My assumption (and I know what that word means ) has been that a commercial laser will arrive from the seller with its power supply twiddlpot preset to the maximum current for the laser tube in that machine. Similarly, a “60 W” replacement supply will arrive preset for more-or-less the correct current for a “60 W” tube from that seller. In either case, there’s no consumer interaction required or expected.
Which seems true for the three HV supplies I have. They all produce about the same 25-ish mA max current, but each has a different twiddlpot setting: the factory techs definitely adjust the pot to produce a specific current. We all know 25-ish mA is how OMTech (and others) wring an alleged 60 W out of a 1 m tube: having a twiddlepot doesn’t preclude overdriving the tube!
Conversely, anyone inclined to tinker inside / “improve” a laser or build one from scratch must do a whole bunch of system-level integration / configuration / testing far beyond the consumer level, so it doesn’t seem unreasonable to expect those folks to perform what’s ordinarily a factory adjustment. Setting the current does not require opening the power supply, doesn’t expose you to high voltages, and uses only common tools. Admittedly, it might require adding a DC milliammeter to the HV circuit, but the OEM supply in my laser included a digital meter and tinkers generally add a meter as one of their first mods.
So it seems reasonable to think the twiddlepot is intended to (and actually does) set the maximum recommended / allowed / peak tube current, with the PWM / analog value scaling the operating current between 0% and 100% of that current. The usual caveat about improving the tube’s lifetime by rarely exceeding 70% of its maximum current translates into a sensible, consumer-friendly meaning of rarely exceeding a 70% “power” level. OEMs setting the twiddlepot to an “optimistic” current just take advantage of the situation.
Choosing a different twiddlepot setting can certainly make the supply harder to use, but what’s the point in that?
Aye!
One would hope that’s the difference between the top-dollar supplies found in high-end commercial machines and the bottom-dollar cost-reduced supplies we have. It would be interesting to measure what goes on in those machines, but my toy budget won’t stretch that far.
Ok, so I have done a bunch of testing with my cheap chinese laser and I found that when I pull out my focus 2-3mm, the dots go away. I created a series of .1 and .3mm wide vertical lines 12 wide 1mm apart. While the laser was running, I slowly defocused until the dots went away. I found that I was about 2-3mm away from my sharpest focus but the dots did disappear. Someone else try this and let me know if it works for you!
No surprise: spreading the same beam energy over a larger area reduces the energy density below the level required to damage the acrylic. The random tube firings still happen, but they no longer produce visible zits.
However, a defocused beam also reduces the beam’s ability to create the damage you want in the filled areas, by requiring either more power or slower speeds, in addition to blurring the edges & fine details.
Defocusing is a useful tradeoff, but it is a tradeoff.
Are these pwm power supplies. Could changing the pulse frequency of the supply alter the spike frequency?
I’m thinking that the frequency modulation of the supply, the laser control modulation frequency are causing interference patterns.
A smoothing capacitor on the power supply output may help.
The PWM input is filtered / demodulated to a DC level, so the carrier frequency is irrelevant as long as it’s above a few kilohertz, as shown by some tests I ran a while ago:
Nope, it’s completely random in both amplitude & time, as shown by those scope traces.
The HV output must switch on and off within a millisecond or so and is already pretty much at that limit, which means further “smoothing” isn’t desirable.
Because the output runs above 20 kV, the cheap and readily available ceramic caps we all use won’t work.
) has been that a commercial laser will arrive from the seller with its power supply twiddlpot preset to the maximum current for the laser tube in that machine. Similarly, a “60 W” replacement supply will arrive preset for more-or-less the correct current for a “60 W” tube from that seller. In either case, there’s no consumer interaction required or expected.