OK, the title doesn't tell the whole story, because I am using a CH stat for a non-standard purpose.
I'd better explain!
I've been having problems keeping the inside of my computer, and the processor, cool - even after adding extra DC fans. So I have solved the problem by fitting one of Mr Maplin's 120mm mains-driven fans in the side of the (tower) casing - which blows quite a lot of ambient air past the electronics, exhausting through the rear.
Works a treat - *except* that it's a bit (audible) noisy, and doesn't actually *need* to run all the time. Now it just so happens that I have a spare Danfoss TWP room stat after fitting a programmable stat on the central heating. So I've mounted this TWP inside the computer case, and wired it to switch the fan on when the temperature is above the set value.
This works quite well *except* for the fact that there's quite a lot of crackling on the computer speakers for several seconds each time the stat switches on or off.
It seems to me that I ought to be able to suppress this noise with a suitable capacitor. Can some kind soul please advise on the spec/value of the capacitor required, and whether to wire it in parallel with the motor, or across the switch in the stat. FWIW the fan motor is described as "impedance protected shaded-pole motor" rated at 22w 140mA (which implies a power factor of about 0.65)
TIA.