Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
AFAICT, that’s the default layout for all similar power supplies.
The H and L pins are the High- and Low-active enable inputs that, when it’s working right, control the laser output. The KT332 controller (and, most likely, all RuiDa controllers) produce a low-active output, so you just wire the controller’s output to the L input and you’re done.
That was the original failure that got me to this point: the power supply ignored its L input and turned the beam on at whatever power the PWM signal on the IN terminal called for. Having that happen was surprising, having it happen with the cabinet lid open was … disturbing.
The P input is intended for the Water Protect signal from the flow sensor on the laser cooling plumbing. When the water is flowing, the IN terminal will be low and the power supply will pay attention to the L input.
The power supply arrived with a jumper between the P input and the G ground / common terminal:
OMTech 60W HV power supply – Water Protect jumper
The jumper holds the P input low = active, meaning the power supply thinks the water is always flowing.
It turns out that the Water Protect signal goes only to the controller. When it’s inactive = no water flowing, the controller will refuse to fire the laser and also sound an alarm. Running the signal directly to the power supply would result in a puzzling failure-to-fire with no diagnostic from the controller.
I removed that jumper and added a (green) wire from the Lid Interlock signal at the controller:
OMTech KT332 controller – Lid Interlock input – added wire
To the power supply’s P input:
OMTech 60W HV power supply – Water Protect as Lid Interlock
In principle, if this power supply fails the same way as the previous one (with its L input always active), then at least it won’t fire with the lid up.
Believing that may display a childish naivety, but at least the thing seems marginally safer than it was before.
That’s the top and bottom of a 40 mm diameter chipboard dollhouse coaster. I made it that small to emphasize the laser kerf: a scant 3 mm across the scorched path on the top and barely 1 mm wide through the bottom, with tabs holding the pieces in place.
The SVG images include the overall frame, as seen above, and the separate pieces for kerf compensation:
Miniature Coaster – on platform
Embiggening the pieces by 0.15 mm all around produces a very snug fit:
Chipboard Kerfs – compensated – composite
I must eventually try that trick with wood, but at least I managed to get the process down without wasting entire veneer sheets.
Yes, there really is a difference between 35 mil and 57 mil chipboard:
Chipboard coaster – 35 mil white vs 57 mil kraft
The thinner leaves (0.92 mm) have one delicate white surface that presents much better color when scribbled with fat-tip colored markers. The thicker frame (1.45 mm) is ordinary kraft chipboard which seems much more durable and looks terrible when colored.
Although it may be a case of gilding the dandelion, a durable kraft frame sets off the petal colors and, being slightly thicker, may also protect them from immediate destruction by sweaty drinks.
We’re talking artsy coasters here, not cheap disposable junk. Right?
So I assembled a coaster from shattered glass in a clear surround with black epoxy atop a mirror base:
Smashed Glass Coaster 2 – mid-layer glass pour
Each fragment sits on a blob of black epoxy that eventually oozed out to fill the gap between the mirror and the transparent layer. You can see the oozing start around the two fragments in the upper left.
A top layer of black acrylic sits flush with the upper surface of the glass, seen here with the protective paper in place before pouring black epoxy into the gap around the perimeter of each fragment:
Smashed Glass Coaster 2 – masked top
Peeling the paper away exposes an almost perfect surface, with the epoxy forming a slight curve between the black acrylic and the glass:
Smashed Glass Coaster 2 – overview
The mirror doubles the number of glass cuboids and their glittery gaps:
Smashed Glass Coaster 2 – fragment detail
All in all, it turned out well, but the epoxy pouring and leveling is tedious.
It might be possible to assemble a coaster upside-down, with the black layer stuck to something like Kapton tape and the fragments carefully aligned in their openings to make the entire top surface a plane. The tape should keep the epoxy from oozing out of the gaps, although a perfect seal may be impossible.
Then fill the gaps with black epoxy, lay the clear middle layer in place, run a dollop of epoxy on each fragment, lay the mirror in place, and hope there’s enough epoxy to fill all the gaps and not enough to make a mess around the perimeter.
With a bit of luck, that wouldn’t require so much hand finishing.
The next coaster must have a perimeter shrinkwrapped around the fragments, if only to break the low-vertex-count polygon tradition.
The petals stand slightly proud of the black top frame, as the colored sheets were marginally thicker than the black sheet, but it looks OK in person. They’re all epoxied to a transparent base plate, so the bottom view is pretty much the same:
Cut Acrylic Coaster – bottom
Because the bottom is perfectly smooth, I think it looks better than the top, which shows irregularities around the petals where the epoxy didn’t quite fill the gaps. There is one small bubble you won’t notice if I don’t tell you about it.
I laid a small bead of epoxy around the perimeter of the base, laid the black frame in place, ran a bead along the midline of each petal shape plus a drop in the round part, laid the petals in place, and hoped I didn’t use too much epoxy. It turned out all right, with only a few dribbles down the edge that wiped off easily enough.
I peeled the protective plastic off the top while the epoxy was still tacky, which pulled far too many fine filaments across the surface:
Cut Acrylic Coaster – frayed top
After the final cure, I managed to scrape most of them off with a thumbnail; I hope to never make that mistake again.
