That resistor (109) (that is a bit undersized and running hot), is only creating a PELV supply for the  HEF4001 CMOS chip (thing with the 14 legs) that is some NOR gates strung up to make a monostable, time constant set by the variable resistor and C105

220 odd of the 230 are dropped, and a diode and a capacitor (104) plus a zener create a low current regulated DC for the timer.

The timer is triggered by the 270k resistor whose number is obscured from the switched live bringing in just under a mA to kick things off.

When the timer is on the 1k5 resistor (107) puts a few mA into the gate of the triac (111) This gives the fan the mains voltage minus no more than a volt or so when its gate is stimulated.

Thanks Mike. Now I  understand. Other motors from the same stable are rated at 110V to 240V, so will work over a range of supply Voltages, but your 1 V drop will not make much difference to the motor then. Does the big Volt drop resistor (109) run hot continually or just when the fan motor is working?


Z.

The timer and its power supply are on all the time there is unswitched steady live present, and the Zener diode makes sure that the voltage on the timer chip is more or less constant by absorbing the surplus current when the triac is not firing, - so in turn the voltage on the dropper resistor is also more or less constant too, at about 220 of the 230V... (i think before it cooked it was probably red red orange, so 22K ohms, about 10mA constant load, a couple of watts, but the colours are burnt so maybe not)

Personally I prefer capacitor dropper designs, as they  run cooler but they are mains frequency dependant,  and still need some series resistance to keep the current from  becoming near infinite during supply switching transients, and on non-sinusoidal waveforms like some inverters they fry completely. A pure resistor is more or less universal.

I suspect this one worked really well on the lab bench and was then boxed into the fan casing without thinking that the airflow had changed.  Much  like cable ratings a 2 watt resistor is only really able to sweat off  the full 2 watts for a given set of ambient conditions,  and must be de-rated when the heat is not free to escape. Two resistors of the same physical size and half the value, in series would have an easier life.

Sometimes you see ceramic or glass spacers on the legs to lift the resistors up a bit, or slots in the PCB, in an attempt to get a few % extra airflow under the device. These are the classic signs that the designer realised they were sailing close to the wind and is trying to improve reliability.

This one didn't realise..

Once the board is charred it conducts a bit, and depending how charred and where, that can start to affect the timer  by leaking charge into or out of the timing capacitor.  In the limit it becomes unusable.



There is a lower power version that uses the spare gates to turn the steady state triac firing pulse into short bursts - unlike a transistor, a Triac stays on once triggered until the next zero crossing so you do not need a continuous turn on signal. By using the low voltage DC cap to provide the short turn on burst you can have a lower average current, and run cooler. This is much like the tank on the loo allows you to use a a half inch filling pipe, and get a short duration flush of full flow in 2 inches diameter - as opposed to installing a fat pipe all the way back to the street main and needing no tank..

Mike.