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Saturday, May 26, 2018

K40 Laser Power System Troubleshooting Guide

So Your Laser Won't Fire?

The laser power and control system in a K40 is a relatively complex and its the most unreliable subsystem of the overall K40. Therefore a lot of time is spent diagnosing and subsequently replacing and repairing parts of this subsystem.
It seems important that we capture what we have learned about troubleshooting and fixing K40's. 


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Is it your laser power or the laser tube that is bad?

Good question! We don't have a definitive way of pointing to a bad tube but we can insure that the control system that drives it is working properly. Well almost. We can verify operation all the way to the input of the LPS but beyond that the HV characteristics of the laser tubes drive makes clear component identification dangerous, uncertain and expensive.
This guide helps eliminate everything up to and including the input of the LPS as the culprit. If everything is good to the LPS then I would:
  1. Replace the LPS 
    1. If you have certain symptoms you can try replacing the LPS's HVT. This is the most common type of LPS failure
      1. Repairing-k40-lps
  2. Replace the laser tube

The K40 Laser Power System Troubleshooting Guide

This guide provides K40 owners a flow chart for troubleshooting the Laser Power Control and Supply subsystem.

The Guide To Troubleshooting

The Elements of a K40 Laser Power Control Subsystem

  • Input power
    • AC plug and fuse
  • Control panel
    • Current control pot or digital controls
    • Laser enable sw
    • Laser fire Test button
    • AC power button
  • LPS
    • The high voltage supply
    • The laser enable loop
    • The local test PB
    • PWM input (L)
    • Local power control (IN)
  • PWM generator
    • The controller that converts gcode S commands to a PWM signal.
  • Laser
    • The laser tube that generates the optical power

Hints of Tube & LPS Failures

If after you check your laser power system with the troubleshooting guide and all is OK here are some additional hints:

Your tube might be bad if:

  • It does not light up at all
  • There is an arc from the anode to the frame. When the tube is bad it provides no load to the LPS. An unloaded (bad tube) LPS will often arch violently to the surrounding case
  • It has cracks
  • There is no current on the meter when the test switch is activated.

Your LPS might be bad if:

  • There is arching inside the supply
  • The mili-ammeter reads erratically at currents above 4ma
  • The current is lower at higher power settings than at lower power settings


If you find any kind of error or lack of clarity please make a suggested change in the comments.

Enjoy and comment,


  1. Hiya, my whole laser system dies including the fan after a two seconds of holding the test button down, then it turns it'sself back on and zero's.
    i'm guessing it is a power supply problem. any thoughts?
    many thanks - Richard

  2. Make sure the power supply rating (wattage) is matched to the laser tube you're using. Sounds like something is drawing too much current etc. Although from a different cause, I used to get a similar end result with an old 1980's synthesizer because the power supply's filtering capacitors couldn't buffer fluctuations in line voltage, which caused them to make their way to the control chip and cause it to freak out and power cycle; so that's food for thought. In line with that, I'd check for burnt capacitors on your power supply just in case they got fried.

    Also make sure your power supply input voltage matches your country. The sticker on my laser chassis says 220V however it's rated for 120V. Very important to get that right too. Undervolting may not damage it, though if you exceed the proper input voltage you'd definitely fry the rectifier. I'm not an electrician or engineer so take what I said with a grain of salt. Good luck.
    (I'm surprised no one else answered this yet).