Power set too high?

New build, everything working.
Then I checked the amps going to the tube.

I have both an analog ammeter and the digital ammeter on the laser power supply (LPS).
My 50W tube is rated for 18ma.
I had the Ruida controller set to 50% power.

Did a pulse and both ammeters showed 20 ma. Hmmm.
Disconnected the LPS from Ruida and fired a test pulse directly from the LPS.
According to the manual, the LPS fires the test at 100% power and the digital ammeter showed 18ma - close enough.

But, I reconnected the Ruida to the LPS and set the power on the controller to 100% and got readings of 25ma - too high.

So, what actually controls the power?

I know you can use the pot access hole on the LPS to adjust the max power.
My max is set ok with the LPS not connected to Ruida. Are these adjustments done with the LPS connected to the controller?

How is the Ruida able to put more amps than the LPS at 100% power into to the tube?


It’s not

All the schematics I’ve seen of the lps show that the test button just does the same thing a ‘high’ on the ‘IN’ does. It should be max power with the test button.

Somethings fishy with your readings or something…

You can check your pwm output… on mine it LPWM1. I have a connector on it for a scope trigger.

You can do it with a voltmeter, at 50% pwm, it should read about 2.5v and 5v at 100% No need for the lps to be energized.

To adjust max current you have to deal with the potentiometer in the lps…

I do all my setups with the Max on the panel set for 50%, I am confident I won’t push anything too far. This also includes most of the power tests. Rarely do I push it to it’s placard 21mA.

Keep in mind there is no real ‘power control’ with a laser. It is lasing or it’s not, there is no 50% lase out of a laser. The illusion is the power/time relationship along with how the human eye/brain work.

The only actual control you have is speed… :crazy_face:

good luck


So Jack,
The question I have is if I tell Lightburn to cut a line at 50% power, it will use 50% of what?
Or modulate what exactly?
Say I have the controller power setting at 10% because I was doing pulses on tape to align my mirrors.

Then I ran the above operation from LB - what power reading do you think I should see on my ammeters?

You said “You can do it with a voltmeter, at 50% pwm, it should read about 2.5v and 5v at 100% No need for the lps to be energized.”
Could you elaborate this for me.
This is with the LPS off and a voltmeter connected between LPWM1 and ground?
How do I set 50% PWM?

Btw, I took those measurements 5 or 6 times, same values as I reported each time.

Thanks John

The easiest mistake to make with an ammeter is putting it across a power supply, instead of breaking the circuit and putting it into the series of components in the circuit. This would be the first thing I would check if the reading was too high.

If the circuit is wired correctly and there’s still too much current it comes back to ohms law.

I=E/R which means the current that you see can be calculated by dividing the voltage by the resistance.

Either the voltage is too high for the expected resistance or the resistance is too low for the given voltage. Rarely both in troubleshooting but not unheard of.

A lower voltage / physically smaller tube could create this problem.

PWM controls the average power through the tube but meter readings over 100% of allowed power show something may be awry.

Any other modifications to the equipment?
Any possible high voltage leaks to ground increasing the current flow?

@JohnJohn correct about the meter needing to be between the cathode and ground. The Anode has the HV applied.

Actually the meter could go ‘in between’ either the supply/tube or ground/tube. The HV side would be rather dangerous for anyone using it… so that isn’t done… hopefully…

Generally speaking it will toggle the pwm output with a 50% duty cycle.

Depends if you are running a job or manually pulsing it.

If you are drawing a vector where the head speed changes, the controller uses the Min/Max power on the layer in association with the ‘Jump-off speed’ of the controller.

Anything at or below the ‘Jump-off speed’ will get low power, it will be ramped up to max power at the set speed.

I originally had ‘start speed’ which is incorrect.

I made a graph from my experience with the Ruida, have no clue how correct it is, but it appears useful.


My tube doesn’t like to fire much below about 9.5%, depending on temperature.

At 10% it will generate a 10% duty cycle of the pwm. I don’t think manual pulse has any relation to the min power.

The console will allow ‘continuous’ or ‘duration’ pulses.

Make sense?


Ohm’s law is not applicable to ‘negative resistance’ devices, like tunnel diodes and co2 lasers.

I have both a hv meter and a current meter. So I watch both of them as much as you would watch a mA meter alone. The red arrow is not pointing at the kV meter :crazy_face:

Sometimes the values shown make me wonder…


First, thanks for the replies.

Second, yes the ammeter is between cathode and ground, and as I said it’s reading the same as the built-in digital ammeter on the LPS.

Third, just to clarify, the connections between controller and LPS:
from Output Plug CN5 Gnd to LPS G, L-Out to TL, and LPWM1 to In.

Third, for Jack, I measured the PWM voltage as follows:
Power 10% = .2
Power 50% = 1.8-2.1
Power 100% = 4+
Voltages varied depending on how long I pulsed. These are rough averages.

Finally, I’m not sure if I made it clear what prompted my initial post.
With the LPS NOT connected to the controller, just 110v line voltage in, and hit the Test button on the LPS I got a reading of 18ma on the built-in LPS ammeter.

Then with the LPS connected to the controller and with a 50% power level set on the controller’s screen I get a pulse reading of 20ma on both analog and digital ammeters.
With 100% power set on the controller a pulse gets 25ma.

