It is in series with the lower switch on the side panel:
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:
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.
The overall doodle shows the electrical wiring and pneumatic plumbing:
The electronics bay now has two solid state relays:
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.
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:
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.
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:
A 50 ms pulse at full power doesn’t quite top out:
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.
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:
The 3.3 V peak at 10 ms is 300 mJ and 30 W:
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:
The 6 V peak at 20 ms is 550 mJ and 27 W, although the on-screen display obscures the top:
Another 20 ms pulse without the OSD produces a peak eyballometrically close to 6.4 V for 580 mJ and 29 W:
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:
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.
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.
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:
Which looks a lot like this 10 ms pulse at 50% duty cycle:
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.
The Box o’ Optical Stuff disgorged an ancient Gentec ED-200 Joulemeter:
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:
A dab of Loctite intended to secure bushings completed the assembly:
I later replaced the nut with a finger-friendly nylon wingnut.
Which allows a measurement setup along these lines:
The white disk atop the sensor is a homebrewed target to indicate the active sensor area and its center point:
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.