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Problem with PIR and LED light.

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Nigel Goodwin

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A guy at work (bosses son) brought me a PIR switch he'd just fitted, saying it didn't work properly with his LED light, and that it kept pulsing the light ON briefly when it was supposed to be OFF.

So I connected the 'suicide lead' to it (mains plug to two croc clips :D) and hung my scope on the output, full peak to peak mains with it either ON or OFF.

So next I connected another piece of hi-tech test gear (40W incandescent bulb with two wires soldered to it - amazingly useful thing to have), and it then worked perfectly with that as a load.

From the previous two tests it seemed obvious there was leakage when it was turned OFF, so I connected a multimeter in series with the lamp - to find 168mA with the bulb ON, and 0.8mA with it OFF.

A quick calculation leads me to believe it has a 0.01uF capacitor across the relay contacts in the PIR, which would give about 0.8mA at 240V.

Obviously the 0.8mA was enough to charge the reservoir capacitor in the LED light until it burst into life, which then discharges it of course.

Sneaky thing to happen though - I'd take it to pieces and remove the capacitor, except it's brand new and sealed - and he's going to take it back.

Looking at the sellers website it actually says 'tungsten lights only', a fact that the instructions don't mention, nor the datasheet :D
 
I had a similar issue when I built my own washing machine but with a triac driven load. The snubber circuit for some reason stopped the triac from turning off under certain circumstances. Heavy loads were fine but the light loads (i.e. inlet solenoids) occasionally had issues.
 
A 5w resistor would absorb most of the 0.8ma, just tell him its an anti frost device.
Some lights havea red led to warn off intruders, how about one of these.
 
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