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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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    The fan is triggered by the voltage across the LED. This does not fall to zero as it would for a filament lamp, as there is a very small capacitance in the switch wiring ,and the LED draws no current at all when voltage falls below some threshold. result the switch line drifts to some tens of volts AC. If you have a high impedance meter you can probably detect this, though the exact value will change as you try and measure it.

    On 50Hz mains the standard fix is a mains rated capacitor of ~ 0.1uF across the light (or in the fan between trigger live and neutral) - now to minimize switch arc, an RC series combo (example) is probably more universal, and is designed for mains use though in most cases a simple cap (example) with enough voltage rating would do.

    Mike.

     

     

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  • 0

    The fan is triggered by the voltage across the LED. This does not fall to zero as it would for a filament lamp, as there is a very small capacitance in the switch wiring ,and the LED draws no current at all when voltage falls below some threshold. result the switch line drifts to some tens of volts AC. If you have a high impedance meter you can probably detect this, though the exact value will change as you try and measure it.

    On 50Hz mains the standard fix is a mains rated capacitor of ~ 0.1uF across the light (or in the fan between trigger live and neutral) - now to minimize switch arc, an RC series combo (example) is probably more universal, and is designed for mains use though in most cases a simple cap (example) with enough voltage rating would do.

    Mike.

     

     

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