# Transformer Wiring Confusion

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#### Tom D

I would like to know exactly what "Center Tapped" means with a Transformer?

Example: If you needed to replace a Power Transformer that has only 2 primary wires and only 2 secondary wires; Could you replace it with a "center tapped" model?

Would the "middle, third or center wire be left open in this case? or is it used as a ground? Thanks for any help.

#### hitsware

the center tap would be left open

#### audioPT

Center tap being left!!

Be sure that the voltage of the center tapped transformer is half of the original

Like this:

Original is 24V
Center Tap must be 12V-0-12V

If not, you will get 48V (24V-0-24V)

Is this ok?

Pedro Martins

#### Tom D

Thanks for the quick replies.

The transformer in question has 2 primary wires and 2 secondary wires. The input would be 120 VAC. The secondary output measures 24 or 25 VAC across the 2 secondary wires. This is a Power Supply circuit that puts out 24 VDC(Regulated) at 500ma.

My confusion seems to be: What rating of transformer would I look for as a replacement? Example: 24 or 25 VAC center-tapped or 12VAC center-tapped? Also; what amp rating would I need; 500ma or possibly 1A or 2 A?

#### audioPT

Well, I think I can explain.

If the original transformer is 24VAC, then any substitution must be 24VAC (without center tap) or 12VAC with center tap. In this last case, use the external wires (leave the center tap).
This way you get 24VAC.
About the Amp rating, if the PSU is going to be 500mA, then the transformer must be, at least 1A.
Why? because in the circuit there are components that absorve energy and the diodes and/or other semi-conductors have some losses.
There are other factors that I don't explain because of the complexity.
Some people use the formula I(transf) = I(out)*1,414.

Did I helped?

Regards

Pedro Martins

#### Tom D

audioPT,

Wow!! Thanks so much. You could not have explained it better! I really appreciate your help. Thanks again.

Tom

#### skaara

Center tapped transformers are commonly used in audio amps... 24V CT (center tapped) transformer has 12V-0V-12V on secondary.. see pic..

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• centertap.jpg
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