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I'm trying to recycle an old cell phone charger to power something else. I've cut the tip off and I see 4 wires... with following colors: red, green, blue, and yellow. I can't figure out which ones are positive and which ones negative. And I don't have a multimeter on hand to test it. Has anyone seen this, or know which is which?

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"An old cell phone charger" - well that narrows it down a bit ;-) (model numbers and pics are helpful for these type of questions) I'm afraid without a multimeter you are struggling, but if we knew the type of charger some pinouts might be found somewhere. The extra two wires make me think it's a USB type charging connection (no actual data) with one of the D lines held at a specific voltage to set charging rate, Apple chargers do this IIRC. –  Oli Glaser Sep 24 '12 at 17:34
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2 Answers

The wire colors in the cable for cell phone charger are probably not standardized in any way. So to solve your problem you really should invest in at least a decent meter of some sort. A digital meter is probably prefereable for most uses but a unit with an analogue meter can be very low cost. Check at Amazon.com using "multimeter" as a search term and you can find units ranging in price from less than 10$ US to ~35$ US (and more), any one of which would be a handy tool for this type of project.

Now all that said you really need to investigate further as to why the cable has four wires instead of two. Sometimes these cables use four wires wired up in pairs for the ++ and -- leads of the charger. Four smaller wires makes a more flexible cable that can be made more "round" than can be made with two larger wires and is one common reason that four wires are used. To see if this is the case in your situation you can perform continiuty checks with your new multimeter on the cut off plug to see if there are pairs shorted together.

Another reason for four wires is that there may be a remote voltage setting resistor in the plug end to adapt the charger output voltage to a specific model of phone. Best way to investigate this would be to cut open the cut off plug end to see how it was wired inside.

Yet another usage for four wires is to support remote voltage sensing at the connector end to permit the voltage regulator in the charger to adjust the exact voltage at the plug end to overcome any voltage drop in the cable from the charger end. Once again cutting open the plug can help to inform what may be going on with the four wires.

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In the Aldi discount around the corner they had a DMM for 6 euro last year IIRC. Probably not the greatest quality, but a godsend if the alternative is no meter at all. A DMM should be your first purchase. –  stevenvh Sep 25 '12 at 18:25
    
If you are in US, Harbor Freight has one for $2.99. I have been using one since >5years for trivial low voltage testing. –  Chetan Bhargava Feb 6 '13 at 23:57
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A guess would be the Red is positive and Green is negative. Do you have an LED, you could figure it out that way. Or even some salt water, as long as it's over 2 volts (any cell phone charger should be,) place both or all leads into the heavily salted water and the negative lead will produces more bubbles and corrode faster, and you can remove the others wires one at a time till you find the positive.

The other two wires are most likely used for feedback.

You could possibly google the phone model and see if you can find a diagram of the plug and see what pins the wires connect to.


Addendum

As my old answer seems to be getting some well deserved, "Don't trust any color..." comments, I feel I should explain the basics of why I stated:

A guess would be the Red is positive and Green is negative.

I'm not too old (at the time of writing this I'm only 30,) but back when I was a child learning by hacking everything I could, and hanging out at Radio Shacks... One of the first things you learn is that Red is positive, and green is negative. At least in the USA's cars, phone wiring, etc.

enter image description here

Then the next thing you get into would be alarm wiring, or for the geek, 25 pair Telco wiring:

Telco Wire

And here is the Telco Color Code for anyone who like myself, installed a ton of "Calling Card" systems, before the internet was popular!

Telco color code

And that is about it!

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I would only trust red to be positive if there's also black for negative. I have never seen green as standard color for negative. Not even in traffic lights. –  Federico Russo Sep 26 '12 at 16:41
    
@FedericoRusso I agree, however it is the most probable option. The next is that green is negative and yellow is positive (like the second line tip and ring.) –  Garrett Fogerlie Sep 26 '12 at 16:46
    
I would never trust ANY wire colour in a random piece of cheap, probably Chinese, electronics. In fact, experience tells me never trust ANY wiring colour in ANYTHING. –  John U Feb 7 '13 at 9:51
    
@JohnU Good point... But back in the day... (Damn I'm getting old,) there was a very well defined wire color code. I'll update my answer when I have the change. –  Garrett Fogerlie Feb 7 '13 at 9:53
    
It's all done to someone's code, but these days that code could just as easily be one from the Elbonian Washing Machine Corporation. –  John U Feb 7 '13 at 11:30
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