Power Control of the K40 LPS

This has been moved from a DM between @jkwilborn & @donkjr to the forum for everyone’s benefit. We will continue it here.

Hi, I’ve had a couple of questions about K40s that I thought you could point me to some information on them.

How does the basic power control, ‘knob on console type’ work, hardware wise…

The simple answer is that the pot sets the duty cycle on a TL494 PWM controller which in turn drives an H switch that drives the HV transformer which in turn charges a voltage doubler. The drive to the HVT is sensed and fed back to the PWM control for current control.
In addition, a 600V power source is derived from the AC line using a very complex multiplying circuit.

More info on my blog: Don’s Laser Things: Search results for LPS circuits

Is there really any difference from one of the lps in the ‘China Blue’ that I have and the K40 other than cost or how it’s labeled.?

I would have to see a photo of the two LPS connectors to be sure. Generally, these 40W supplies are the same. The 50-60W I have seen have an upgraded HVT but the PCB circuits are the same. That said, I have seen a few new LPS designs that are not the same as the typical K40. I have also seen 80W configurations with the same circuitry and 2x HVT’s.

If it sets the pwm, how is the controller able to change it for power control?

In my recommended configuration the LPS power is controlled by PWM from two places;

  • the controller connected to L with a PWM signal
  • the pot connected to IN controls the internal PWM signal

The resulting PWM for the system is the product of the two PWM values.

Does this function the same if they replace the K40 controller with a different one.?
I would assume they both have some kind of control via the PWM from the controller.

To my knowledge, the stock K40 controller only dithers and does not control power from the L pin. It turns the beam on and off with the L signal while the power level is controlled with the pot.
I am not sure how this changes if the stock controller is driven by the newer versions of compatible software.

More on my blog: Don's Laser Things: Engraving and PWM Control

I’ll have to look through your stuff and see if I can find a simple schematic… I’ve looked at the tl494 controller, so I’m somewhat aware of its functionality.

My schematic is posted on this forum but I did not create a simple or summary schematic. I can create one if you think it would help.

Aware of the hv lps and it’s basic operation, but haven’t really examined the current feedback area.

The current feedback mechanism has always been somewhat of a mystery to me. The current feedback seems open loop to me in that it is not actually measuring the current through the tube. Rather it indirectly measures the current in the output stage. When the tube (a negative resistance device) discharges the current waveform is a complex avalanche of current. I am not clear on how a closed-loop approach would be implemented or perhaps even necessary. I ended up figuring that since it works it must be sufficient.

Did you have a specific purpose for asking these questions? Just interested or are you wanting to use this information for making changes etc. Knowing the purpose would help me better answer your questions.


Thanks for moving the thread… Sorry I’ve been out for a while.

I’m just trying to get the relationship between how the current is controlled. I’ve watched my hv meter and have a slight handle on what it must be doing while I’m running certain types of jobs, more concisely, how I’m loading the tube. Having both meters let me relate, somewhat.

Right now I’m trying to add on a 600kOhm on the end of my hv meter resistor to tap about 1V to 1kv scale on the scope or in the ballpark… I’d like to see what its really doing. Would also give me a dependable number for a response time of the lps.

Thanks, take care

FYI: a generic discharge profile:

1 Like

Have you seen this with any kind of time reference?

I’m assuming the ‘townsend regime’ is were you don’t want to operate for any extended period…?

How much time is it from start to finish?


I do not have a time base for this in the K40 or for Co2 lasers in general.
I spent a lot of time when I did the initial work on K40 LPS to try and see this waveform.

My intention was to measure the response time of the marking engine to try and better understand engraving. This was promoted by the models I had built which suggested that conventional Co2 lasers do not have a response time that would result in good resolution. Note: the model did reveal better PWM values which are now used in most K40 setups.

I did not complete this quest for the following reasons:

  1. I did not like working on the HV end of this thing. I had multiple close calls with arcs and decided it was not worth being electrocuted and/or destroying equipment (scope).
  2. To get a meaningful response measurement you need to look at the optical output. I did not find a sensor that met the response time, power, and $$$ goals.
  3. Needed to sync the scope down to a single pixel and just did not get motivated to set this up since the K40 was obviously working … and # 1 & #2 were not resolved.
  4. Sarbarmultimedia did some work looking at the output with a scope and HV probe that showed the waveform and that satiated my curiosity.
  5. I decided to use printed patterns to measure the printed effect of response time on the image. Don's Laser Things: Measuring Laser Marking Quality

@43.51 of this video: RDWorks Learning Lab 97 Understand Your HV Power Supply via Florida - YouTube


I think I’ve schemed up a way to hook the scope to the hv tap… Hopefully I’ll get to it tomorrow.

If I get anything useful, I’ll post a screenshot of it, I think the scope can do that.

Wondering about number 2 or the response of the optical beam. Did you find any kind of sensor?

It might be worth frying a photo transistor to test this… cost wise…

Watched his supply video a while back, thanks for posting it.


1 Like

I did not find a sensor. Those I looked at in the right frequency range were slow and expensive.

1 Like

I have ‘broken’ the line going to the uA meter and inserted a 640K resistor.

Figured it would give me about a 1V to 1kV. I didn’t think I could watch it drain off… Kind of kills this idea… at least with a scope.

The pwm output is purple and the yellow is the voltage.

You can see where the line starts to go up that 1mS pwm pulse from the console. It’'s a single 1mS pulse at a low percentage.

Doesn’t really tell me much except I need to get at the power end :frowning:

Might waste a photo transistor and see what happens… never ‘lased’ a semiconductor.


1 Like

This was the test setup I built just before I abandoned the testing: