This has been discussed Ad-Nauseum please bear with me.
I have my Omtech set at 75% Max Power on the Ruida. I seldom ever go above 65% power in Lightburn.
Am I correct in thinking that my max power at the laser is actually 48.75%.
What is confusing to me is at 65% power in LightBurn I read 18mA. According to OmTech the 60W/50W should not be run above 20mA. Has my tube deteriorated that much? I don’t even run it every day.
In my opinion it is not correct.
If you have not down-calibrated your power supply for your tube, then there is no limitation in the power that comes from LightBurn, even if you have limited it in your Ruida controller. There is no max limit in the controller, but it would be nice if there was, for those without an accessible pot in the power supply.
Thank you Bern,
I see on the Omtech site they are giving 15mA to 18mA . Told me 20mA 2 years ago. lol
Merry Christmas to you and your family!!!
Btw, 55% corresponds to approx. 16 mA on my 60Watt tube, and that is my max. I have done tests and it clearly showed that 16+% has no additional effect in the depth of cut.
Thank you for your Christmas greetings, the same to you and your loved ones.
Yep,
My reading is 55% to 16mA
I bought some new material that is 3.5mm in thickness. Screwing up a lot of settings I had on items.
Maple veneer both sides with MDF core. I have a lot of skipping in the cut on bottom side with a 4" lens if I do not go to 65% power and the focal is at 16.1mm. That sets the beam in the exact center of my cut depth according to Ramp test.
If your machine is sold as 60 Watt. machine, then we both probably have the same machine.
If you are reading an analog mA meter and you wish to know the to know the real on value of what you are reading…
At 55% pwm you are only reading current for 55% of the time… So if you want to know the actual current it’s roughly 100%/55% * 16mA or 1.818 * 16. That make the current during the on period 1.818 * 16(mA) = 29.08 mA when the tube lases…
Do you mean that with this calculation, which I do not dispute, that "normal 30mA ammeters cannot be used as a directional instrument because they do not reflect the values that are in the lists from the manufacturers?
It’s simpler than that, because Ruida controllers use 20 kHz PWM that’s filtered into a nice DC-ish tube current. The meter is reading the DC-ish current, so dividing that by the PWM setting gives you the power supply’s maximum current setting.
The current through my 60 W OMTech looks like this:
The 20 kHz PWM signal (cyan) is at 50% and the tube current (green) is a constant 15-ish mA (there’s a baseline offset) while the L-ON signal (magenta) is low.
The digital meter on the side of the power supply, which is pretty much the same as your analog meter, also reads 15-ish mA.
The power supply’s internal trimpot arrived set for 25-ish mA, which is hot for a tube allegedly intended for “maximum” currents around 15 mA, but it’s the official OMTech replacement.
A plot of current vs. PWM shows it’s reasonably linear for larger PWM:
All those currents are the DC-ish equivalent as seen on the meter:
https://softsolder.com/2022/08/03/omtech-60-w-laser-replacement-hv-power-supply/
Do you mean for ‘adjustments’?
All I’m saying is that with a changing voltage/current that the mA meter will be constrained by physics…
If you measure a 50% ttl pwm signal you will read about 2.5V and we know when it goes high it’s 5V, so it works like any other meter would work.
If it’s being energized for 1/2 the time, it will read 1/2 of the energizing potential… that’s the way it works…
When your tube is ‘tested’ they set the lps to a fixed value of your tubes maximum current … They then test it at different pwm values with the fixed current.
This is why you really need to set the limit within the lps… The tube will draw whatever current the supply will produce…
IMHO, I think these ‘tubes’ are tougher than most realize… Most of these lasers are running the tube with way too much current… yet they don’t ‘die’ overnight…
No Jack, it’s probably a translation misunderstanding on my part. I will try to explain myself differently. My understanding of the necessity and use of a 30mA ammeter is that the manufacturer prescribes a maximum mA value for a certain tube and I try to stay below this value. The wisest thing in this context is, as you and others also write, is to reduce the output of the power supply (2.5V=50%…). However, in some cases it is not possible or people do not dare to put a screwdriver in a high-voltage blind hole 
I use my ammeter that I put on the machine on the first day to check that my LB setting of about 55% matches 16mA. It is this 16 mA I am referring to.
