OMTech 60 W Laser: Key Lock FAIL

The orange indicator I added makes the key lock switch position on the OMTech 60 W laser perfectly obvious:

Laser Power Lock Indicator - installed
Laser Power Lock Indicator – installed

It is in series with the lower switch on the side panel:

OMTech Laser - rocker switch lit
OMTech Laser – rocker switch lit

Although I would have labeled those switches differently, the “Control Switch” handles the 120 VAC line voltage to the HV power supply. As you’d expect, when its light is ON, the power supply is also ON and the laser is ready to fire.

Those two pictures show the situation after I turned the laser power on a few days ago: key lock switch OFF, HV laser power supply stubbornly ON.


The “Control Switch” still does what it should, so I can shut the HV supply off when it’s not needed, but the key lock switch has definitely failed ON.

As far as I can tell, the moving contact bar jammed at the bottom of its travel against the terminals. Pulling the switch out of the laser jostled it enough to release the bar and it’s now at the top of its travel:

OMTech Laser - key lock - side view
OMTech Laser – key lock – side view

If it failed once, it’ll fail again.

OMTech’s Customer Support agrees it shouldn’t behave like that; a replacement should arrive in a few days.

OMTech 60 W Laser: Dual-path Air Assist

A tweak to the air assist plumbing of my OMTech 60 W laser produces much the same result as Russ Sadler’s Super Ultimate Air Assist, with somewhat less plumbing and cheaper Amazon parts:

OMTech Laser - air assist - plumbing
OMTech Laser – air assist – plumbing

The overall doodle shows the electrical wiring and pneumatic plumbing:

Dual-path air assist diagram
Dual-path air assist diagram

The electronics bay now has two solid state relays:

OMTech Laser - air assist SSRs
OMTech Laser – air assist SSRs

The front SSR turns on the air pump when the controller activates the STATUS or AUX AIR outputs; the diode between the (-) terminals acts as wired-OR.

The rear SSR turns on the solenoid valve whenever the AUX AIR output is active. The diode turns on the other SSR to start the pump.

When the laser cutter is idle, both the STATUS and AUX AIR outputs are inactive, so the pump doesn’t run and the solenoid is closed.

The controller has a front-panel AUX AIR button that turns on its eponymous output, which turns on both the solenoid and the pump. I have turned it on to verify the circuitry works, but don’t do any manual cutting. I never was very good with an Etch-a-Sketch and the laser’s UI is much worse.

LightBurn includes an Air Assist setting for each cut layer, which should be OFF for engraving layers and ON for cutting layers. Basically, you go through the Material Library and tweak all the values , then It Just Works™ when you assign that material setting to a particular layer.

The solenoid valve must be a “direct acting solenoid valve“, as the air pump produces about 3 psi and cannot activate a “self piloted” solenoid valve. When the valve is open, the pump can push about 12 l/min through the plumbing to the nozzle:

OMTech Laser - air assist - 12 lpm flow
OMTech Laser – air assist – 12 lpm flow

That’s noticeably lower than the 14 l/min without all the valves and additional plumbing.

The flow control valve is a manually adjusted needle valve to restrict the engraving air flow to maybe 2 l/min, just enough to keep the smoke / fumes out of the nozzle and away from the lens, when the solenoid valve is closed.

I set the controller to delay for 1 s after activating the air pump to let it get up to speed. There’s an audible (even to my deflicted ears) rattle from the flowmeter when the air assist solenoid opens.

The paltry 12 l/min seems to promote clean cuts and 2 l/min doesn’t push much smoke into the surface around the engraved area.

So far, so good.

Gentec ED-200 Optical Joulemeter: Oscilloscope Comparison

The little DSO-150 oscilloscope has a 1 MΩ || 20 pF input with a 200 kHz bandwidth that should be entirely adequate for the OMTech laser’s millisecond-scale modulation signals from the Gentec ED-200 Optical Joulemeter. There is, however, only one way to be sure:

Gentec ED-200 - scope test setup
Gentec ED-200 – scope test setup

The two scope inputs are in parallel, so the joulemeter over on the far right sees a 500 kΩ load, half of the specified 1 MΩ load, with at least twice the capacitance. If the two scopes display pretty much the same result, then it’s good enough.

