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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 light was installed last year and didn't appear to have a problem on it's own. The issue only arose when the fan was fitted this week, which was why I assumed the problem was caused by an interaction between light and fan. The switch wire is only about 1M long so any pickup would be very small, although all of the connections are in the fitting so that won't help. I had considered using a two pole switch and using one pole to turn the light on and the other pole for the fan. This isn't such a tidy solution but the materials are readily to hand. I would have specially order a suppressor. It does beg the question that if there is an inherent issue with the light why the manufacturers don't include something. The light was chosen and installed by a local sparky (the property was tenanted at the time) so presumably it isn't top quality.  

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    The light was installed last year and didn't appear to have a problem on it's own. The issue only arose when the fan was fitted this week, which was why I assumed the problem was caused by an interaction between light and fan. The switch wire is only about 1M long so any pickup would be very small, although all of the connections are in the fitting so that won't help. I had considered using a two pole switch and using one pole to turn the light on and the other pole for the fan. This isn't such a tidy solution but the materials are readily to hand. I would have specially order a suppressor. It does beg the question that if there is an inherent issue with the light why the manufacturers don't include something. The light was chosen and installed by a local sparky (the property was tenanted at the time) so presumably it isn't top quality.  

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