Want advice on a brew temperature controller
What they want is something that will maintain a constant temperature inside an insulated box they are building (about 3x1x1m), which the vats will go in to brew.
My idea is to use an arduino controller to switch a Solid State Relay and turn the heater on and off to maintain a set temperature. I want to try using low frequency PWM (at about 5Hz) and a proper PI control loop to get really accurate control, rather than just using a standard thermostat action.
I'm thinking that given that the relay should switch on every half-cycle (100Hz), this will give me at least 20 levels of control. (Probably more because as the PWM frequency drifts against the mains frequency, the number of on cycles will jump up and down by 1 around an average value.) I decided on this method rather than using a phase angle controller because they are costly and need external components (e.g. a line filter) to work properly.
As far as I can see, this switching method should give me pretty good control over the temperature, but it would be good to know if anyone can see any problems with it - I'm designing this more from theory than experience.
One thing I'm not sure about is how to measure the temperature. I'm thinking of using an SPI chip rather than try to calibrate a thermistor, but these cost about 4 pounds, so I'm wondering if I could save a bit of money by using a simpler method.
Another question is about safety - any advice on things I should do to ensure it's safe would be welcome. I've never done a mains powered project before, so I want to be really careful.
I've written up some notes on my proposed design here.