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Old 03-23-2012, 02:35 PM   #1
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How do I run a 24vdc solar attic vent with 120vac?


I have a 24vdc / 0.7A solar vent fan in my garage roof and I'd like to be able to run it at night by flipping a standard wall-mount light switch.

I'm thinking of buying a properly rated AC/DC transformer and simply wiring it in parallel with the output from the solar panel. Will this work, or is there a risk of damaging the solar panel, the DC fan motor, or the AC/DC transformer?

Thanks in advance!

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Old 03-23-2012, 02:44 PM   #2
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How do I run a 24vdc solar attic vent with 120vac?


It will not work. You must disconnect the solar before turning on the line powered DC.

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Old 03-23-2012, 02:52 PM   #3
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How do I run a 24vdc solar attic vent with 120vac?


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It will not work. You must disconnect the solar before turning on the line powered DC.
Thanks joed, care to elaborate?
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Old 03-23-2012, 02:57 PM   #4
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How do I run a 24vdc solar attic vent with 120vac?


You could use a three way switch.
Connect the fan tot he common.
Connect the panel to one traveller screw.
Connect line power 24v to the other traveller screw.

That way only one of the line can be connected at once. Flipping the switch will select between one of the sources. You will still need a switch on line powered adapter unless you want it running 24/7.
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Old 03-23-2012, 03:32 PM   #5
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How do I run a 24vdc solar attic vent with 120vac?


Good suggestion joed, but I don't understand why the below won't work. Will it damage something? Are there diodes needed somewhere? A relay or two?


Last edited by czissman; 03-23-2012 at 03:34 PM.
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Old 03-23-2012, 04:43 PM   #6
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How do I run a 24vdc solar attic vent with 120vac?


Because I don't know what happens if you apply voltage to a solar panel. A solar panel is designed to be a supplier of power not a receiver of power, which it will be at night.
Use a three way switch to select one or the other.

How do I run a 24vdc solar attic vent with 120vac?-solar.jpg
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Last edited by joed; 03-23-2012 at 04:50 PM.
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Old 03-23-2012, 07:29 PM   #7
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How do I run a 24vdc solar attic vent with 120vac?


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Because I don't know what happens if you apply voltage to a solar panel.[/ATTACH]
You may not know, but it is known: Solar panels are like leaky diodes. They will withstand their rated voltage applied to them while dark, but will leak some current through the panel. When used to charge batteries, a diode is required to prevent the battery from being discharged slowly when the panel is dark. You probably don't need a diode in series with the panel, but it would be a good idea. Your panel may already have one in it.
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Old 03-24-2012, 05:01 PM   #8
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How do I run a 24vdc solar attic vent with 120vac?


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You may not know, but it is known: Solar panels are like leaky diodes. They will withstand their rated voltage applied to them while dark, but will leak some current through the panel. When used to charge batteries, a diode is required to prevent the battery from being discharged slowly when the panel is dark. You probably don't need a diode in series with the panel, but it would be a good idea. Your panel may already have one in it.
So one way to skin this cat with a degree of certainty is to install as my diagram indicates but with diode(s)? Do I need one or two, and do you have a suggestion as to which diode I should buy?

Any other thoughts out there?

Thanks for everyone's replies
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Old 03-24-2012, 07:23 PM   #9
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How do I run a 24vdc solar attic vent with 120vac?


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So one way to skin this cat with a degree of certainty is to install as my diagram indicates but with diode(s)? Do I need one or two, and do you have a suggestion as to which diode I should buy?

Any other thoughts out there?

Thanks for everyone's replies
You probably don't need the diode since there's no battery to drain. If you want to, install a single diode in series with the positive lead of the solar panel. Any 1A diode should be fine. Radio Shack still carries them. The lowest common voltage rating is 100V, so that won't be a problem. The cathode (marked with a stripe) goes towards the motor, the anode (no stripe) to the solar panel. While you're at it, you might as well install a second diode in series with the 24V adapter so the solar panel doesn't have to backfeed it (there could be some leakage that direction, but probably not much). That way the adapter and the solar panel will be completely isolated from each other, but will both power the fan independently.
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Old 03-24-2012, 08:57 PM   #10
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How do I run a 24vdc solar attic vent with 120vac?


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You probably don't need the diode since there's no battery to drain. If you want to, install a single diode in series with the positive lead of the solar panel. Any 1A diode should be fine. Radio Shack still carries them. The lowest common voltage rating is 100V, so that won't be a problem. The cathode (marked with a stripe) goes towards the motor, the anode (no stripe) to the solar panel. While you're at it, you might as well install a second diode in series with the 24V adapter so the solar panel doesn't have to backfeed it (there could be some leakage that direction, but probably not much). That way the adapter and the solar panel will be completely isolated from each other, but will both power the fan independently.
Perfect, that's just what I needed, mpoulton... thanks! It's amazing what you can learn in places like this.
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Old 03-24-2012, 10:34 PM   #11
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How do I run a 24vdc solar attic vent with 120vac?


I would use a dpdt switch. I wouldn't chance any interconnection between the solar panel and xformer.
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Old 03-25-2012, 06:35 AM   #12
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How do I run a 24vdc solar attic vent with 120vac?


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I would use a dpdt switch. I wouldn't chance any interconnection between the solar panel and xformer.
Why? Two diodes provides complete isolation between the two sources. It's like a switch, but automatically uses whichever source is providing a higher voltage at the moment.

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