A 50 ms pulse at half power looks the same on both scopes:

  • Gentec ED-200 - 50 ms - DSO-150
  • Gentec ED-200 - 50 ms - Siglent

A 50 ms pulse at full power doesn’t quite top out:

  • Gentec ED-200 - 11V 50ms - DSO-150
  • Gentec ED-200 - 11V 50ms - Siglent

Given that the pulse duration should be less than the detector’s 1.5 ms risetime, using a 50 ms pulse is absurd. Right now I’m just looking at the overall waveform and detector range, not trying to get useful numbers out of the poor thing.

All in all, the DSO-150 will do just fine.

OMTech 60 W Laser vs. Gentec ED-200 Optical Joulemeter: Long Pulse Duration Power

The Gentec ED-200 Joulemeter is severely underqualified to measure the OMTech 60 W laser’s beam power, because the laser’s 1 ms minimum manual pulse width isn’t much shorter than the sensor’s 1.5 ms risetime and the maximum beam power is far too high for the sensor’s health. With that in mind, I set the PWM power to 50% = 30 W (grossly too high) and looked at the peak output voltage for a series of (far too long) pulse widths:

Rounding the detector sensitivity to 11 V/J says the 1.3 V peak at 5 ms corresponds to 120 mJ and 24 W:

Gentec ED-200 - 60W 50pct 5ms
Gentec ED-200 – 60W 50pct 5ms

The 3.3 V peak at 10 ms is 300 mJ and 30 W:

Gentec ED-200 - 60W 50pct 10ms
Gentec ED-200 – 60W 50pct 10ms

The 3.4 V peak at 15 ms is 310 mJ and 21 W suggests the PWM power output is not nearly as constant as one might expect, although the pulse width looks fine:

Gentec ED-200 - 60W 50pct 15ms
Gentec ED-200 – 60W 50pct 15ms

The 6 V peak at 20 ms is 550 mJ and 27 W, although the on-screen display obscures the top:

Gentec ED-200 - 60W 50pct 20ms OSD
Gentec ED-200 – 60W 50pct 20ms OSD

Another 20 ms pulse without the OSD produces a peak eyballometrically close to 6.4 V for 580 mJ and 29 W:

Gentec ED-200 - 60W 50pct 20ms
Gentec ED-200 – 60W 50pct 20ms

The KT332N controller in the OMTech 60 W laser has a pulse duration setting showing tenths of a millisecond, but (based on some additional measurements) the beam power can vary by 25% for successive pulses in the low millisecond range, so the pulse width resolution doesn’t seem to provide useful control.

Despite the over-long pulses, the calculated power corresponds surprisingly well with the nominal laser output power.

The 1 ms pulses used in LightBurn’s Dot Mode are consistent enough to punch essentially identical 0.2(-ish) mm holes in manila paper to mark the graticule:

OMTech 60W laser - beam alignment - focus detail - 2022-03-22
OMTech 60W laser – beam alignment – focus detail – 2022-03-22

They’re on 0.25 mm centers, with slight variations showing the difference between stepper resolution and positioning accuracy. The shorter graticule lines have three holes on one side of the center lines and four on the other, despite the design’s 1 mm length on both sides; I think there’s a missing dot on the side where the head starts the line, perhaps due to a picket-fence error.

The large beam hole came from two 10 ms pulses, one at the focal point and another 10 mm lower.

Kukoke Outlet Timer: Over-powered Zener Diode

If the title seems familiar, it’s because there’s no visible difference (apart from the “brand name”) between the Enover timer that failed a little over a year ago and the Kuoke timer that recently failed:

Kukoke timer - overview
Kukoke timer – overview

That’s what it looked like after the repair. Prior to that, it’s just a blank display with no response to any inputs.

Given identical hardware, the overheated phenolic PCB under the Zener diode came as no surprise:

Kukoke timer - zener heat death
Kukoke timer – zener heat death

As promised, though, this time I epoxied a brass shim heatsink to the new diode in hopes of cooling it enough to live long and prosper:

Kukoke timer - zener heatsink
Kukoke timer – zener heatsink

I suppose I must now preemptively affix heatsinks in the two surviving timers, because we all know how their stories will end.

Gentec ED-200 Optical Joulemeter: Specs

The Gentec ED-200 optical joulemeter from the Box o’ Optical Stuff is so thoroughly obsolete that no datasheet exists for it anywhere online:

Gentec ED-200 - measurement setup
Gentec ED-200 – measurement setup

The best I could come up with, after many dead ends, is a 2001 capture from at with the barest hint of specifications:

Gentec ED-200 specs
Gentec ED-200 specs

The Max Energy Density spec suggests longer pulses are allowed to deposit more energy, probably because more time gives thermal diffusion an opportunity to spread the heat across the target; at CO₂ laser wavelengths that may not apply.

