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# Not Sure Which is Positive Wire - AC to DC Adapter

#1
07-17-14, 11:08 AM
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Join Date: Apr 2009
Posts: 17
Not Sure Which is Positive Wire - AC to DC Adapter

I want to power an old computer fan off of an AC to DC adapter. I know I'm going to need to splice the wires using wire nuts. Is the striped/ribbed wire on the adapter positive or the ground/negative? I tried looking it up and keep seeing different answers. I don't know what to believe.

Also, does the wattage of the power adapter matter?

#2
07-17-14, 11:45 AM
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Join Date: Mar 2006
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Hook it up and if it runs backwards switch the wires. DC fans will run without harm in either direction.
does the wattage of the power adapter matter?
Yes, it must equal or, ideally, slightly exceed the fan wattage.

#3
07-17-14, 11:49 AM
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There is no standard but much of the time the striped wire is positive. I always confirm the polarity with a volt meter on the wires. Also make sure your power supply is actually DC. Some only step down the voltage and leave the output AC.

The wattage of your power supply does not matter much as long as it is more than the motor will draw. If it is too small it may end up burning up the power supply but being bigger is not a problem.

#4
07-17-14, 01:28 PM
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Join Date: Apr 2009
Posts: 17
Thanks.

The fan indicates it's 12v, DC, .35A. There's no indication of watts, but if my calculations are correct then 12v x .35A = 4.2 watts, correct?

I have several adapters to choose from. Here's the specs on two of them:
Input: AC 120v~0.25A 60Hz
Output: DC 12v 0.75A Max

Input: AC 120v 60Hz 15w
Output: DC 18v 500mA

Would there be an issue with either of those? I'm not sure what it means that the first one has .25A input. Would that be bad?

#5
07-17-14, 01:56 PM
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Well, if the fan is 12V...then don't use the 18V supply. The input on the first is no issue, all you need to concern yourself with is the output.

#6
07-31-14, 10:13 PM
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Join Date: Apr 2009
Posts: 17
So this project is not going like I had planned. The fan is too darn loud and that is completely unacceptable. I know I can buy other fans that should be much quieter, but I'm not ready to go that route yet (long story).

Would it be safe to use a 6v DC adapter with this 12v DC fan as long as the adapter is over 350ma (the fan is .35a)? I keep reading contradicting info on this. Some websites indicate doing this will burn out the motor in the fan. Some say it will cause the fan to draw more amps which will cause overheating. Some say it's perfectly fine to do this. So naturally, I am very confused.

#7
08-01-14, 03:57 AM
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Match voltage and amps should exceed the fan.

#8
08-02-14, 11:17 AM
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Try the fan on the lower voltage and see if it operates satisfactorily for your application. Sometimes fans won't even start on excessively low voltages.

#9
08-05-14, 02:58 PM
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Join Date: Apr 2009
Posts: 17
I tried the fan with a 6v 400ma adapter (it's labelled as a Class 2 battery charger for Power Wheels) and the fan works great. It's much quieter, but still moves a significant amount of air. In fact, it still might be too powerful, I'm not sure.

So, would there be any safety issues with using this adapter? I'm concerned about fires and damage to the fan. I was actually thinking about seeing if there was a way to encase the wiring where the wire nuts are in some sort of metal box in case there was a loose or bad connection there. This fan will be used around pets so I want to make sure they can't bother anything.

#10
08-05-14, 09:48 PM
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I'm concerned about fires and damage to the fan.
Is this a fan that will be running 24 hours a day ?
I wouldn't worry about fire or covering the wirenuts. You could put a few small cable ties on the wires to hold the splice tight to the wire.

Damage to the fan can't be guaranteed. You are running it on a lower voltage then it was intended for. Myself.... I wouldn't worry about the fan. If it craps out down the road replace it.

This fan will be used around pets
I hope you put some kind of guard over the blades.