So my question is, are the mA readings from my external ammeter useful for this purpose or not.
When I set the power output in LightBurn to, for example, 5mA, does the tube still draw max current because psu is not reduced? I’m just trying to take some confusion out of this topic for those of us who are not high voltage technicians
No.
The power supply filters the PWM and sets the tube current to the corresponding DC-ish value as a fraction of the maximum possible current set by the internal trimpot.
The trimpot in the HV power supply in my laser is set to 25 mA (more or less). The supply demodulates the 50% PWM input to drive the current at a constant 15 mA (more or less); it should be closer to 12 mA, but the current probe I use has a dramatic baseline shift.
That’s what you see in the oscilloscope screen above.
Setting a PWM value of 100% will produce a constant tube current of 25 mA, which you probably don’t want to do very often.
Setting a PWM value of 10% should produce a constant tube current of 2.5 mA, but the actual current waveform will be an ugly mess that generally produces that value as an average. The actual peak currents have nothing to do with the trimpot settings, seem to be around 100 mA, and are “just the way it works”.
Practically, PWM values between 25% and 99% produce reasonable current waveforms. How hard you want to drive the tube will determine the upper limit and how the tube behaves at low power will set the lower lImit.
The mA meter is the only ‘tangible’ evidence we have to referencing the power we’re using. So yes it’s a great tool, but most of these lasers don’t even come with one. Many people use these without a mA meter…
All I’m advising is that you understand what it is you’re reading and that entails how the meter itself works…
Although an led laser doesn’t operate like a dc excited laser you can point out similarities.
The pwm on an led laser turns on when it goes high and off when it goes low. So at 50% pwm the laser is on 100% for 50% of the period (frequency of the pwm). At 1kHz pwm it would lase 100% for .05mS (period is 1mS).
Lots of ‘things’ happen to a tube laser before it’s actually lasing, one of the arguments about running at low power levels causing damage.
Since the people who manufacture and test these tube do so by setting the lps to the maximum allowable current for the tube and make their measurements from there, indicated to me I need to control the lps current itself.
When I was setting up mine, Russ Sadler advised me to set it for 50% pwm and set the lps to 1/2 the maximum current. This worked out well and the percentages in lightburn line up properly …
I have read people here that purchase a 60W tube and the manufacturer recommends a maximum current of 10 or 12 mA… I remember reading in my documents that my 50W had a limit of 21mA, so I wonder…
I found this on one of the laser sites… my tube is 880mm in length, it has a measured output of 44W with a Mahoney meter.
Thanks to you both, it’s nice to get things sorted out.
It is a little new to me that there is also a danger in too low operation of co2 tubes, but it sounds reasonably logical.
There are (unfortunately) many lists with different recommendations of max power for co2 tubes. It also confused me at first, so I did a “run-in test”. It showed that after 18 mA there is no further effect, in fact it goes back a little at 19 mA and then keeps the curve straight. - That’s fine with me, 16 mA is my safety limit.
Btw, my original manual that came with the machine from OMT says 30mA at 100% but recommends using no more than 95%. It will have killed my tube in zero times without getting anything for the extra 14 mA. 
The “60 W” tube in mine has nary a hint of a recommended / maximum / tested current anywhere on it. OMTech installs a power supply with a 25+ mA maximum current, suggests ~15 mA typical current, and recommends not exceeding 70% PWM = ~17 mA very often for longest tube life.
Because the thing is barely 1 m long, it’s certainly a 50 W tube overdriven to 60 W. Perhaps their recommendations make sense.
Mine came with a ‘tube test’ sheet. It had 21mA on it… that’s why I chose that value… When I hear them saying much lower, I wonder… 
I’ll see how long this lasts running it this way. Next one I buy will probably be one of these…
It would be nice to have a laser pointer… 
You are not the only one that has stated similar experiences with these tubes.
If you go anywhere and examine a tube, the higher the power the longer the physical length. The physics, from my meager understanding, is how long the tube length determines where the ‘amplification’ of the beam is actually obtained.
Many people have stated that over a certain mA, there is little to no gain and sometimes there is a loss in output …
If only the current mattered, you could run a 40W at 80W, maybe for 1/2 the time or less … ?
A mA meter is about the only thing we can measure…