Heater coil construction

(Backdated: Nov 10, 2016)

Today I built the heating system for our extruder. I made a new heater coil out of about 1 foot of Nichrome wire and crimped it to the end of a length of 18AWG speaker wire (in order to handle the high current required). I hooked it up to a 30V 17A FET on our small high-power prototyping board and added a new conductor in our control connector to switch its gate.

The three things we’re now managing on the board are our large extrusion motor, our smaller spooling motor, and the heater coil. These all require a fair bit of power, so this prototyping board has a lot of large FETs and heavy-gauge wire on it. Eventually we’ll have to hook this up to a more permanent installed power supply, but for now it works just fine with a lab power supply.

In assembling the heater, I also tried to include a temperature control system. After a long and almost entirely fruitless search, I found a lone stray J-type thermocouple in the EE stockroom. I placed it against the side of the extrusion barrel, and wrapped over it with Kapton tape, simultaneously fastening the thermocouple junction and providing electrical insulation for the bare heater wire, to prevent a short-circuit through the aluminum barrel.

Once I had the thermocouple attached, I tried running the heater coil to heat the barrel. While the coil did in fact put out a lot of heat, I wasn’t able to get a meaningful reading out of the thermocouple. I realized that it would need some sort of amplification, so I built a quick non-inverting amplifier with a gain of 1000.

After quite a few hours of fruitless fiddling with wires, I still failed to get any meaningful result our of the thermocouple, despite buffering its leads with an instrumentation amplifier. It looks like we’ll need to measure it with thermistors after all.