Safe Maximum PWM with CO2 Lasers

I was using my K40 with its stock controller and K40 Whisperer to cut solid vector patterns into wood and getting great results. Since switching to a Mini Gerbil board and Lightburn the quality deteriorated a lot. I discovered this was a result of the default PWM rate, which is optimised for grayscale.

Every time I increased $28 I got better results until at $28=14 I get no output. Nothing beyond 5 or 6 was documented in anything I found, but I’ve since seen that the max value (15) is 80KHz, with 13 being 45KHz.

Some forum posts suggest that 20KHz is the max rate you should run a CO2 tube at, although the Trotec site suggests 60KHz.

Can any offer some insight to this? I’m worried about damaging the tube and would be interested to know more.

Many thanks, Tom

If you have a glass tube laser, you should just use the analog output of a controller for the power curve.

Even if you had software or a controller that supported PPI (that’s fast switching of the laser on and off while cutting aka Pulses Per Inch), that is a hard On/Off 5v signaling to the PSU.

This frequency setting you are referring to is something different and is designed not for glass tubes. It is for RF excited lasers (RF lasers). You should have it set to 20kHz and leave it. Otherwise, all you’re going to accomplish is disruption to the laser PSUs designed ability to effectively convert that PWM signal into an analog 0-5v signal.

If you’re seeing a difference from playing with that number on a glass tube, it is because your laser PSU is failing to accurately convert the non ideal or expected input.

Ruida controllers, for example, have and suggest the use of the analog output for this reason.

Many thanks Jeff.

With my Mini Gerbil board you disconnect the 0-5v pot that was on the stock control board and use the 5v PWM output to control the power level on the PSU. The documentation for GRBL & Mini Gerbil suggest that varying the PWM rate with $28 affects the grayscale curve, with the default 1KHz value allowing full grayscale but higher rates allowing less dynamic range but greater accuracy, which appears to be the case.

I’m very new to all this though so may have have misinterpreted this, but it seemed that at higher PWM rates it was giving me results that were comparable to the equivalent power level set on the pot that it ships with. It seemed to stand to reason that if it was moving quickly and strobing slowly then it would miss fine detail, which is what I was experiencing.

For a glass tube powered by a normal 'ole power supply, the reality is that your Laser PSU has a G (ground). It also has the 5V input that is essentially the power on signal to the LPSU. You have an IN that accepts either a “digital” PWM or an Analog signal that is used to trim the aforementioned 5V signal from 0 to 5v (which in turn regulates the LPSU’s current to the laser tube).

Thanks again Jeff.

I’ll take it up with the guys at who make and sell the Mini Gerbil board, because as things stand I’ve spent hours tweaking settings and getting nowhere near the quality I was getting with the stock board, which is frustrating to say the least.

I suspect that the precise, low-powered engraving we’re after for this project just isn’t as well suited to PWM control. I could get pin-sharp results with the same artwork on the stock board but the Mini Gerbil seems to need to run slowly or at higher power to get comparable resolution, which then goes too far through the material.

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