I just bought a bathroom fan with a timer, that runs when the bathroom light is on and a few minutes after that. I had come across older ones that used a resistive dropper, and got very hot. They used 10% as much power when switched off as the fan used when running.
When turned off, the electronics is just waiting for an input signal, which is a mains voltage, so it hardly needs to be sensitive.
In the new fan, there is a large capacitor, which I assumed was a capacitive dropper. Here is what it looks like.
I checked and I found that the fan took 1.4 W all the time, which was far more than I expected. I looked under the board and found that the capacitors were in parallel with the mains, and there was a resistive dropper under the board. The dropper was ten 8.2k surface mount resistors, in a 5 series, 2 parallel array, totaling 20.5 kOhm. The circuit was:-
I haven't included the trigger circuit. That feeds straight into the timer part.
I can't work out what the capacitors are there for. The motor is an induction motor so I wouldn't have expected the to be much RF interference from it.
To cut down the power and the heating of the circuit board, I changed the circuit to this:-
This now works fine and takes so little power that it doesn't show on a plug in power meter with a resolution of 0.1 W. I calculate that the power is a bit under 0.1 W
I'm trying to work out why the fan was made with an inefficient resistive dropper. Any ideas?
When turned off, the electronics is just waiting for an input signal, which is a mains voltage, so it hardly needs to be sensitive.
In the new fan, there is a large capacitor, which I assumed was a capacitive dropper. Here is what it looks like.
I checked and I found that the fan took 1.4 W all the time, which was far more than I expected. I looked under the board and found that the capacitors were in parallel with the mains, and there was a resistive dropper under the board. The dropper was ten 8.2k surface mount resistors, in a 5 series, 2 parallel array, totaling 20.5 kOhm. The circuit was:-
I haven't included the trigger circuit. That feeds straight into the timer part.
I can't work out what the capacitors are there for. The motor is an induction motor so I wouldn't have expected the to be much RF interference from it.
To cut down the power and the heating of the circuit board, I changed the circuit to this:-
This now works fine and takes so little power that it doesn't show on a plug in power meter with a resolution of 0.1 W. I calculate that the power is a bit under 0.1 W
I'm trying to work out why the fan was made with an inefficient resistive dropper. Any ideas?
