Switch Select PWM or Potentiometer power control on K40

Added by Techrat about 11 years ago

I have now had my K40 running with the LaOS board for a short while and am wanting to get the PWM set-up but was thinking that it will be ideal to be able to switch between manual (Potentiometer) control and PWM control. I have put together a schematic (See below) to accomplish this goal that I will appreciate feedback on. This is my first time using KiCad/eeschema so don't judge the schematic too harshly on formatting ;)

The one issue that I am a little concerned about is that when the switch is between positions, I believe that the laser will go to full power. Since I don't really envision switching back and forth while running the laser, I am not too worried about it. Anyway, please let me know of any major errors and if I happen to test it out prior to hearing back from anyone, I will post my results.


Replies (2)

RE: Switch Select PWM or Potentiometer power control on K40 - Added by KalleP about 11 years ago

Very smart diagram.

Ground traditionally towards the bottom of schematic but otherwise good.

I would suggest you do not switch the ground signal at all, a floating ground could cause you more problems that switching time.

Only the control in pin need be switched.

To minimise glitches it might be practical to place a small capacitor (0.1uF at a guess) to ground that is appropriate for the source impedance (in the region of the pull up or POT resistances) and the PWM frequency on the laser side of the switch. This will maintain the power level at the one input value while it switches to the other. Also you could have a resistor (pull-up/down as appropriate) on the laser side of the switch to bias the input to the low power side (either GND or 5V depending on laser PSU) about 10 times larger than the source impedance, this would cause the laser to see low power if both inputs become disconnected or temporarily interrupted during switch-over.

What I thought of doing was to use the pot on the dash to scale the LAOS output so I could manually select any fraction of the LAOS output. With an option of a switch to select either the live-end or the wiper of the POT if you wanted to switch between LAOS or LAOSxPOT setting.


RE: Switch Select PWM or Potentiometer power control on K40 - Added by Techrat about 11 years ago

Thanks for the feedback and alternative ideas!

I had considered only switching the control pin but upon review of the schematic I had thought that if I do not switch ground as well, I will be setting up a voltage divider on the source side of the PWM at the LaOS board with some current flowing through the POT to ground. Perhaps this will be negligible depending on the impedence of the POT but I did not have the pot available to evaluate when working on the schematic.

The Pull-Up resistor (on my power supply) at the laser side of the switch is a great idea and I definitely will implement it. I will look at adding the Capacitor(s) as well.

Thanks again for the feedback and I definitely will report back on the finished product. If anyone has any additional input, I will welcome it.