# Fan switched on with light...?



## Bazbro (May 9, 2005)

I wanted to automatically switch on my turbofan roof vent when I turn on the shower room light. I ran the earth return from the light switch/fitting through a relay that switched the fan on, but, although the fan came on OK, the fluorescent light wouldn't, not even a flicker.

Any idea why this won't work, and how I can make it?


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## cabby (May 14, 2005)

could well be a voltage drop, not enough to start the light up with.You could change to another type of light.

cabby


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## Bazbro (May 9, 2005)

That's difficult, cabby, because the tube is in an opaque plastic column that supports the swing-out basin. Nothing easy! And I can't move the wiring to the switch (easily) because of a difficult, tight bend that includes the convoluted waste pipe. D'oh!  

What seemed an easy, straightforward job...isn't!! I just couldn't understand why a direct connection to earth (albeit through a relay) would stop the light working. It got the fan going though!


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## cabby (May 14, 2005)

well can you run a single feed direct to both to see if they will both work bypassing the relay, this of course is just to test only.maybe to see if my thought was correct as well. :wink: 

cabby


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## Bazbro (May 9, 2005)

Yes, I've done that and they both work independently. And the fan works when connected to the relay and switched on via the light switch. I think you're right when you say "voltage drop" not leaving sufficient voltage for the fluorescent starter. We're talking about just 12 volts here. Ah, well. A nice idea while it lasted!


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## cabby (May 14, 2005)

can you change the switch, fit a 2 stage switch, this will turn the light on first then the fan.Or a delay switch/relay that would give the light time to turn on then bring in the fan.

cabby


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## Bazbro (May 9, 2005)

That's a good idea, cabby.

The switch is a minute, thin affair that pushes into a very thin slot on this aluminium column, so finding a 2-stage switch replacement might be difficult! But maybe worth investigating. I'll have a look at what RS offer.

This all started because there's virtually no natural light in the shower/toilet, so the light and the fan will both normally be required. It was just too easy to connect the two together and make life easier! :roll:


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## cabby (May 14, 2005)

Can you not place a switch elsewhere, just leave the original switch in the on position.then put a two stage switch in a sensible place.

cabby


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## listerdiesel (Aug 3, 2012)

Not sure if you've worded that correctly, but if you've wired the LIGHT earth return through the relay coil, then the relay will probably work as the current required is much less than for the light, but you're asking two devices in series to run off 12V, which they won't.

Might have misunderstood your wording, in which case apologies.

Peter


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## 113016 (Jun 5, 2008)

Not sure if you have thought about if and when you go off grid :?: 
This is a time to save all the battery power that you can and a fan will use quite a bit. We have even removed unnecessary light bulbs
 :lol:


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## Bazbro (May 9, 2005)

Thanks, Peter. That's exactly what I've done and... it won't work!

The fan has its own, separate On/Off switch, *Grath*, so we're still able to control it independently. In fact, in the absence of my little 'fix', this is how we're controlling things.

It's clear from what *Listerdiesel* says that using the existing switch won't work. Running more wiring and another switch is becoming too complicated for what I'd hoped would be a nice, simple 'fix'. What I had wondered was whether I'd missed something and Peter's explained that you can't run two devices in series, so I had missed something! You never stop learning.

Thanks guys.


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## listerdiesel (Aug 3, 2012)

So what do you have easy access to?

If the lighting switch is on the earth feed, you can still put the fan earth connection with the light earth connection and switch those together, but you will need to feed the fan with 12V.

Let us know what you can lay your hands on and we'll see if there is a quick solution.

Peter


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## Bazbro (May 9, 2005)

You have a PM, Peter.


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## listerdiesel (Aug 3, 2012)

Replied, I think I see a solution, but will be working this weekend.

I'll be back in touch.

Peter


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