Laser triggering, but no power?

Sorry if this has been asked a million times before, can’t see it from my research!

I’ve just bought 2x K40 cutters second hand. One with the digital control panel had a broken/cracked laser tube, the other fully functioning (according to seller) but with an older case and analog pot control.

So I took the “good” laser unit from the older machine and placed it in the newer, digital control, machine. Laser fires fine, purple plasma, but there does not appear to be a beam exiting the laser. With laser goggles on I tried holding some brown tape between the laser and first mirror, no scorching or cutting. Tried on 30% and 80% power. Is there a warm up period? Any help appreciated.

Welcome to Maker Forums!

Analog controls are better than digital controls. The ammeter on the analog unit actually measures current; the digital display is of “PWM duty cycle” which controls current, but in a way that changes as the tube ages. Our first recommendation to everyone with a “digital” unit is to add an analog ammeter. “100%” on a digital display is too much for a K40 tube.

No, there is no warm up period. The LPS (Laser Power Supply) might have been damaged from running it with the cracked tube, .

Have you seen the “K40 Intro” at the top, which links into the whole #k40:intro category? It’s full of useful information, including about why you want an analog ammeter! :relaxed:

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Ah, that makes sense, I will add a analogue Ammeter to ensure current is known. Will take a read through the intro docs.

I’ve had a look over the system and the LPS, using a fault finding guide, all looks ok. The fact the the laser is firing but has no power, makes me think that the laser needs replacing? Although would have thought there would be enough power, even in a used laser, to burn or mark paper? Just seems really strange it is firing and can see the plasma and the plasma intensity changes with an increase in PWM, so that seems to be working.

A ma meter is a good idea even for normal operations because only knowing the programmed power level [the digital panel] does not tell you what the actual load on the tube is [the ma meter].

Those symptoms do seem a bit unusual for a totally dead laser.

Can you see the end of the tube [power off the machine of course] and is there any damage etc to the exit lens? Is the lens cracked or delaminated.

What coolant and temp are you running?

Do you hear any crackling or hissing noises when it fires.

Post a video of the tube firing.

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As far as I understand it, the glow discharge excites the nitrogen molecules which bump into the CO2 molecules and these excited CO2 molecules then emit the photons.

If there aren’t enough CO2 molecules left, you won’t get any significant output. You still get the light show (the glow discharge), but nothing happens.

So, it appears that the seals on that tube are still fine, but the gas mix is spent. Since it’s a sealed DC/glass tube, the gas can’t be refilled. The tube needs to be replaced.

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The end of the tube looks ok, see pic in link below, along with video (as new user, couldn’t upload)

I swapped the laser tube back into the machine with the ammeter and the same problem occured. The pot also seems dodgy, so mA jumped to 20mA during test. Doesn’t seem to be any noises when firing. Thanks for any help.

Any idea how old that tube is?
During the test what did the ma read, that is other than the times it jumped to 20ma?

Is there something plugging the water jacket?

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Not sure how old the tube is, was 2nd hand, so suspect it could be in need of replacing and the seller hasn’t been 100% honest. The water was flowing ok, that’s just some mould on the inside of the tube I think.

Once I got the pot set, it seemed scratchy, it was consistently 20mA. It seemed to be all or nothing.



That explains what I am seeing, think I will try a new tube. Thanks for the response

Yea, the cheap potentiometers they use for these machines aren’t very reliable. They fail pretty often.

There should be a label on the tube. With these no name tubes, a lot of the info is usually missing, but at least the manufacturing date is often on it.

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Thanks for the help, 2019 is the date on it. I’ll order a new one and a pot. Is there any benefit to going to a 50W tube? Appreciated I may need to mod the enclosure. Aim is to cut 5mm acrylic. If the 50W is a must for this, might invest whilst changing the tube.

Tube power is a function primarily of length. A 50W tube needs a case extension. This has 20,000 volts to it, so the extension must be metal and grounded. This can be life or death, so don’t mess around with things like PVC pipe.

More on replacement tube here:

Air assist is often more valuable than more power.


More power of course reduce the # of passes while cutting
You will have to modify the enclosure and get a more powerful LPS.

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Thanks everyone for your help, think i know what to do now.

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New laser tube in and is firing and cutting paper when tested. I’ve also got an ammeter to fit, thanks all for the help.