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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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    If the timer circuit only needs a tadd to switch on semiconductors then an input on the T terminal can cause it

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    What is strange though is that when all of this is happening the fan should be on overrun and running continuously, but only appears to run when the light pulse is on.  

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    I have tried a 0.1uF 100R unit and it made no difference. Led flashes all the time the fan permanent live and neutral are connected whethet switch live to fan is connected or not. Is it worth trying higher capacitance?

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    I have tried a 0.1uF 100R unit and it made no difference.

    Just to ask the silly questions first - what was it connected between?

       - Andy.

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    SOLVED I went through the wiring from end to end and discovered that the incoming neutral had a brown sleeve indicating it was switched live. Hence light has been running with sw live and neutral transposed which only became apparent recently when the fan was instaĺled.. Worrying, as the lights were installed by a ‘qualified electrician’ as part of an EICR who insisted that new lights were required ‘to comply with the new regs’.

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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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    I started with Switch live to Neutral at the LED. Then Sw Live to earth, Permanent Live to Neutral, Sw  live to permanent live then Sw live to Neutral at the fan.

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    mapj1 Threw me completely. I thought that I had effectively replaced or bypassed all of the cables and had already replaced the switch. it was only when I realised that the switch cable was the only one that I hadn't replaced that I started looking more closely in that area. I had also had some strange voltage readings (high neutral to earth) at one time which I hadn't properly investigated.

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