Capacitive power supply - remote control fan

[Harry Bloomfield]

I have just got around to a serious look at this, originally my assumption was that it was the remote failing to work properly..

It is a large portable electric fan, three speed, timer to go off, variable direction, with a random function. Control is via IR remote or buttons on the fan. As the control IC it uses a SC8206 18 pin, with green LED to indicate function and a tiny piezo buzzer.

The original problem was it sometimes failed to respond to the remote, which became worse over time. Then the buttons on the unit itself became iffy.

It uses a capacitive type power supply, with an X rated cap from the mains, followed by diodes and resistors, a smoothing cap, then a zenor. Which is supposed to provide 5v to the control system. With nothing running, no LED's lit, it does provide the 5v, but that falls to around

3v when anything is powered, any current is drawn. I can turn it off via a button on the unit, the remote will then work to turn it back on, but no further response to remote - obviously because the rail voltage has fallen to 3v.

Prace your bets on the likely failure mode? X rated cap, diode rectifier, smoothing cap maybe? To be fair, the smoothing cap looks fine and is rated 16v.

I'll get back to it with some serious diagnostics in a day or two.

[The Natural Philosopher]

If it is under voltage, but otherwise working the answer is to add a little more capacitance to the 'X rated capacitor'.

You can solder a smaller cap across it, or replace it with a 10-20% larger one.

If the zener starts smoking, you have gone too far.

[Bob Eager]

I'd go for the smoothing cap. The X rated capacitors are usually only there for interference suppression, and in any case they usually fail with magic smoke.

[The Natural Philosopher]

Not in the case when you want a wattless dropper from the mains without involving a coil.

[Brian Gaff]

Dried out capacitors. These so called lossless droppers using a capacitor can be prone to the cap simply going low capacity, or in the case of an old torch charger, going short and catching fire instead. Brian

[Harry Bloomfield]

It happens that Bob Eager formulated :

No, in this case it is used as a 'voltage dropper', in series with the load.

[Bob Eager]

Ah, then it's a toss-up really! Probably more likely to be that I guess.

[Harry Bloomfield]

It happens that [e-mail] formulated :

Possibly, I bought the fan around ten years ago.