With the platform lowered as far as it goes, the ED-200 is 130 mm below the laser nozzle where the beam diameter is about 6 mm for an area of 0.3 cm². Ignoring the ideal Gaussian beam profile by smearing 60 W uniformly across the circle gives a power density of 200 W/cm², which means the laser pulse must be less than 0.5 W·s / 200 W = 2.5 ms to stay inside the power density limit.

I sincerely hope Gentec overbuilt and underspecified their detector.

Also, there’s a useful overview document from, wherein it is written:

The Voltage Response
The result is a voltage pulse that rises quickly with the response time of the device to a level proportional to the laser energy (Figure 2). It then decays exponentially over a longer period of time that is a function of the pyroelectric device and load impedance. Figure 2 also shows that there is a longer recovery time to return to the initial state of the detector. This is a function of thermal phenomena and is not affected by the load impedance as are the rise and decay times. The integrated pulse energy over this period is proportional to the peak voltage.

Pulse Width Versus Rise Time

Usually the applied laser pulse must be shorter than the rise time of the detector for all of its energy to be represented by the peak voltage. Pulse energy received after the detector voltage has peaked will not be fully integrated into that value. For very long pulses, the peak voltage will actually represent peak power rather than pulse energy.

Gentec Energy Detectors, page 2

Figure 2 shows the overall waveform:

Gentec Energy Detectors - Figure 2
Gentec Energy Detectors – Figure 2

Which looks a lot like this 10 ms pulse at 50% duty cycle:

Gentec ED-200 - 60W 50pct 10ms
Gentec ED-200 – 60W 50pct 10ms

The pulse was 10 ms long, much longer than the 1.5 ms ED-200 risetime spec, but the overall shape looks right. Dividing the 3.3 V peak by the detector’s 10.78 J/V calibration value (11 J/V works for me) says the pulse delivered 300 mJ = 300 mW·s. Dividing 300 mJ by 10 ms gives 30 W, a beam power astonishingly close to the expected value.

The OMTech laser has a nominal 60 W output, although the tube life drops dramatically with regular use over 70% = 40 W. Power does not scale linearly with the laser tube current displayed on the power supply milliammeter, with the maximum value presumably preset to the tube’s 20 mA limit producing 60 W. The 20 kHz PWM duty-cycle chopping applied by the controller should linearly scale the average power downward from there.

It looks like the ED-200 might deliver reasonable results for millisecond-scale pulses at low PWM duty cycles, but it was obviously intended for much milder lasers.

On the other paw, it’s fully depreciated …

Gentec ED-200 Optical Joulemeter: Accessories

The Box o’ Optical Stuff disgorged an ancient Gentec ED-200 Joulemeter:

Gentec ED-200 - measurement setup
Gentec ED-200 – measurement setup

It’s an optical pyrometer producing, sayeth the dataplate, an output of 10.78 V per joule of energy applied to its matte black absorber. Whether it’s accurate or not, I have no way of knowing, but aiming the business end toward the sun and waving my fingers over it produced a varying voltage, so there was hope.

It has a 1/4-20 socket on one side and my spare magnetic mount expects a 3/8 inch rod, so I drilled a suitable hole in a suitable aluminum rod and cut the head off a suitable bolt:

Gentec ED-200 mounting rod - parts
Gentec ED-200 mounting rod – parts

A dab of Loctite intended to secure bushings completed the assembly:

Gentec ED-200 mounting rod - assembled
Gentec ED-200 mounting rod – assembled

I later replaced the nut with a finger-friendly nylon wingnut.

Which allows a measurement setup along these lines:

Gentec ED-200 - measurement setup
Gentec ED-200 – measurement setup

The white disk atop the sensor is a homebrewed target to indicate the active sensor area and its center point:

Gentec ED-200 target - scorches
Gentec ED-200 target – scorches

The 1 mm graticule lines give a jogging suggestion to hit the center, assuming you (well, I) manage to hit anywhere on the target at the first shot. The beam is supposed to fill most of the central region, which is obviously not going to happen here, and it must not be focused to a pinpoint. The previous owner (or his minions) put a few scars on the surface and I expect to make similar mistakes.

Early results look encouraging:

Gentec ED-200 Joulemeter - first pulse
Gentec ED-200 Joulemeter – first pulse

The SVG image as a GitHub Gist:

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I added the two mounting ears in anticipation of putting the joulemeter in the beamline between the mirrors to measure their loss.