The power supply shown has the circuit shown in the figure, and transformers of 5 + 5, 6 + 6, 7.5 + 7.5, 9 + 9 V and up to 12 + 12 V can be used according to the desired voltage at the output. We observe that the no-load voltage (open) is the peak voltage of the secondary of the transformer, that is, a much higher value that can reach 18 V in the case of 12 V transformers. 15 V transformer with a maximum of 9 V secondary must be used. The maximum current of this source is given by the transformer. For this configuration we recommend a maximum current of 500 mA. The capacitors must have working voltages at least 2 times higher than the secondary voltage of the used transformer, as they charge with the peak voltage.
