Posts Tagged Sewing
Stripping the components from the back of a “5 W” COB LED gets it ready for action:
Jumpering the pads with nickel strips harvested from various NiMH and lithium cells restores the original contact pads to service:
A bit of bandsaw artistry produced a replacement for the OEM LED bracket:
The epxoy bonding the LED to the heatsink happens a few paragraphs ahead in this story, but the view justifies it. The 2 mm hole just to the right of the 3 mm SHCS aligns the heatsink to a pin in the machine’s frame, ensuring it doesn’t twist around under vibration.
The view from below (in a mirror on the machine’s bed) shows the COB LED just barely fits in the opening:
I screwed the bare heatsink into the Juki, applied double-stick tape to the COB LED, aligned LED with opening, and stuck it in place. Back in the shop, I traced around the LED to figure out what part of the heatsink needed removing, introduced it to Mr Disk Sander, and contoured it to match the LED.
Clean everything with denatured alcohol, put the heatsink on a glass plate, and clamp it to the height gauge:
Butter up the LED PCB with JB Kwik epoxy, having previously masked the contact pads (with masking tape!) to prevent oopsies:
Raise the height gauge, align LED & heatsink, lower height gauge to squish epoxy into an even layer, raise slightly to ensure the aluminum heatsink doesn’t short the nickel strips, and fast forward a few hours:
Peel off the masking tape and solder a cable in place:
The transparent doodad around the cable is a PET clamp snipped from a consumer electronics clamshell package, then punched and folded to suit. It didn’t work particularly well, so more rummaging will be required.
Foreshadowing: all this went swimmingly and looks pretty good (in a techie sort of way), but I’ve been running a nasty cold (stipulated: there being no pleasant colds). Building While Stupid is never a good idea, as the part of your brain in charge of telling you you’re about to do something catastrophically wrong is the first thing to go.
More to come …
For the record, Juki thinks this SMD LED provides enough light around the needle of Mary’s TL-2010Q sewing machine:
A detailed look at the active ingredient:
The 30 Ω resistor drops exactly 2.0 V, so the white LED runs at 67 mA.
We think it’s a glowworm, compared to the COB LED bar across the back of the arm:
Changing from 2.0 Ω to 2.2 Ω produces a noticeable decrease in light, so 10% steps around 2 Ω seem to be about the right increment. The COB LED strips claim 6 W at 12 V = 500 mA nominal, so they’re running well under the spec.
A bag of G4 COB LEDs arrived from halfway around the planet:
Those are “5 W” and “4 W” cool white modules, respectively, with another set of 4 W warm white looking pretty much the same. There’s no provision for heatsinking, which makes the wattage seem suspect; halogen G4 bulbs run around 20 W, for whatever that’s worth.
The silicone overlay becomes nearly transparent when seen through an ordinary desktop document scanner:
Highlighting the PCB copper pours shows 18 LEDs arranged in three series groups of six LEDs in parallel:
The “smart IC” touted in the writeup turns out to be a bridge rectifier for AC or DC power:
The SMD resistors on all 15 modules measure 27.6 Ω, more or less, and seem randomly oriented face-up or face-down. I assume that one is face-down; maybe it’s just unlabeled on both sides.
Back of the envelope: there’s no way it will dissipate 5 W. The bridge drops 1.4 V = 2×0.7, the LEDs drop maybe 9 V, leaving the resistor with 1.6 V to pass all of 60 mA, so call it 700 mW.
With 12 VDC applied to the pins, the bridge drops 1.6 V, the LEDs 8.2 V, and the resistor 2.2 V, with 80 mA through the whole affair dissipating just under 1 W.
Cranking the supply until the current hits 200 mA puts 15.7 V across the pins for a total dissipation of 3.1 W, burning 1.7 W in the LEDs and 1.1 W in the resistor.
Cranking the supply to 21.3 V drives 410 mA, dissipates just under 9 W total, produces a curl of rosin smoke from the PCB, and maybe delaminates the silicone around some of the LEDs.
OK, now I have a crash test dummy.
Given complete control over the application, I’ll strip everything off the PCB and bond it to a heatsink of some sort. With 6 LEDs in parallel, 120 mA (6 × 20 mA) total current might be reasonable and 200 mA (6 × 30 mA) probably won’t kill the things outright. Plus, I have spares.
An external 18 Ω resistor should suffice. Perhaps a pair of 6 Ω SMD resistors on the PCB, with fine-tuning through an external resistor. Call it 250 mW apiece: don’t use little bitty SMD resistors.
With the Juki TL-2010Q all lit up, it seemed reasonable to apply the same technique to the Kenmore 158 sewing machine a few feet away:
In an ideal world, I’d match the COB LED module to the opening under the machine’s arm, but module length isn’t a free variable, so it sticks out a bit on both sides.
