Hotplate

pcb-hotplate

Soldering hotplate on a pcb. Fed by a DC power supply, and controlling with any microcontroller.

How to build one

The board has essentially three parts, that can be used somewhat independently. Not all components are usually necessary. Most of the PCB is the heater area, just a "heater coil" from copper traces. There is also a MOSFET driver to control the heater, as well as temperature-sensing circuitry, both easily connected to your microcontroller of choice. Anything with an analog input and digital output can do the job.

This project is a bit "raw", and anyone who builds it should have some understanding of electronics and what's going on. An external PID controller for your preferred heating profile is necessary, and is not provided here (though there are plenty of open-source examples for it, including ones with fancy displays and controls).

MOSFET driver

I used an IRF540N, but lots of N-channel TO-220 mosfets with sufficient voltage and current capabilities will work fine. The flyback diode (D1) is probably unnecessary, the heater should have almost no inductance. DC power can be connected via the a screw terminal (J2), automotive-style blade terminals (J3+J4) or banana plug screw terminals or soldered wires to the plated holes (H1+H2). The transistor is along the edge of the board if you want to mount a heatsink to it, but it may not be necessary.

Using without mosfet driver

If you have some other means of controlling power, connect DC negative/ground to H3/J5 instead of H1/J3 to just drive the heater.

Temperature sensing

The board has space for two thermistors, one on the bottom side of the board (TH1, with a little "heat spreader" metal area) and one on the top (TH2). Measuring on the top should be more accurate, but will also use up some of the available space. Thermistors in glass bead packages with something like kapton tape to hold it in place works fine. You can use either thermistor, or both or none of them. I used both and the bottom side reads a few degrees colder, but this can be compensated for in software.

The reference "pullup" resistors (R1/R2) should be of some reasonable value for the thermistor and heat range. For a regular 100K NTC thermistor, 2.2K or 4.7K (and maybe a nice 0.1% one) works well. If you don't populate them, you can have them externally and if so there is no need to connect VCC.

Pinout

Apart from the power terminals, there is one pin header for control signals to connect to a microcontroller.

1. Signal ground for the heater control (same as "heater ground" for the mosfet driver)
2. Heater control signal (mosfet gate)
3. VCC (+5V or +3.3V as appropriate), only used for the thermistor voltage dividers
4. TEMP1 (TH1, bottom side)
5. TEMP2 (TH2, top side)
6. Signal ground for thermistors

If you are using both the mosfet driver and the thermistors, the two signal grounds (pins 1 and 6) will probably be the same. If so, you can tie them together with the JP1 solder bridge (and optionally only wire one of them).

License

This project is licensed under the GNU General Public License (GPL) version 3.0.