiometer hidden on the side. If this is set very low this might cause your low output. As you have tested on two different power supplies this is probably not the case but at least worth looking for.

I did look for one. Normally there is a small hole where you can insert a screwdriver into and adjust the current, but this power supply has no obvious current adjustment potentiometer. If I take apart the case, I can see a potentiometer, bit have been hesitant to adjust it because I cannot find any documentation for the power supply (MYJG-50W). I will try adjusting it to see if it is for the high voltage side or the low voltage side. Thank you Brian.

That is a great idea actually. I do have the old tube. I will give that a try and see what current levels I get with it. Thanks Andreas.

You are looking at it correctly, that’s why there are different power supply’s for different tubes, generally, the more powerful the tube, the longer it is. The longer it is, the higher the “R”, so more voltage required to make a certain current flow. When you get a 40W power supply for a 40W tube, it should be able to drive max current through the tube.

The fact you bought a bigger supply would mean you would not be able to get near 100% PWM without over driving the tube, so you would set a limit in the laser setup window.

My power supply has a screw, but it is only usable over a narrow range. I set it up so that 99% gives me 25mA, which is the max for the extended life spec of my tube. So even if you had one, I don’t think you could dial it up or down much. I did get mine to go very non linear when I adjusted it too far.

What I am going to try next is to put the old tube back in and test it to see what kind of current I can draw on it to compare with the current draw of the new tube. I am ordering a high voltage probe and will test the voltage from the power supply to see what it is putting out. I will also pull out my laser power meter and see what the actual laser power output at the laser head is (should have done that in the first place, but I was assuming that the current reading is as good a measure of power output as a laser power meter is.

I will then open up the power supply and see what that potentiometer in it actually adjusts; it adjusts either the high voltage output or the 24/5 volt outputs. If it does adjust the high voltage side, then it may fix the problem.

This will take me a few days to accomplish, but I will get back to the forum with my results as I am sure others have similar power output issues and maybe the troubleshooting steps I am taking can be of help those with the same issues. Thank you very much Joel.

I don’t think you need to mess with any LPSU adjustments to get this sorted out. The only reason I adjusted mine was to reduce the power output. If I wanted to limit my power to 25mA, I was maxing out at about 80% on the PWM. I adjusted the LPSU so that I got 25mA at 99%.

The things you want to check are good, leave the LPSU alone.

Hi Everyone,

I just wanted to give an update on the power issue I have been working through.

I got a high voltage probe and tested the laser at various power levels (25%, 50%,75%, 99%). What I discovered was that the power supply stopped increasing the output voltage to the laser tube at a power setting of 50% where it started capping at 13,000 volts and 8 mA. At 99% power the voltage was around 13,800 volts and 8.5mA. So, a power setting between 50% and 99%, there was virtually very little increase in output voltage and current.

I decided to see how much current the 5 volt and 24 volt rails on the power supply were drawing and found out that the 24 volt rail was drawing 6 amps on a rail that was rated for 2 amps. I purchased a separate 24 volt power supply rated for 8 amps and put everything drawing 24 volts on the separate power supply. With the new 24 volt power supply installed, the output voltages and currents for the tube increased to 14,800 volts and 10 mA at 99%. The output voltage and current were still going flat at a power setting of 50% but removing the 24 volt load off the tube’s power supply did increase the tubes output power 1.5 mA, but I was not satisfied because the tube is rated for 18mA max, 14 mA for longevity of the tube; I needed another 4 mA for the full 14 mA/80% power output for the tube. I did try to put the old tube in the system, but the CO2 charge in it was way too low to produce an output. Additionally, I did not open up the power supply to play with that potentiometer, which I had no idea what it did.

Although the power supply I installed was rated for 20,000 volts and 18 mA, it certainly was not producing that in reality. I decided to order a 60 watt power supply that could fit in my already tight case and have just finished installing it.

I will be testing the new 60 watt power supply and the laser’s output power over the next few days and will report the results once complete. I would have waited to complete the test before posting this update, but I am approaching the 30-days-since-last-reply thread locking policy and I wanted to make sure the lessons I learned from this issue are put out to the community in the hopes it can help someone else with the same problem.


I’m not sure why the 24V supply would have anything to do with the laser output.

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