Need help activating an exaust fan from two diff type switches - AVS Forum
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post #1 of 8 Old 06-07-2013, 04:38 AM - Thread Starter
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Hi All,

Not really sure which forum to post this is...

I want to be able to have a 120VAC exhaust fan power on from two different sources. The first would be a timer of some sort to cycle the fan on/off 2x (or more) per day for a set time (say 20min). The second would be when ever I turn on my HT receiver, which would be from the 12VDC trigger. I would like to have this second source (the 12VDC trigger) be the main source, meaning whenever the RCVR is ON, it will over-ride the timer.

Any ideas on how this would work and possibly recommendations on hardware to accomplish this?

Thanks all,
Rob

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post #2 of 8 Old 06-07-2013, 10:39 AM
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Just put the contacts of the 12 volt relay in parallel with the contacts in the timer.


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post #3 of 8 Old 06-07-2013, 10:46 AM - Thread Starter
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Quote:
Originally Posted by olyteddy View Post

Just put the contacts of the 12 volt relay in parallel with the contacts in the timer.


Hmmm, never thought about that.

So technically the fan would be plugged into two different feeds of 120? One turned on/off from mechanical timer and the other turned on/off by a 12VDC trigger activated relay?

What would happen if the 12VDC trigger was supplying 120VAC, but then the mechanical timer switched to ON (or vise-verse)? Then you have the two sources of 120VAC going to the fan.

If this is a safe solution, it would literally be the most simplistic!

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post #4 of 8 Old 06-07-2013, 11:45 AM
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O'teddy's suggestion is a good one. I do something similar with a timer I use for water heating and circulation. Instead of a 12v trigger I have a wall mounted switch to override the timer, but the concept is identical. IIRC I found a schematic on this site: http://waterheatertimer.org/index.html - I just did an admittedly very quick search for relevant wiring diagrams, but couldn't find one that is an exact fit - if interested, check it out, you might see what I'm referring to.

The key thing is to feed the fan from the same electrical source (such as a common outlet), and to make sure that you keep your live & neutral wires correctly matched/paired - as long as you do this, the fact that electricity is arriving simultaneously on two separate paths (timer & 12v trigger) is irrelevant - it's just the same as connecting two positive wires in parallel from a plug leg to the same side of a motor.

Given the potential for mayhem, I definitely wouldn't recommend that you do this if you're not 100% confident, but once you get your head around the wiring layout it's pretty straightforward. Or you could use a 12v solution throughout.....
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post #5 of 8 Old 06-07-2013, 11:52 AM - Thread Starter
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Quote:
Originally Posted by DGF View Post

O'teddy's suggestion is a good one. I do something similar with a timer I use for water heating and circulation. Instead of a 12v trigger I have a wall mounted switch to override the timer, but the concept is identical. IIRC I found a schematic on this site: http://waterheatertimer.org/index.html - I just did an admittedly very quick search for relevant wiring diagrams, but couldn't find one that is an exact fit - if interested, check it out, you might see what I'm referring to.

The key thing is to feed the fan from the same electrical source (such as a common outlet), and to make sure that you keep your live & neutral wires correctly matched/paired - as long as you do this, the fact that electricity is arriving simultaneously on two separate paths (timer & 12v trigger) is irrelevant - it's just the same as connecting two positive wires in parallel from a plug leg to the same side of a motor.

Given the potential for mayhem, I definitely wouldn't recommend that you do this if you're not 100% confident, but once you get your head around the wiring layout it's pretty straightforward. Or you could use a 12v solution throughout.....

Looks like I have my answer.

Thanks guys! Much appreciated!

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post #6 of 8 Old 06-07-2013, 03:00 PM
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I meant you would have one power source, but two switches.


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post #7 of 8 Old 06-07-2013, 04:35 PM
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I think we're talking about the same thing, but in my installation the "single power source" is a double electrical outlet - the timer sits in one socket and the 12v trigger box would be located in the other, equating to your "two switches". Either can power the fan - if they both happen to energize the circuit at the same time, no problem.

I scouted around a bit more to find a good wiring diagram to illustrate what I'm talking about, but still can't locate the exact one I originally used for inspiration. The closest I can find is here: http://waterheatertimer.org/Timer-with-remote-control.html - take a look at the "Timer with remote control" section that's pretty near the top of the page. If you substitute your 12v trigger for the "Woods 32555" remote box, you'll get the idea (and there are a couple of other variations on this theme lower down).

Dave
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post #8 of 8 Old 06-10-2013, 01:03 PM
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Quote:
So technically the fan would be plugged into two different feeds of 120?

No, you're just putting two switches in parallel.
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