The reason for my confusion is that I read somewhere, can’t find it now, that when you fire a pulse with the LPS Test button it uses 100% power. Maybe not so?
So I was seeing 100% = 18ma with the LPS alone and 100% = 25ma using the Ruida.
Thus my questioning what was going on.

Unfortunately the access to the LPS potentiometer is blocked in my current configuration so I have to do some extracting to get at it.
Not exactly sure how to adjust the power but I will start by taking a few turn CCW to reduce the power and start taking readings until 100% power set at the controller produces a reading around 18ma.
Does that sound right?


Ruida to lps look fine.

The console needs to have the set laser to 99.9% (won’t let you set it for 100% I don’t think and the duration on ‘continuous’.

Since there is no laser firing, hold the pulse down until the meter stops moving. It will settle down to an RMS value of the waveform.

100% power, there is no pwm generator inside to control the ‘power’ of the tube. The schematics I’ve seen force ‘L’ low and ‘IN’ high resulting in full power.

I don’t use 100% power on anything except having fun…

If it’s setup properly, you can do most things at lower power levels.

Increased safety for you and the machine.

When it’s all setup, then play around with the higher settings :crazy_face:

All things are pretty much up to debate, but I’ve used this process a few times.

Pretty much the KISS principle.

Assuming I want 100% as say 18mA.

I set my Max on the console to 50%, adjust the ‘pot’ for 9mA with the pulse key held down.

Your tubes output should be pretty stable or ‘reasonably’ linear in the mid range of the output.

Mine is actually set to 11, I can overdrive my tube to 22mA but that’s not what I’d advise… I’ve driven it to 21mA, it’s max, supposedly…

Keep in mind that if you have a materials library and you twist this pot, all of your ‘previous’ data, based on % power is invalidated.

Make sense?

My supply is mounted like this. The hole to the pot is on it’s right side, so it’s facing the bottom of the machine. Have to do it by ‘feel’… I’d suggest you use a non conductive screwdriver…

Good luck


Thanks Jack,

Exactly how my LPS is configured. After alot of blind feel - that screw’s slot is might shallow - I have adjusted my Max Current at 100% (ok 99.9) to 16ma which is the recommended max operating amperage for my tube.

I know I could have just calculated x% power = y amperage but now I don’t have to worry about overdriving my tube.

This LPS was set at the factory to deliver 25ma at full power to satisfy the big tubes.
I ok now with what I’ve got so I need to get going on making some various material cut/engrave specs.

As the Brits say, I’ll consider this “sorted”.


Great, take care and good luck


This is interesting and i’ll kick this around. The reference you provided says:

The negative resistance issue is just that at some point on a transfer curve the dV/dI slope becomes negative.

I think the whole point of High voltage and PWM is to get the device out of that part (or into the sweet-spot) of the transfer curve. It’s intriguing to think that the additional current draw could be caused by the tube acting in the wrong region of the transfer curve.

The question i was attempting to answer is, What controls the power?
I believe the voltage is a given parameter for the tube. I intuit that the laser operates at a low resistance (current maximum) in and around where it functions. Can a laser tube draw more current in a failure mode? I don’t know this part of the physics.

Putting the mA meter in the ground path is wise and i’m glad you mentioned it. I have questions about the kV meter - are you reading the pre PWM supply voltage or the PWM average voltage across the tube?

From what I can make out of the schematic, I believe it’s a current control set by the potentiometer when it comes down to it.

Keep in mind the lps is either off or on (pwm.) My current was set too high for my tube.

A negative resistance device just means that a change in voltage doesn’t constitute a relative change in current. As you raise the voltage the current may drop… Doesn’t mean that it actually is a negative resistance…

There are actually a group of stages the tube goes through to lase… Unfortunately I’m not a physicist either.

It’s just a dropping resistor from the anode, the very simple meter circuit. The connectors are setup so I can just ‘remove’ it out of the circuit and reconnect the HV directly.

Resistor stack in machine… Ignore the circled current meter…

Good luck


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I gave you the wrong item to change, please check out the correction.

The Jump-off speed is what I should have stated.

Good luck…

Sorry about that…


I have adjusted 4 different power supply units, including 2 like yours. I always pull the supply out of the machine to adjust it. The adjusting screw is very small and has no " feel " when you adjust it. When I pull the power supply I mark the slot alignment as a reference and then turn the screw. Most I have adjusted take between 3 to 4 full counter clock wise turns to adjust properly. I always adjust my power supplies Ma to 100% of the tube rating.

Kind of a tough way to do it. But whatever works for you.

I put an insulated (usually ceramic) screwdriver and ‘feel’ the slot. Make the adjustment and the test it.

Mine was also a couple three ‘turns’ too high. Most of these appear to be twenty turn pots.


This is the day that I’m glad my dad had all the tools to fix old television sets including the tuning screwdrivers for the high voltage side. I think they’re Nylon or fiberglass.

Thanks for the heads-up on the 20 turn pots. I occasionally miss that fine point.

The good part is the it ‘turned’ the correct direction to rise or lower the power… wasn’t right to left…

You would have figured it out pretty quick… :crazy_face:


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