Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
An upcoming project calls for cutting dozens of lengths from a spool of 550 (pound tensile strength) all-nylon paracord, which means I must also heat-seal the ends. Cold-cutting paracord always produces wildly fraying ends, so I got primal on an old soldering iron tip:
Paracord cutting – flattened soldering iron tip
Bashed into a flattish blade, it does a Good Enough job of hot-cutting paracord and sealing the end in one operation:
Paracord cutting – results
Setting the iron to 425 °C = 800 °F quickly produces reasonably clean and thoroughly sealed cut ends.
Obviously, I need more practice.
Yes, I tried laser cutting the paracord. Yes, it works great, makes a perfectly flat cut, and heat-seals both ends, but it also makes no sense whatsoever without a fixture holding a dozen or so premeasured lengths in a straight line. No, I’m not doing that.
The entire control panel of our longsuffering Kenmore gas range became increasingly erratic, eventually reaching the condition where touching the upper right corner would blank the display, touching the lower right corner would restore it, and gently touching the temperature knob might elicit an F2 or F4 error code on the display. Given the symptoms, the old adage “It’s always the connectors” sprang unbidden to mind; I was pretty sure the oven temperature sensor had nothing to do with it.
Pulling the thing apart reveals the PCB across the back of the control panel:
Kenmore oven control – PCB overview
Note that all of the external connections arrive on the white power supply PCB attached over the main PCB.
A closer look shows one of the two groups of wire interconnects between the two boards:
Kenmore gas range – rear PCB
There’s a similar group hidden behind the hulking transformer.
Removing the two obvious screws and easing the PCB out of the red plastic latches made the problem instantly obvious:
Kenmore gas range – failed solder joint
Yeah, that broken solder joint would definitely be touch-sensitive!
The solder joints in the other group also show signs of fatigue:
Kenmore gas range – broken solder joints
It’s of interest only the upper joints on the power supply PCB have fractured. Perhaps those ends of the wires were hand-soldered separately from the other ends in the main PCB?
Resoldering both ends of all the wires restored perfect operation:
Kenmore gas range – resoldered joints
For the record, the Kapton tape I laid over the entire control panel 2-½ years ago continues to protect the slightly cracked membrane over the pushbutton switches:
Kenmore oven control – Kapton tape cover
Gotta love yet another zero-dollar appliance repair …
It is in series with the lower switch on the side panel:
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.
Whoops.
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
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.
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
The overall doodle shows the electrical wiring and pneumatic plumbing:
Dual-path air assist diagram
The electronics bay now has two solid state relays:
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.
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 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
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.
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
The 3.3 V peak at 10 ms is 300 mJ and 30 W:
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
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
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
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.
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
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
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
I suppose I must now preemptively affix heatsinks in the two surviving timers, because we all know how their stories will end.
The Smell of Molten Projects in the Morning 