For example, this would generate five square waves:
The bars are 10 pixels wide, so scaling the image at 254 dpi makes them 1 mm wide:
As before, the first and last bars are 100% (white), with 0% (black) bars just inboard. The other bars are 10% and 90% to stay a little bit away from the 0 V and 5 V limits. I set Lightburn to invert the colors so that 100% = full power and 0% = beam off.
Engraving the pattern at 100 mm/s makes each bar 10 ms wide and the risetimes and falltimes are easy to see:
[Edit: Clicked the wrong picture.]
Although it’s a bit handwavy, a 1.5-ish ms risetime suggests a single pole (ordinary RC) time constant τ = 700 µs = 1.5 ms/2.2, so the controller’s output filter cutoff would be around 200 Hz = 1/(2π τ).
The laser tube current looks a little slower than that, so there’s a definite tradeoff among engraving speed, edge crispness, and power level.
The PWM signal does not appear in that scope shot, because it runs at 20 kHz and is a blur at 20 ms/div.
It’s worth noting that the tube current has large startup spikes at low power levels in both PWM and analog control, so the spikes are generated internal to the power supply and have nothing to do with the PWM input signal.
Another test pattern using constant power:
At 10% power the analog output is about 0.5 V:
At 50% power the analog output is a constant 2.5 V and the tube current settles at a constant 12-ish mA, about half of the power supply’s maximum 25 mA:
Obviously, controlling the laser power to intermediate values using an analog signal does not involve switching the current between the supply’s minimum and maximum values: there are no PWM pulses involved to do the switching.
I suspect the analog output comes from the PWM signal run through an internal low-pass filter similar to the one in the power supply. Based on the PWM frequency measurements and squinting at the rise / fall times, the analog filter cutoff is probably around 1 kHz.
Other than bragging rights, I don’t see much advantage to using the analog signal in place of PWM.
Laser cutter controllers generally set the tube current (and, thus, beam power) through a digital PWM signal to the HV power supply. Confusingly, the same power supply input terminal can receive an analog signal controlling the output current. Both signals have the same 0 to 5 V range.
I set the KT332N controller for a 200 ms pulse when poking the front-panel button, which is long enough to show any interesting behavior, and changed the PWM using its awkward controller interface. LightBurn provides access to the “vendor settings” which include the PWM frequency, which I set as needed:
So, we begin by varying the PWM frequency with a constant 50% PWM …
The default 20 kHz:
The upper half of the scope screen shows the entire 200 ms pulse, with the small slice near the middle appearing zoomed across the bottom half. The readout just above the buttons along the bottom gives the measured PWM percentage and frequency. The green trace shows the tube current is about 12 mA, half of the power supply’s maximum 25-ish mA.
The Tek current amplifier has plenty of thermal drift that I have not attempted to compensate, so always eyeball the average current with respect to the baseline around the pulse in the upper half of the screen.
No trace of the 20 kHz PWM signal appears in the tube current, which runs at a constant 12-ish mA for the duration of the 200 ms pulse.
Increasing the PWM frequency to 100 kHz (!) produces no change, although I cranked up the zoom timebase to better show the PWM pulses:
Reducing the PWM frequency to 10 kHz produces very small ripples in the output current corresponding to the PWM cycle:
At 5 kHz the tube current becomes sinusoidal, with an average around the same 12 mA produced at higher frequencies:
The sine wave current is about 90° out of phase with the square wave PWM, although much of that must come from delay through the entire power supply, rather than just an RC low-pass filter.
At 2 kHz the tube current takes on a decidedly lumpy look:
At 1 kHz there’s definitely something odd, perhaps a resonance, going on inside the supply, although the average current remains 12 mA:
At 500 Hz the PWM is slow enough that the tube current resembles the output of an integrator, rather than a filter:
At 100 Hz, the digital PWM signal is so far below the filter cutoff that it’s behaving as an analog input, with the tube current ramping between minimum and maximum:
The current has regular full-on glitches halfway through the “off” part of the PWM signal, so running at absurdly low PWM frequencies does not prevent them. Also note that the PWM signal does not control the current at the same speed as the L-ON enable signal, due to the low-pass filter rolling off the transitions.
Now, holding the PWM frequency constant at (the absurdly low) 100 Hz and varying the % PWM duty cycle …
At 30% PWM, the output current becomes triangular due to the low-pass filter:
At 99% PWM, the output stays at the power supply’s 24 mA maximum output, with small downward ramps marking the 1% off times:
Some observations for this HV power supply, which seems typical of similar supplies sporting other “brand names”:
A PWM frequency below 10 kHz introduces output current variations due to the power supply interpreting the PWM waveform as a somewhat analog input, rather than a purely digital signal. This effect increases as the frequency decreases.
An Arduino-speed digital PWM near 1 kHz will be interpreted as an analog signal, with the tube current varying significantly around the PWM signal’s average analog value. It does not control the current in an on-off digital manner.
Due to the effect of the low-pass filter, the PWM signal cannot switch the tube current between “full off” and “full on” at any frequency. The current will always follow a ramp with a slope controlled by the filter rolloff, so low PWM inputs will have low peak currents.
The bucket contained all the water to start with, so with the icemaker and laser tube empty, all the water is back in the bucket. Getting all the bubbles out of the laser tube takes a while after the pump starts running, so I stuck a check valve on the laser output tube in the icemaker’s reservoir:
Which, after a few days, developed a slow leak, once again emptying the reservoir.
There being no way to dismantle the valve for analysis and cleaning, I just cut it apart:
Lo and behold, a small tangle of thin fibers had found its way into the valve:
Which held the silicone disk ajar and let the water slowly leak backwards through the valve.
I have no idea where it might have come from, but a simple filter seems like a good idea. Given that the pump produces pretty nearly zero pressure, anything fancier than a coffee filter in a funnel would present too much back pressure.
Or, with three more valves in the bag, I can wait to see how long it takes for another tangle to arrive …
We have accumulated enough measuring spoons (typically from garage sales) to dedicate them for specific purposes, which means keeping them from wandering away:
The design is simple enough:
The slot is a rounded rectangle about 2 mm larger than the spoon handle in both directions, inside a rounded rectangle large enough to put the handle just clear of the jar. The curved side comes from outsetting the jar lid OD by a millimeter (for the double-sided foam tape), then subtracting that circle from the holder.
So, yeah, they’re custom-made for the spoon and jar in hand.