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Interaction between Ventilation fan and LED light

Tim Truscott

I was asked to investigate an issue, an extractor fan had been installed in the loft above a bathroom, connected to an LED light fitting in the bathroom. When the light is turned on the fan runs, when the light is turned off the fan ‘pulses’ and the light ‘strobes’. ie both are on for ~1 sec then off for several seconds repeatedly. There are two bathrooms (family and en-suite) with identical fans but different LED light fittings. Both fans work correctly with the en-suite light, both fans exhibit the same problem with the main bathroom light. I believe that my tests have also excluded any local wiring faults 

I have raised the issue with the fan manufacturers and they say they have only seen this once before and was cured by fitting a ballast resistor (presumably in the light). I don't really want to do this as it negates the energy saving of the LED (and will generate unwanted heat). I suspect some strange interaction between the LED driver circuit and the fan timer circuit, either due to power factor or resonance (the light flash is similar to a faulty fluorescent light), in which case a coil or capacitor might be more appropriate.

Has any one else seen this? If so what did you do to solve it?

 

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    wallywombat: 
     

    I can understand how capacitive coupling would cause intermittent flickering of an LED (the LED's circuity presumably includes a capacitor which charges up via the leakage current and when high enough, causes the LED to light briefly). But a fan is just a big coil of wire with a low static resistance. Surely there couldn't be enough leakage current to power the fan for a second?

    I would guess that the fan has a permanent live, and a sense wire to the lamp so the fan knows when to turn on.  If the fan has an overrun timer, that's the most likely way to wire it.

    So it only needs enough voltage across the lamp to trigger the little bit of electronics that tells the fan it needs to turn on.

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  • 0

    wallywombat: 
     

    I can understand how capacitive coupling would cause intermittent flickering of an LED (the LED's circuity presumably includes a capacitor which charges up via the leakage current and when high enough, causes the LED to light briefly). But a fan is just a big coil of wire with a low static resistance. Surely there couldn't be enough leakage current to power the fan for a second?

    I would guess that the fan has a permanent live, and a sense wire to the lamp so the fan knows when to turn on.  If the fan has an overrun timer, that's the most likely way to wire it.

    So it only needs enough voltage across the lamp to trigger the little bit of electronics that tells the fan it needs to turn on.

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