Electronic – use the center tapped transformer as a non-center tapped transformer

You can create a bi-polar supply from a transformer without a center tap as long as you can live with just half wave rectification for the positive and negative supply rails.

Here is a schematic that shows it is feasible. Note that I left out the filter caps so that you can see the half wave rectified waveforms.

Here are the waveforms of the two output voltages:

Half wave rectification places some requirements that you add bigger capacitors to the output to smooth the DC voltage to less ripple than would be needed if you did full wave rectification. It can also be beneficial in this case to use a higher transformer secondary voltage so that more ripple on the capacitors would not cause your down wind voltage regulator to drop out cycle by cycle.

Note that it is conventional to do full wave bridge rectification on an untapped transformer secondary. Unfortunately you cannot get two bridge rectifiers to work on a single untapped transformer secondary to get a bipolar output.

A center-tapped transformer has three connections on the secondary, where a non-center-tapped transformer has only two. Transformer datasheets should indicate this by showing a schematic symbol of the transformer.

The output voltage of a transformer is always given as the RMS value. The peak voltage is about 1.4 times the RMS voltage, and after a bridge rectifier and filtering the DC voltage will be 1.4 volts less than the peak-to-peak voltage.

A 30 volt center-tapped transformer with a bridge rectifier will produce about 40 volts DC. If you use the center tap as the common, or ground, terminal, that gives you +/- 20 volts.