The Smell of Molten Projects in the Morning

Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

The New Hotness

  • CO₂ Laser Tube Current vs. Analog Control

    Up to this point, the Ruida KT332N controller has set the laser power supply current from the PWM terminal:

    Ruida KT332 - PWM laser control wiring
    Ruida KT332 – PWM laser control wiring

    The blue and purple wires go off to the oscilloscope I’ve been using to measure how the controller and power supply behave.

    The L-AN terminal produces an equivalent analog signal:

    Ruida KT332 - analog laser control wiring
    Ruida KT332 – analog laser control wiring

    The power supply accepts both analog and PWM signals on its IN terminal, so no rewiring was needed on that end:

    OMTech 60W HV power supply - terminals
    OMTech 60W HV power supply – terminals

    This test pattern came in handy again:

    Gray bars
    Gray bars

    The pattern has white bars on the left and right edges as markers. I invert the pattern in LightBurn so that white produced 100% PWM and black produced 0% PWM.

    The L-AN output produces 5 V for 100% power and 0 V for 0% power, with other power fractions spread out in between:

    Tube Current - analog - gray bars - 10 ma-div
    Tube Current – analog – gray bars – 10 ma-div

    The traces:

    • 1 X axis DIR, low = left-to-right (yellow)
    • 2 L-ON laser enable, low active (magenta)
    • 3 L-AN analog voltage (cyan)
    • 4 tube current – 10 mA/div (green)

    Engraving that pattern in scrap acrylic looks like you’d expect:

    Analog mode acrylic engraving
    Analog mode acrylic engraving

    There’s little trace of the discrete intensity levels in the acrylic trench and the scan interval is a rather coarse 0.2 mm.

    The analog-mode current looks remarkably like the PWM-mode current for the same test pattern:

    Tube Current - grayscale bars - 100mm-s 10ma-div
    Tube Current – grayscale bars – 100mm-s 10ma-div

    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:

    Pulse Timing Pattern - 1 mm blocks
    Pulse Timing Pattern – 1 mm blocks

    At 10% power the analog output is about 0.5 V:

    Tube Current - analog - 10pct 250mm-s - 10 ma-div
    Tube Current – analog – 10pct 250mm-s – 10 ma-div

    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:

    Tube Current - analog - 50pct 250mm-s - 10 ma-div
    Tube Current – analog – 50pct 250mm-s – 10 ma-div

    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.