Modding popcorn popper: shut off heater while keeping fan on

Question

So, I'm trying the age-old tactic of roasting coffee with a popcorn popper, and while it works, I would like to have better control over the heat applied in order to prolong the roasting. I've seen other mods to popcorn poppers, but the schematics for this one are bugging me.

I've got a bifinett 1200w popcorn popper, and to the best of my ability, this schematic represents how the internals are wired (I'm not an electrical engineer so I might be wrong). Note: I'm missing a capacitor connecting the two AC wires.

This is an image of the "Fancy Bridge Rectifier", which connects to a motor working as a fan:

And this is a picture of the heating element, with the labeled HL, HT, and HR points:

I would like to accomplish two things, both related to controlling the heat while leaving the fan always runnning:

1. Add a switch to, at least, the main coil, so that I can control the heat. This would be my main objective. However, from the schematic, I don't see how I can turn off the main coil without turning off also the fan and secondary coil. Plus, it seems to me that the main coil lowers the voltage that is passed on to the rectifier and secondary coil, so I don't see how to go around it.

2. Later I would like to control the main heating element with a solid-state relay so that I can control the temperature through an ESP8266 I have that is measuring temperature with a thermocouple positioned inside the heating chamber. This one I have no clue how to wire up, but this is also more for "future work", right now I want to accomplish 1) first.

Answer 1

This popcorn popper uses essentially the same circuit as many hair dryers- the elements form a dropping resistor for the DC fan, so you can't turn the heater off without affecting the fan. Image from here. Ignore VD and the switch, that's for the "low" position.

What you could do is provide a separate DC supply for the fan motor and disconnect it from the bridge rectifier assembly. To avoid affecting the remaining circuit you could replace the fan with an equivalent resistor (that may not be necessary but it's safer since the short element may otherwise get too hot).

To do this you need to figure out the fan voltage and current rating. It may be marked on the motor, but if not you could (carefully) measure it.

For switching the remaining elements you could use a relay or an SSR. It's a trade-off the relay will have limited life (but for a coffee roaster maybe that's not a big deal), and the SSR will require a big heatsink and/or a fan of its own. You could also use a relay and a resistor to switch a triac which is pretty much the same as an SSR. It wouldn't hurt to add another thermal safety fuse, especially if you're depending on firmware to turn the fan on whenever the heater is on.

The unit will have been designed, tested and certified not to cause an unsafe situation if the fan motor fails, but you're increasing the chances of that if you use firmware, and making other modifications so extreme care is called for. The current arrangement ensures the fan is always powered whenever the heater is on.

Edit: DPDT selection (CAUTION, this requires one side of low voltage supply to be tied to the mains). It would also be possible to use a somewhat higher (like 1.5V more) voltage and switch the input to the bridge rectifier), but I have a suspicion the fan motor voltage will turn out to be something convenient like 12V at 5-10W.

^{simulate this circuit – Schematic created using CircuitLab}

Answer 2

You can maybe use a capacitive dropper to power the fan rectifier. picking the right part here is impossible without knowing what current and voltage the fan needs,