A project log for Heating Controller

Tempted to call this FrankenHeatController or something, but this time round doing it a little bit better.

h3liosphanh3liosphan 10/25/2021 at 21:190 Comments

IRLZ44N's have arrived, I've successfully built a test circuit to prove the MOSFET is doing it's job.

I told a small lie last log entry - I had dealt with MOSFETs before, trying to trigger a bright LED, but it was totally unsuccessful! Didn't even consider at the time putting in a pull down resistor to tie the gate to ground!

This time round I had read a lot about it, so I had a basic mock-up circuit on a breadboard. Interesting to read that the GPIO to Gate resistor isn't there to limit voltage into a MOSFET, it's only there to delay the 'charging' of the FET - still, the advice I read is to put one in at rough

After a lot of trial and error I achieved success turning the MOSFET on and off, thereby shutting down the Servo motor completely - as previously mentioned, some servos will make your life difficult if they demand too much current in a short period of time, especially with something as 'delicate' as an RPi.

So the standard advice for inductive loads such as a motor, is to put a flyback diode in to catch voltage spikes during start / stop of the motor, and save your MOSFET and/or GPIO pins on the Pi from being fried. Call me risky but so far I've not needed one, so is not on my list to add one - does a Servo motor come with one to protect RC radio gear perhaps?

I'm using a 10k resistor from Gate to ground to pin it low.

I'm also using a ~250ohm resistor from GPIO pin to Gate to slow the activation of the MOSFET by a tiny amount - my thinking at least is to ramp up the MOSFET activation, hence energy usage, slower than if it were on instantly, saving brownouts - is this good!?

Aside from that, it's all just standard connections - the Servo is high, MOSFET under it, servo negative terminal connected to Drain, then Source goes straight to ground.

Toying with the idea of putting some decoupling caps in, but nobody else has mentioned needing them for a MOSFET, and my circuit works without.

Now on to planning the physical form, and building a small MOSFET breakout board from proto-stuff.