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.

7 thoughts on “CO₂ Laser Tube Current vs. Analog Control

  1. I got some LED drivers (e.g.PT4115 or ZXLD1360) that have a dim/adjust pin that takes either pwm or analog. Looks like, internally they turn it into a reference voltage for their own pwm. I assume your black box has a similar logic inside. – Coming from China it might even be a LED driver driving a Power MOSFET ;)
    From the ZXLD1360 datasheet:
    “The device contains a low pass filter between the ADJ pin and the threshold comparator and an internal current limiting resistor (200kV nom) between ADJ and the internal reference voltage. This allows the ADJ pin to be overdriven with either DC or pulse signals to change the VSENSE switching threshold and adjust the output current. The filter is third order, comprising three sections, each with a cut-off frequency of nominally 4kHz.”

    1. A third order filter: egad!

      I’d expect no more than a simple RC filter. Wonder what forced them to such great lengths?

  2. Maybe even an active filter: just Ctrl+C Ctrl+V, they already had a bunch of opamps in the design.
    But I think, the problems are more likely in the output. Looking at LED drivers I managed to reverse engineer the secret design guidelines for pwm drivers from the College of Affordable Engineering: “Look at the datasheet for the recommended values of L and use a quarter of that. If the output still kind of works, you’re done.”

    1. Guy I used to work with came from RCA, back when they actually made televisions, and was in those engineering meetings that wouldn’t end until they had taken 25¢ out of the manufacturing cost of the chassis. If it pretty much worked after you took a resistor out, then it didn’t really need the resistor.

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