Hello!
I am trying to help fix a friend's old waffle iron where the thermostat completely broke down. While doing so we decided to upgrade the iron and add a lcd display with a rotary encoder for navigation. This is the design I came up with:
From that I made this pcb:
The idea of this circuit is to use the arduino as the brains of the system measuring the temperature of the iron with a PT-100 and the Max-31865. The arduino will act as a PID controller (probably overkill, but it's fun to play around with) and determine the power to the heating elements in the waffle iron. The temperature will be set by the user with the encoder and that information will be displayed on the lcd display. With this circuit I would be able to controll the power to the waffle iron with either phase control or zero crossing, although as I've understood it, zero crossing is best if the response of the system is slow which a heater is. The iron draws around 9A when on full power so all components on the mains side should be able to handle at least 10A. Now that is the part which I am not to confortable with: the high voltage. Now made the trace thickness of the power carrying lines 4mm and I also added traces on both bottom and top layer. Will this be enough of should I add wires/solder (order without soldermask) once I get the pcb?
I am also wondering about the clearance. Looking at
this for 330Vpk it shows an external clearance of 2.5mm and coated clearance of 0.8mm. Does that mean that between pads, which are not coated, there needs to be 2.5mm clearance between them, and between traces only 0.8mm?
The last thing I'm worried about is the cooling of the TRIAC. Now looking in the
datasheet for ACST1210 it shows it will generate 11W of heat at 10A. It's thermal mass is 1.5 C/W and the heatsink I
selected has a thermal mass of 3.7 C/W without a fan. Now am I right to assume that (1.5 + 3.7) * 11 + 20 ≈ 80C so that should be good?
Any other suggestions/remarks are also welcome! Mostly I just want to make sure I am making a temperature controllor and not a smoke generator