how to use a multimeter to find positive or negative?

Question

I'm not completely electrically illiterate but I have never gotten the hang of using a multimeter.  Last year a halloween prop was broken.  The head (which contains two dc servos; one for the eyes and one for the mouth) became disconnected from the body.  The problem is that the wires disconnected from their weak point which is where they had been joined to the wires in the body which are color coded differently (this is a purchased prop, not one I made, and getting info from the manufacturer has been next to useless).  The two pair of wires in the head are obvious (two complete sets each containing a black lead) but where they connect its just four random seperate colors: purple, red, green and white.  I imagine there is some way for me to figure out with a multimeter which of the two leads are positive and which are negative, as well as which positive goes to which negative.  Any help would be apreciated.

James · Accepted Answer

So you have four useless color wires powering two motors that are red and black? By "servos" I am going to assume you mean motors, since there are only two wires per;  the problem would be significantly more complicated if they are servos.

If the motors turn in both directions you are up for a bit of a challenge. If they only turn in one direction, it's pretty easy. With power off, and the wires separated so they're not shorting, look for two and only two that are connected (either zero ohms in resistance mode or a beep in continuity mode). Make sure you check in both directions to rule out the case below. These are likely the negatives and it doesn't matter which one you use where (as they're connected!) The other two you will have to guess. If it's thrashing its head around and blinking slowly like it's having trouble staying awake, then you probably guessed wrong. :)

In the case where the motors reverse, you likely have four "half-h bridge" drivers in there. The good news is, if this is the case, you can't screw it up. If you have the wrong connections it will just move erratically. You can arbitrarily choose one wire to be red#1 and try the other three until the motor moves the way you remember. That leaves only the other pair. If they reverse, it probably doesn't matter if you mix up red and black.

If you take a photograph of the board where the random wires attach and post it here or in sci.electronics.repair, someone can probably offer more assistance.

billrussell42 · Answer

The leads are not positive and negative such as you would get from a battery, where a multimeter could be used, but a more subtle meaning. for a servo motor, the + and – are only for convenience, so you know, together with the motor documentation, which way it would move if your applied + to +, and then you would know that it moved the opposite way if you connected + to –.

Your best bet is to connect them up any which way and see what happens. If it moves the wrong way, reverse two of the leads. 

.

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how to use a multimeter to find positive or negative?

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