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DIY Hotplate

I was once blessed with about 1kW worth of 808nm unmounted laser diode bars. Now, if you are not familiar with diode bars, they come as a piece of Gallium Arsenide that is about 1cm long, 1mm deep, and .2mm thick. Needless to say, they need to be soldered down to a piece of copper to help dissipate the immense amounts of heat produced by them. Due to the sensitive nature of the diodes, they must be soldered using a low melting point solder, on a hotplate, and very carefully.

But, I didn't have a hotplate. So originally I took a 2" peltier and used a thermal epoxy to hold it down to a smallish CPU heatsink, and a piece of aluminum. Hooked it up to a source of 12v/3a and it worked great for the first ~25hrs. But ultimately one of the internal traces melted and it didn't draw any current. So I took it apart and messed with it for a little while, which gave it another few hours of life. But is soon died again and the truth was that it was dead.

So, as to make the best of the remaining 8 hours left on the extended weekend, I started looking around to see what I could make a hotplate out of. I had my thermal enhanced epoxy, computer psu, and some K-type thermocouple wire, which turned out to be the prefect resistance (about 1ohm/foot) so I decided to get to work.

First, I took some Teflon insulated wire I had, (no melting wire for me this time) and soldered it to the thermocouple wire (2 elements, a 30w to keep it warm, and a 150w to heat it up to operating temp) and since I couldn't find my Kapton tape (high temp electrical tape that sticks to everything) I wrapped the joints in some plumbers Teflon tape.
heating element solder joints

Then I used some CA to hold the wire down to the hotplate from my peltier experiments, and some loctite 384 thermal epoxy (it is supposed to soften at 90c, so the CA is there to hold the wire in place) to transfer the heat from the wire to the hotplate. As I went I put cellophane tape to keep the glue in place and allow me to work it in. Once I was done I sandwiched it between another 1/4" aluminum plate and clamped it between 2 small vices, and hooked up the 1ohm winding to the 12v rail of a computer psu. Then I had the insight to embed a small K-type thermocouple in with the power wires so I could keep an eye on the temp.

Next I let it bake at 220f for 20 minutes (one tv show minus commercials) I pulled it all apart, took off the tape, and cleaned it up a little.
hotplate-top hotplate-bottom

The thing left to do was attach it to a heat sink (because I need to be able to cool down the diodes after I solder them) but I wanted to keep the heatsink somewhat cool so I could move the assembly around. The solution was to loosely tape the hotplate to the heatsink, which created a joint that had a horrible thermal conductivity--but was still enough to cool the plate down in a few minutes.

Hotplate with a 50w 808nm diode that is in the process of being soldered:
hotplate with laser diode

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