Electronics Workbench

CO₂ Laser Tube Current: Test Wiring

Having seen some rather bizarre laser tube current waveforms from the replacement power supply (and an equivalent Cloudray supply I bought as a backup) in the OMTech 60 W laser, I finally got A Round Tuit for a closer look.

I tapped three signals from the Ruida KT332N controller by the simple expedient of crunching wires into the output terminal clamps along with their original ferrules:

KT332N controller - Tube Current test connections
KT332N controller – Tube Current test connections

From top to bottom:

  • X axis DIR: low = left-to-right motion = toward X+
  • Laser L-ON: low-active laser beam enable
  • PWM: pulse-width modulation laser power control

Those three cables pass through a small hole in the cabinet to the left of the hatch on their way to channels 1, 2, and 3 of the scope.

The PWM signal (cyan, channel 3) isn’t particularly useful, but a quick look confirmed it is an active-high signal ticking along at 20 kHz, with a duty cycle corresponding to the selected laser “power”:

Tube Current - 40pct PWM first detail - 250mm-s - 10ma-div
Tube Current – 40pct PWM first detail – 250mm-s – 10ma-div

The bottom trace (green, channel 4) is the laser tube current, as monitored by a Tek A6302 Hall-effect current probe around the tube’s cathode (low voltage return) lead:

HV laser power supply - current probe setup
HV laser power supply – current probe setup

This time around, I poked a bight of that overly long wire through the hole in the cabinet (just above the power-line earth ground terminal) so I could keep the probe outside the cabinet and close the hatch.

Minus the PWM signal, the scope looks like this:

Tube Current - 40pct - 250mm-s - 5ma-div
Tube Current – 40pct – 250mm-s – 5ma-div

The top trace (yellow, channel 1) is the DIR signal, with a high-to-low transition triggering the scope when the X axis begins moving from left to right.

The second trace (magenta, channel 2) is the L-ON laser enable; the high-voltage power supply drives current through the laser tube only when L-ON is low.

The third trace (green, channel 4) is, as above, the laser tube current. The Tek AM502 amplifier sets the gain, with the scope channel always set to 10 mV/div with a 50 Ω input impedance, so I must put the current scale in the screenshot file name (which becomes the caption here).

With all that in mind, the next few posts will make more sense … and I can remember what I did.