transformer question

Hello, I took this transformer from an old alarm clock and it has 2 red wires and a black. I used my multimeter and connected black to common and red to positive and each red wire gives me an AC voltage of 7.5. Can anyone explain what is the significance of having 3 wires and if possible an explanation of how it works?

Google "center tapped transformer".

Connect them in parallel and you have twice the amps.
Use one as return and use the other as supply (aka, put them in series) and you have twice the voltage.
Use black as GND, use just 4 diodes and get a dual rail supply (around +10V and -10V) which is using the full sine.

Aka, it's way more versatile :slight_smile:

Thanks for the responses. I will google and do some more research. Thanks

I did notice with some research the first 2 letters on the transformer are ST which means (Signal transformer) . So can it be both a center tap transformer and Signal transformer? Thanks

tjones9163:
I did notice with some research the first 2 letters on the transformer are ST which means (Signal transformer) . So can it be both a center tap transformer and Signal transformer? Thanks

A signal transformer is designed for devices that use very little power. Just made to set an wait for something to happen and then "signal" that that something has happened. An AC powered smoke alarm, or a door opening alarm for example. Or a clock that just sets and runs and sometime may run a buzzer or alarm for a few seconds.

So, you can use it to power something that requires only a small amount of power. And DO NOT EVER short the two end wires of a center tapped transformer and power it up. The smoke will certainly be released!

Paul

septillion:
Connect them in parallel and you have twice the amps.

No! !!, not with centre tap transformer.
How can you?
If you use the black as one lead and join the two reds you will short the windings out.
The two red wires are anti-phase, not in-phase.
Tom... :o

@TomGeorge, you're right. If it's center tapped you can't. 0:-)

Thanks for the responses, just so I'm getting it straight,
I can use the middle (black) wire as a (common) and I must use separate wiring circuits for each hot wire and common ground? Since the sine waves are not in phase, I must use them separately? I have although watches videos where you can tie together the 2 hot (red) wires and double the voltage. Can someone tell me the name of transformers where you can double the voltage by connecting the 2 Red(hot) wires? Are there categories of transformers and what are their attributes?

Separate as you can't connect them together. But you can use only the red wires to get double the voltage :slight_smile: (and thus double the power).

septillion:
Separate as you can't connect them together. But you can use only the red wires to get double the voltage :slight_smile: (and thus double the power).

I think if you "doubled the power" out of the secondary, you also double the power input on the primary of the transformer and that would get pretty hot!

Also, for the OP, the center tap should be on the middle winding layer of the transformer secondary. That is vastly different than being connected to the exact center of the secondary wire. So, measuring the resistance of your transformer black wire to one red wire should be different than the resistance to the other red wire, since one copper wire is longer, but has the same number of transformer turns.

Some low voltage, high current transformers, will have only a single secondary winding and will have the center tap connection connected to the physical center of the copper wire and the middle of the turns count.

Paul

tjones9163:
Can someone tell me the name of transformers where you can double the voltage by connecting the 2 Red(hot) wires? Are there categories of transformers and what are their attributes?

Just use the two red wires and ignore the black.
Measure the voltage on your transformer, red to red.

Or a transformer with two separate secondary windings.

Tom..... :slight_smile:

Hi,
This might help.

Tom... :slight_smile:

septillion:
Connect them in parallel and you have twice the amps.
Use one as return and use the other as supply (aka, put them in series) and you have twice the voltage.
Use black as GND, use just 4 diodes and get a dual rail supply (around +10V and -10V) which is using the full sine.

Aka, it's way more versatile :slight_smile:

You can't parallel a single center-tapped winding.

@krupski, did you even bother reading any further before posting? :wink:

I did admit I was a bit to quick / didn't had my thinking hat on and posted something stupid. :confused: But mentioning it again 7 posts later isn't smart either :wink:

@Paul_KD7HB, yeah, so? If you stay under the rating of the transformer that's just what you get. But I did assume it's a symmetric center tapped transformer aka the same current rating for both wires. Should have mentioned that.

septillion:
@krupski, did you even bother reading any further before posting? :wink:

Sorry, no I didn't. I see that you mentioned it before I did. My bad.

No hard feelings :wink: