transformer
I have what I think is a centre tap transformer with 0V and 230V inputs and 0V, 20V, 0V, 20V outputs.
Is connecting the two middle outputs to each other safe and will there then be a difference of 20V AC between the inner and each two outer outputs and also a difference of 40V between the 2 outer outputs?
It's a Common Mode Choke integrated in the RJ45 connector.
If there is a voltage spike on an incoming line the common-mode choke will prevent it damaging the silicon by transferring the energy of the spike so that it appears on 10, 11 and 12 together i.e. there isn't a differential spike.
Each half can still supply 250mA. So you 2 12V outputs @250mA.
A common way to use a center tap transformer is to provide postive and negative supplies with 4 diodes.
Be careful about using a bridge rectifier from each half as I see in some circuits, that won't work as some of the diodes short out others.
The total power output depends upon how the transformer was rated but it probably is good for 250mA DC output from each of the positive and negative supplies.
Best Answer
If you have a transformer with two identical 20 volt secondaries, then you can connect them in parallel to get twice the current of a single secondary, or you can connect them in series to get either twice the voltage out or a center-tapped transformer with 20 volts on either side of the center tap.
Here's how to find out what's what:
1) Connect the primary to the 230 volt mains, or to some lower AC voltage if you're not familiar with working around high voltages.
2) Connect one wire from one secondary to one wire of the other secondary.
3) Measure the voltage across the other two ends of the secondaries.
4) If you measure a very low voltage and you connect those ends to each other, the windings will be in parallel, but if you measure about 40 volts, the wires you've already connected will be the center tap of a 20 - 0 - 20 secondary.