Here’s the heating circuit I plan to use snaffled from here:
The Load is an SMD resistor about the same size as the MPU6050 chip, attached to it with thermally conductive sticky tape. It needs to run near its power capacity (= melt down) when loaded with ≈ 5V. SMD resistors of the right physical size have max. power settings ≈ 1W meaning 0.2A will trigger meltdown. A resistance ≈ 5 / 0.2 = 25Ω will serve the purpose perfectly
The mosfet needs to have a low internal resistance itself, so the heat is generated primarily by the resistor. The 2N7000 shown has a wide range of specifications depending on the manufacturer. The best I’ve found is 2Ω, so the mosfet will drop 5V * 2 / (2+25) = 370mV and 0.37 * 0.2 ≈ 0.075W. That’s about 12% of the chip’s maximum power rating of 625mW.
Alternatively, a BS170 has a 1.2Ω source gate resistance, dropping ≈ 240mV and ≈ 0.05W – 6% of BS170 specified 830mW.
The GPIO is PWM driven so that resistor meltdown is avoided with feedback from the MPU-6050 internal temperature sensor.
There are a few niggles left:
- the phone charger battery bank I use to power the Raspberry Pi needs to be able supply up to another 200mA but in pulses – will it cope?.
- Will the Raspberry Pi polyfuse cope with this extra power drain?
I guess I’ll just suck it and see.