Back Emf

[Rimmer]

Hey Crossy tell me about back EMF

At night when I turn off one particular ceiling fan one of the outside security floods turns on by itself, stays on for the prescribed length of time then goes off. I assume it's back EMF that's causing this. Don't believe it's a problem but I am intrigued.

The floods are 150w motion sensor type wired on a seperate circuit in to a TP board. The fans are all on a seperate circuit.

Strange huh?

Edited October 18, 2008 by Rimmer

[Tywais]

Any inductive loads, in this case the motor fan, will produce a back EMF pulse when switched off. This can also produce a broadband RF radiated spike. So two possibilities. One it is radiated RF triggering the sensitive front end of the detector or an induced pulse into the power line. The circuits may be separate but they are common at some point in the wiring.

Is that fan the closest one to the detector which would suggest RF sensitivity? One way to verify which mode is triggering it is to put the sensor circuit on an UPS that is not plugged into the power line. If it still triggers the sensor then it is RF radiated emissions. If not, then it is power line induced noise. Possible a surge arrestor/emi-rf filter on the power line to the sensor AC (if induced noise). Or the same across the fan if radiated to clamp the back EMF.

[Rimmer]

Thanks Tywais so that's it

Must be RF as the fan is, as you suggest the closest one to the light sensor, maybe 2 1/2 mtrs only, none of the other fans have any effect on any of the other 5 security lights. What about if I re position the sensor?

Doesn't bother me, can you confirm that it's not a problem please?