As you might expect, acrylic plastic’s pure saturated colors wipe the floor with Sharpie-scribbled white chipboard:
Chipboard coaster – rounded petals – front vs back cut
The black frame makes the whole thing overly dark, so the next attempt should use white or perhaps a transparent layer atop a mirror base.
While I had the hatch open, I thought it would be interesting to look at the HV supply’s current waveforms:
HV laser power supply – current probe setup
The Tek current probe over on the right measures return current through the cathode wire, the point in the circuit where you might be tempted to install an ordinary analog (moving-coil) panel milliammeter, oriented so (conventional) current returning from the tube will produce a positive voltage.
Unfortunately, an analog meter isn’t up to displaying anything meaningful for this nonsense:
HV laser power supply – 5 mA-div – 50 ms 10 pct pulse
Admittedly, that’s a 50 ms pulse, during which an analog meter would barely twitch. The vertical scale is 5 mA/div, so the highest peaks exceed 35 mA, more than twice the tube’s recommended “14-15 mA”.
A closer look at the pulse startup waveform:
HV laser power supply – 5 mA-div – 50 ms 10 pct pulse – detail
HV laser power supply – 5 mA-div – 50 ms 10 pct pulse – tight detail
That’s at 10% PWM, close to the threshold below which the laser just won’t fire at all. The power supply must ramp up to produce enough voltage to fire the tube while simultaneously limiting the current to prevent the discharge from sliding down the negative resistance part of its curve.
Apparently this supply isn’t quite up to the task.
A 10 ms pulse at 50% PWM gives the supply enough time to stabilize the current:
HV laser power supply – 5 mA-div – 10 ms 50 pct pulse
The 14-ish mA at the tail end of the pulse (note the baseline offset) matches my previous 13 to 14 mA measurements as closely as seems reasonable. That 2 ms of hash on the leading edge suggests the start of each cut or engraving line will be a bit darker than you might expect.
Another 10 ms pulse, this time at 99% PWM:
HV laser power supply – 5 mA-div – 10 ms 99 pct pulse
The peak 24-ish mA matches the previous measurements. Note that the peaks in all the previous pictures exceed the 99% PWM current level.
AFAICT, all PWM values below about 25% produce equivalent results: random current spikes with unpredictable timing and amplitude. Changing the PWM value does not affect the (average) tube current or laser output power in any predictable way.
Some samples to illustrate the point, starting with a different 50 ms pulse at 10% PWM than the first one up above:
HV laser power supply – 5 mA-div – 50 ms 10 pct
A 50 ms pulse at 15% PWM:
HV laser power supply – 5 mA-div – 50 ms 15 pct
A 50 ms pulse at 20% PWM:
HV laser power supply – 5 mA-div – 50 ms 20 pct
A 50 ms pulse at 25% PWM:
HV laser power supply – 5 mA-div – 50 ms 25 pct
Now, that last one is different. After the hash during the first 8 ms or so, the power supply actually produces a stable 5 mA beam current, which is roughly what I measured using the power supply’s meter.
However, the other three are pretty much identical: the 10% PWM pulse does not delivers half as energy as the 20% PWM pulse. The waveforms may be different, but not in a meaningful or consistent way: the two 50 ms 10% pulses are different, but you’d (well, I’d) have trouble separating them from the 20% pulse.
To summarize:
The first several millisconds of any pulse will consist of randomly distributed spikes with very large tube currents.
For PWM values greater than 25%, the tube current will settle down to the corresponding current after 5 to 10 ms. Before the current settles down, the tube will be firing those random spikes.
For PWM values less than 25%, the tube current never settles down: the entire pulse, no matter how long, will be short, high-intensity spikes, without a consistent DC-ish level.
No matter what an analog meter might show.
I have no way to know if this power supply is defective, but I’ll certainly ask …
I think the Total job laser on time line says the power supply failed after firing the laser for a little over eight hours. The OMTech manual says the laser tube should last 1000 to 2000 hours (low vs high power), which suggests I should stock up on power supplies.
Its replacement just arrived:
OMTech replacement HV supply
It (bottom) seems to be a knockoff of the original ZYE Laser supply (top), with a similar model number and a “serial number” resembling a date from last year. All the connectors matched up, which isn’t too surprising.
The three most interesting inputs:
L = controller’s active-low L-ON enable output
IN = controller’s PWM output
P = jumper to G (circuit ground) — not water flow sensor
Also note the two AC power-line terminals directly adjacent to the TEST button, then consider insulation and stand-off distances before poking the button with your index finger.
The power supply has a digital current meter, so I plotted output current against PWM input:
Laser Power Supply – mA vs PWM – overview
Taking more points at the low end, with vertical bars indicating single-digit flicker on the meter:
Laser Power Supply – mA vs PWM – 0 to 20 PWM
I have little reason to believe the meter reading indicates the true current with any accuracy and I know CO₂ laser output power does not scale linearly with the current.
But it’s cutting again, which is a step in the right direction.
The Smell of Molten Projects in the Morning 