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Old 10-21-2012, 01:30 AM   #1
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DC Switch help!


I am working on a DC project and have hit a snag. I want to power a 12V fan using an AC to DC transformer. It has 3 leads, 2 yellow and one black. I can use the 2 yellow for 12volt, or one yellow and the black for around 6 volts. Never should all 3 be connected together or it will short out the transformer.

I want the option to use either the 6V or 12V leads using a switch to make my choice. I want to only have one switch, so there could never be a chance of shorting it out. I instantly thought of a DPDT toggle, but I don't think that will give me a separate choice between voltages.

Any thoughts on what switch to use? Are there any rotary switches that would fill this void?

In case anyones wondering, I am making a magnetic stir plate using a computer fan, 25 OHM potentiometer, ac/dc transformer (case mount style) and putting it in a project box from radio shack.

Think of what I want as a high low selector, then the potentiometer as a fine tuner.

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Old 10-21-2012, 01:52 AM   #2
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DC Switch help!


AC to DC transformer? All the transformers I have seen are AC to AC....to get DC you need a rectifier circuit.

So....assuming you actually have an AC to DC converter (transformer + rectifier), you need to determine which one of those yellow wires is common. But, I'm willing to bet that it's AC...so there will be no common.

So...assuming you found the common....or it's AC....One of the yellows is your common....that gets connected directly to the common on the stir plate.

Your other yellow is 12v....the black is 6v.

So...take your typical toggle switch.....one yellow goes to the NO terminal...the black goes to the NC terminal...and the stir plate wire goes to the wiper of the toggle switch.

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Old 10-21-2012, 03:29 AM   #3
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DC Switch help!


Maybe we call them different things, here is a link to what i have. http://www.radioshack.com/product/in...tent=CT2032230

I want a switch to select between the center tap 6V and the 12V. Is that what you are talking about or was that just an on off switch you mentioned?
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Old 10-21-2012, 03:35 AM   #4
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DC Switch help!


Ok, I think I get it now. I was making this more complicated than it has to be. I'm still new to the world of electronics, trying to educate myself. Just sucks everytime I search something I get 1000 links wanting to sell me a switch instead of teach me all the different ways to connect them.

Thanks again! I appreciate you taking the time to help a new person to the site.
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Old 10-21-2012, 07:41 AM   #5
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DC Switch help!


Why the 12 volts and not all A/C?
That transformer is A/C to A/C.
Going to need an inverter to convert A/C to D/C.
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Old 10-21-2012, 08:48 AM   #6
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DC Switch help!


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Originally Posted by Calopolus View Post
Maybe we call them different things, here is a link to what i have. http://www.radioshack.com/product/in...tent=CT2032230

I want a switch to select between the center tap 6V and the 12V. Is that what you are talking about or was that just an on off switch you mentioned?
Your link is to a transformer (AC in --> AC out). You need a rectifier circuit to convert that to DC. The rectifier circuit wouldn't be too complicated... a diode or two, a capacitor, and probably a couple of resistors.

The term inverter is commonly used to describe a device that does the opposite: DC in yields AC out.
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Old 10-21-2012, 09:05 AM   #7
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DC Switch help!


You need to look at your computer fan...is it ac or dc (i'm betting DC). If so, you will need a rectifire like Dave said......any reason you can't use a computer power supply?

And your potentiometer needs to be a pretty good size....at least the wattage of the fan.
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Old 10-21-2012, 09:07 AM   #8
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Opps, convertor not invertor. I should know better.
http://www.amazon.com/s?ie=UTF8&keyw...dc%20converter
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Old 10-21-2012, 09:17 AM   #9
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Originally Posted by joecaption View Post
Opps, convertor not invertor. I should know better.
http://www.amazon.com/s?ie=UTF8&keyw...dc%20converter
Inverter, converter, diverter......what ever it takes......
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Old 10-21-2012, 10:49 AM   #10
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DC Switch help!


Well thats embarrassing! The one I bought in store is AC to DC. The stirrer is already built and works great. I just wanted to add the ability to switch between 12V and 6V to have more control over the fan speed. I moved recently and threw out all the computer parts I had. I had so many! Hardly ever used anything more than the occasional screw. Now I could have used a power supply, case fan and hard drive magnet! Thats ok though. I got all the parts I needed, and enjoyed building this. I could have bought a used one on eBay cheaper, but whats the fun in that!


Anyhow, I had worked my zillinonth hour of overtime when posting that yesterday, I hope I didnt confuse anyone too much with my posts!

Edit:
I'm going to go take another look at this thing. I know all the package said was transformer. I swear it said DC output, but I also thought the link I posted said the same thing.... If I'm pumping AC into that fan, it probably won't last long!

Last edited by Calopolus; 10-21-2012 at 10:55 AM.
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Old 10-21-2012, 11:06 AM   #11
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DC Switch help!


Thank you all for the help, as you all knew before me, its pumping out AC. Just went a took a closer look. Thought for sure I had the right stuff, glad you all set me straight. I can't believe it actually runs that fan!

I saw the convertors there, but I wanted something completely enclosed with just a short power cable coming out of the back. I can't even tell you guys how red my face is right now.....

Came for switch advice, learned about way more!

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