LED dimming and ac fan wierd problem

[jtd]

Hi all,

I have a series of LED lamps 23 w each powered by individual constant current ballasts. The outputs are pwm modulated with a transistor. The ballasts are always on. There is also a ceiling fan with a regular leading edge phase dimmer and a switch in series.
The problem is that when the fan is switched on/off using it's switch, the LED lamps will turn on for a brief instant. This inspite of shorting the base-emitter (after isolating the uc).
I suspect that the fan is dumping energy into the mains and two back to back 4007 diodes should solve the issue.
But could there be a better method?

 

[(*steve*)]

I'm not sure what a pair of back to back 1N(?)4007 diodes would do.

Some sort of snubber network might be more appropriate.

 

[jtd]

Back to back diodes would clamp any voltage above the diode PIV.

 

[(*steve*)]

Probably not.

You might consider some sort of transient absorbing device.

The problem with rectifier diodes is that their PIV rating is a minimum. They may not break down until you get a much higher voltage. In addition they are not designed to operate in this mode and may be damaged.

Best get something that it designed for the purpose.

But yeah, the general concept *may* be applicable.

Check out snubbers too. The problem may be caused by a high dv/dt (rapidly changing voltage), not necessarily just by high voltage alone.

 

[duke37]

You can get transient suppressors which look like a ceramic disc capacitor which are placed directly across the mains. These have a high resistance until a certain voltage is reached when they conduct. You need to chose one suitable for your mains voltage.

Computers have mains inlet filters (or a least they should have), you could get one out of a dead computer power supply. My washing machine had one until it exploded, now I put up with clicks on the radio.

Distribution boards are sold to run computers, the better ones have surge protection and filtration. This may be the easiest and safest way to go.