I recently purchased a 273-1512 transformer from radio shack which I plan to use to run a hobbyist electric motor I have built.

The transformer is said to be both a 12 or 25 volt model. Primary coil has two wires that come from home outlet, the secondary has two yellow wires and one black wire. The unclear instructions say for 25 volts coming out of the secondary (which is what I want) I need to connect the two yellow wires and I don't need the black wire at all. But connect the two yellow wires to what? Connect them to each other? Connect them as positive and and negative to my load?

I can't get my mind around this. I know I need a circuit for anything electric to work. That makes it seem to me that I'd connect the two yellow wires together so they'd add up to 25 volts and that I would use a black wire for the negative. I think I've already burned out one of these transformers doing that and don't want to do it to my newest one.

Is it possible I connect one yellow wire to the positive side of my load and the other yellow wire to the negative side of the load, then just dead end the black wire with electrition's tape? This is very unintuitive for me. Is this the way to get 25 volts out of this transformer?

Thanks,
Diderot2

 

If you need 25 volts, just use the two yellow wires, tape/cap off the black one. The transformer puts out AC voltage, there is no positive or negative.
If you need DC voltage you'll have to rectify the transformer output, that is easily handled with a rectfier. You'll have to give us more info on what you want before we can advise further.

 

Gerty,
You've blown my mind. It is true I've mainly been reading about and studying DC but I thought the two prongs in an AC wall socket were positive and negative, too, to complete a circuit.

Anyway, here is what I'm doing. I've built a little electric motor from PVC pipe, copper pipe, a steel rod for two coils for electromagnets running perpendicular and through the center of the PVC tube, and earth magnets on either side. The copper pipe is the commutator with brushes at both the front and back. The commutators have electrician's tape covering half the circumference in the front side and the exact opposite half of the circumference on the back half. The way it is wired it is supposed to reverse the current direction twice for every time the shaft revolves one full turn. The reversal shifts the attraction and repulsion forces of the magnets and along with the inertia of the spinning coils it will spin quite energetically on 25 volts DC, but not 12V.

However instead of running it on four 6V batteries I thought I'd run it on a transformer (which I further thought converted AC to DC). But in any event I need wires to complete a circuit the way it is set up.

For more clarity, you can see the project I imitated at http:/www.howtolou.com/

Lou has a video there called How to Build a Motor. That is what I did.

The bottom line question is this: can I hook up the two yellow wires coming out of the secondary coil to what I called positive and negative loads, before? Or do I need to add something to my transformer (perhaps a diode of some kind) to get it to work?

Thanks again,
Diderot2

 

You ought to read up about the difference between DC and AC.

http://www.allaboutcircuits.com/vol_2/index.html

If your motor runs on DC then you need a DC power supply.
Your transformer runs on AC and produces AC between the two yellow wires on the secondary winding. There is no +ve and -ve.

The voltage (and current) reverses direction 60 times a second (or 50 times in countries where the AC mains is 50Hz).

To convert the AC voltage from the two yellow wires to DC you need a bridge rectifier which is four diodes connected in a special configuration. This is shown in the tutorials.

 

This may help you calculate your desired voltage, and it's a good visual for rectifiers.
http://www.hammondmfg.com/pdf/5c007.pdf

 

Folks,
Thanks to all for your comments and your attempts to help. This all seems too complicated for me currently however. I will need to study AC quite a bit before being able to choose the correct components for a bridge rectifier, so I am postponing this all for now.

Appreciate the suggestions, however,
Diderot2

 

I wanted to thank some folks on this forum for their help. Forum members may recall a couple of weeks ago I was trying to build a small transformer to power the DC DIY electric motor I had built. Today, I put the transformer parts together and it worked!

Specifically I wanted to give a shout-out to Gerty and MrChips for their comments.

By the way, I connected a center-tapped RadioShack transformer capable of 25V and used a bridge rectifier. Actually, considering the difficulty I had with doing a clean soldering job I'm surprised but it all worked out okay -- the motor has been humming along today and that's what counts.

Appreciate the help guys,
Diderot2