Some old multimeters have a high resistance scale including an internal circuit powered by a 22.5 V battery. