Archive for February 7th, 2019

Kenmore 158: COB LED Light Bar

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:

Kenmore 158 COB LED - installed
Kenmore 158 COB LED – installed

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.

As you can see from the reflections on the base, this machine already has LEDs over the needle and in the endcap:

Needle LEDs - bottom
Needle LEDs – bottom

They run from a 12 VDC 18 W power supply with an adjustable boost converter producing 18 V for the nominally 21 V LEDs:

Needle LEDs power supply - interior
Needle LEDs power supply – interior

I replaced the coaxial power plug with a DE-9 connector:

Kenmore 158 COB LED - power supply
Kenmore 158 COB LED – power supply

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:

Kenmore 158 COB LED - Power supply DE-9 pinout
Kenmore 158 COB LED – Power supply DE-9 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:

Kenmore 158 COB LED - endcap wire routing
Kenmore 158 COB LED – endcap wire routing

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:

Kenmore 158 COB LED - machine assembly
Kenmore 158 COB LED – machine assembly

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.


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