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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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    Ah. Well that was unexpected. I'm sorry to have led you the wrong path, and maybe the Cap and R are not needed after all this time - but keep them, as you will get the flickering lights problem from wiring capacitance at some point.

    The 0.1uF should be enough to pull the voltage below 10V, which is well below the never trigger level for almost all LEDs, against cable of about 0.005uF, or if you prefer other units ~ 5000pF, this represents something  over 50m of typical T and E cable. There are very few installations that have long enough parallel switched wiring to need more than 0.1uF. The last one for me was a Worcester Bosh boiler that kept firing up even when off at the thermostat.

    Mike.

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  • 0

    Ah. Well that was unexpected. I'm sorry to have led you the wrong path, and maybe the Cap and R are not needed after all this time - but keep them, as you will get the flickering lights problem from wiring capacitance at some point.

    The 0.1uF should be enough to pull the voltage below 10V, which is well below the never trigger level for almost all LEDs, against cable of about 0.005uF, or if you prefer other units ~ 5000pF, this represents something  over 50m of typical T and E cable. There are very few installations that have long enough parallel switched wiring to need more than 0.1uF. The last one for me was a Worcester Bosh boiler that kept firing up even when off at the thermostat.

    Mike.

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