Old 06-08-2014, 04:02 PM
  #1  
Jamesbeat
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Join Date: Jun 2014
Location: NY, USA. Originally Birmingham, UK
Posts: 85
Default Replacing the lamp in my Kenmore Model 84

The light from my Kenmore's bulb was dim and yellow, and it threw off a lot of heat.

I decided to upgrade to an LED lamp, which will be brighter, whiter, and cooler.
It also has the advantage of having a ~50,000 hour lifespan, which means that it should never 'blow' or need to be replaced.

Here are the old guts:



The led is a Cree (XR-E I think). It is a 'high CRI' version, which means that the spectrum of light it emits is similar to natural daylight.

I'm powering it by USB.
USB is ubiquitous nowadays, so a power supply will always be easy to find. I'm using an old phone charger.
Using USB also has the advantage that it is a standard receptacle and voltage, so it is unlikely that anyone will ever manage to fry it by using the wrong power supply.

I wanted to drive the 3.5v LED at around an amp, so I calculated that I would need a 1.8ohm protection resistor.
I didn't have one that low, but I did have two wire-wound 1ohm resistors. They are a little big, but they are 10w each, so they should run nice and cool. The extra 0.2 ohms is insignificant.

I cobbled together an aluminum heatsink/mounting bracket, and attached the resistors to it with epoxy and a cable tie. The cable tie was really just to hold it together until the epoxy set, but I left it on or good measure.

I then put a dab of thermal compound on the bracket and epoxied the LED to it.
I covered the legs of the resistors with insulation, and soldered them to the pads of the LED.
I didn't use a reflector, because the bare LED gives off a smooth diffuse light.
I positioned the emitter low down, so that it is where the tip of the original bulb was.

The new guts:



All that remained to do was to screw the heatsink to the machine, and solder the original lamp wires to the other ends of the resistors.
I had to use plumber's flux to solder to the original wiring of the machine because the copper was so oxidized.

I disconnected the other ends of the lamp wires from the line voltage connector(!) and soldered them to a USB cable, being careful to observe correct polarity.

The original switch for the light works exactly the same as before.





It works!




I gave it a 'burn in' of a few hours to check that the heatsink is working properly and there are no problems with overheating.
The LED and the resistors were still cool to the touch after being on for around five hours.

The light is MUCH better than before.
It's pure white rather than dingy yellow, and I can actually see what I'm doing now. I can also thread the needle a lot easier.

It looks a little too bright in the photos, but that's just because it's difficult to take a realistic photo of a light source.
In real life it is very bright, but not bright enough to be uncomfortable to look at.

Ok, flame-suit on I guess...
Attached Thumbnails 01.jpeg  

Last edited by Jamesbeat; 06-08-2014 at 04:13 PM.
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