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?
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.
I understand that you mean a transformer like the one shown in the following figure:
This transformer has a primary (terminals 1 and 2) and two secondary (terminals 3 and 4; terminals 5 and 6). I understand what you want to do is connect the two secondary in series, to obtain an output of 20-0-20 V.
In order to make this connection, it is vital to know the relative polarity of each winding. In the figure, the plotted point indicates the terminals having the same polarity at any given instant.
This means that when the terminal 3 is positive, it is also the terminal 5. With this data it is possible to connect terminals 4 and 5 for what is shown in the following figure:
If connect the terminals that do not have the same polarity, the total voltage (between the end terminals) will be very close to zero volts. This is because the magnetic flux generated voltages in phase opposition which are subtracted from each other.
To determine the polarity of the windings must inject a known signal, not necessarily power, since the measurement is performed with the transformer without load. Using an oscilloscope, you can determine which terminals have the same phase of the injected signal, and therefore correspond to the homopolar terminals.
I hope my answer is useful and has correctly interpreted the question!
