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.