They run from a 12 VDC 18 W power supply with an adjustable boost converter producing 18 V for the nominally 21 V LEDs:
I replaced the coaxial power plug with a DE-9 connector:
Thpse 1/4 inch QD connectors on the AC power are marginally OK in this situation, as they’re tucked under the sewing table out of harm’s way. The other end of the AC line cord burrows into the sewing machine’s guts and isn’t easily removed, so this was the least-awful place for a connection.
The LED connector pinout:
The black cable comes from my lifetime supply of lovely supple flexible 28-ish AWG 9-conductor serial cables with molded-on male connectors.
I used some silver-plated / Teflon-insulated coaxial cable for the COB LED wiring. It burrows into the guts of the machine through a gap above the presser foot lift lever, then joins up with similar cables from the other LEDs routed through the (grossly oversized) heatsink fins:
The cables meet the repurposed serial cable inside the arm, following the original route of the 120 VAC wires formerly lighting the glowworm incandescent bulb in the endcap:
What’s not obvious in that picture: the cables pass under two stamped steel guides and through two stamped steel clamps, each secured to the frame by a cheese head screw in a tapped hole. They definitely don’t make ’em like they used to!
A 2.0 Ω ballast resistor produced the right amount of light, dropping 780 mV to run the LEDs at 390 mA and burning 300 mW. This supply produces 12.0 V at that current, so the COB LEDs run at 11.2 V and dissipate only 4.4 W.
The lower output voltage (compared to the supply on the Juki) is probably the result of the higher load from the SMD LEDs lighting up the area around the needle. We cranked up their voltage to match the COB LEDs, so they’re surely conducting more than the original (guesstimated) 50 mA apiece = 300 mA total. I have no convenient (pronounced “easy”) way to measure either their current or voltage; when the light’s good, it’s all good.
The other Kenmore 158 machines will eventually get the same treatment, but not right now.
The COB LED module claims to run at 12 V and 6 W, so it expects to draw 500 mA. First pass measurements showed 500 mA happened at 11.6 V:
The 12 VDC supply actually produced 12.1 V at 500 mA, so a 1 Ω 1/2 W resistor should produce the right current:
Which it did, but the Customer Base judged 6 W to be far too much light. A 2.7 Ω resistor seemed too dim, so we settled on 2.2 Ω:
For the record, a 2.2 Ω resistor drops 980 mV and dissipates 440 mW, probably too close to its 500 mW rating. The supply produces 12.2 VDC at 450 mA, so the LEDs run at 11.2 V and dissipate 5 W; the heatsink remains pleasantly warm to the touch.
The hot melt glue anchoring the pin header won’t win any prizes, but it sticks like glue to the Kapton tape and, in any event, there’s not much to go wrong in there.
A cardboard cover hides the ugly details:
And then It Just Works™:
As evidenced by the glove fingertips, she does a lot of sewing and I’m glad I can shed some light on the subject …
The wires to my earlier LED lights on Mary’s Kenmore 158 produced one absolute requirement: the Juki TL-2010Q lights must not have any external wiring. Some experimentation showed putting the COB LED module across the rear of the arm, just over the opening, would spill enough light to the front:
Juki’s teeny OEM SMD LED in the endcap, just above the far side of the needle, casts a dim glow over her left hand. Although they deem it sufficient, I’ll fix that in the near future.
The machine’s power supply and drive motor live inside a plastic cover on the rear of the machine, just to the left of where the LED lights will attach to the arm:
For future reference, a detailed look at the PCB:
The yellow-and-blue pair come from the AC power line switch. The brown-and-blue pair carry +120 VDC from the bridge rectifier (left of their connector) to the motor driver. The white-and-blue pair carry filtered 120 VAC from the PCB to the bulky transformer below the motor.
I snipped the white-and-blue pair, added Y connections, and threaded the wires through the vent slots to the 12 VDC power supply:
If I had to do it again, I’d cut the white-and-blue pair an inch further away from the transformer, so as to move the butt splice connectors around the corner of the frame, rather than across the back of the transformer frame. The flanged screw boss pretty well fills the space left of the transformer and made it difficult to arrange the new connectors.
The 12 VDC 18 W LED supply attaches to the 120 VAC lines with 1/4 inch quick-disconnects, making it possible, if not easy, to completely remove the cover and LED power supply. You’d install dummy plugs in the vacant QD sockets to keep the AC out of harm’s way.
There’s just enough space to the right of the PCB enclosure to route the LED wires around-and-down to meet the wire nuts. They’re not the most elegant connectors you’ve ever seen, but wire nuts are impossible to confuse with the QD connectors on the AC line.
With that in hand, the power supply almost looks like it grew under the spool flange:
In an ideal world, the label would be right-side-up, but ya can’t have everything. The wires had to be where they are, primarily to avoid snagging on fabric passing through the machine.
The green-and-black PET braid covers the AC wires to make them a little less exposed, but it’s surely unnecessary. I gently singed the braid ends to prevent unraveling.
The COB LED supply wires emerge through a slot filed in the cover:
Next step: LED brightness tweakage.