steves wrote:Tim
Thanks for all the info you share here.
I understand the basic concept of PWM, but have a couple questions for you or Bart about the actual in service implementation.
You mention that your laser is at max power at a current of 18mA - over what time scale is the max power a limit? Since PWM is either on at 5v or off, I'd expect you to be getting 25mA when it's on, and 0 when it's off. When you set it to 60% you'll get an average of 15mA, but peaks of 25ma.
Now, obviously, the current doesn't build instantaneously, and if the PWM cycling frequency is high enough it may not reach 25mA at all. But it seems to me if you could current limit your power supply to 18mA you wouldn't have to be concerned about overdriving your tube at any power setting or for any portion of the PWM cycle.
So the questions are:
1) What sort of time scale is the power limit applicable to? Is it OK to have the 'on' portion above max power as long as the overall average stays below the limit? Will the tube last longer if we limit the max current so that a 100% duty cycle equals the power rating of the tube?
Thanks,
Steve
twehr wrote:In other words, the PWM logic in the ps is converting the PWM signal into a corresponding analog signal. So the part of the ps that sends power to the tube is not really seeing 5v and 0v, it is seeing 2v and acting accordingly. (Again, may not be accurate description of reality but certainly is functionally the way it works.)
BenJackson wrote:It's highly unlikely that the PSU is interpreting the PWM logically. Doubly so because the input is an analog input. What's really going on is that the PWM is driving some kind of simple analog filter (probably a plain RC) which removes the high frequency component of the PWM and leaves the DC.
steves wrote:Tim
I guess I didn't word my question accurately enough - my point is that when you call for 60% power through PWM you're calling for 100% power 60% of the time. If 100% power is too much for your laser, is that going to be a bad thing? I'd guess that if the PWM frequency is high enough and the tube's sensitivity to being overpowered is on the timescale of minutes, using 60% to get to your max power is fine. If a tube could be blown in milliseconds at 100% power, then 100% for 60% of the time is going to be a bad thing if you have a low PWM frequency. As Ben pointed out, your PWM freq might be as low as 400HZ.
That's why I was thinking an optimal solution would be to current limit the PSU's output to 100% power at a 100% duty cycle.
So I guess I'm really asking a question about the actual in practice behavior of the tubes versus a simple power calculation.
Can't touch the theory/practice subject without a favorite quote:
"The theorist invents his companions, as a naive Romeo imagined his ideal Juliet. The experimenter's lovers sweat, complain, and fart.” James Gleick
Steve
r691175002 wrote:The point of PWM and the number is that you are supposed to have discrete flashes from the laser. This is more efficient, repeatable, and control is more linear. Also pulsing reduces charring on some materials. This is because you get the advantage of high power instantly vaporizing the material, but since the laser turns off there is no excess radiant heat to brown the surrounding material.
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