Hi, what is the convention for voltages in a center tapped transformer?

For example, I am looking at the product here that lists the item as 25VA, 25 C.T., 1A

I interpret this as a 25W secondary capable of delivering 25V @ 1A across the complete secondary winding OR 12.5V @ 2A across one of the secondary windings relative to the center tap.

Thanks

 

I interpret this as a 25W secondary capable of delivering 25V @ 1A across the complete secondary winding OR 12.5V @ 2A across one of the secondary windings relative to the center tap.
Thanks

Correct.
So what is the confusion?

 

Working with a 2A load on half the winding is safe power-wise but depending on the wire gauge used for the secondary, It may run warm and the 12.5V winding may deliver less than 12.5.

 

I interpret this as a 25W secondary capable of delivering 25V @ 1A across the complete secondary winding OR 12.5V @ 2A across one of the secondary windings relative to the center tap.

Correct.

Not exactly, as the would double the I²R heating due to secondary winding resistance and possibly overheat the winding.
(Double the current through 1/2 the resistance is twice the power dissipation).
To keep the heating the same, the current should be no more than the √2 of the full winding rating or 1.4A.

If you want only 1/2 the total voltage then you should try to balance the current between the two halves, such as using a two-diode full-wave rectifier.

 

I bought a transformer several years ago in Bangkok from a well-know distributor that was labeled 24V CT, but it was actually 24 volts for each side of the winding. In other words, you might be mislead as I was.

 

@k1ng 1337 Double check the wire gauge used!!

 

I'm building something to the effect of this linear power supply:



A bonus question: What is the purpose of D9 & D14?

As for the transformer, the maximum rating of LM317/337 is 1.5A and I have selected a 2A full bridge rectifier. I don't expect to draw more than 1A in any case so it appears the 1A transformer I posted is appropriate whether single or dual supply mode is used.

The transformers I'm looking at are from a local business that has served me well. Unfortunately the datasheet does give much more than a description and dimensions. I'm open to suggestions on the idea. This is my first mains AC project.

 

I'm building something to the effect of this linear power supply:

Okay, imbalance is not a problem, since the transformer output load is always balanced for either output,

it appears the 1A transformer I posted is appropriate whether single or dual supply mode is used.

But you can't draw 1A from both outputs at the same time, since the transformer has to derated near 50% due to the high peak RMS currents drawn by the capacitor-rectifier supply.
So the output ± total current should be limited to a about an amp DC to avoid overheating the transformer.

 

Okay, imbalance is not a problem, since the transformer output load is always balanced for either output,
But you can't draw 1A from both outputs at the same time, since the transformer has to derated near 50% due to the high peak RMS currents drawn by the capacitor-rectifier supply.
So the output ± total current should be limited to a about an amp DC to avoid overheating the transformer.

Can you elaborate on "both outputs at the same time"?

Is this to say independent positive and negative loads can draw 500mA max at the same time?

 

Can you elaborate on "both outputs at the same time"?

The sum of both output currents should not exceed about 1A thus, for example, 0.7A from the plus output and 0.3A from the negative output.

Is this to say independent positive and negative loads can draw 500mA max at the same time?

Yes.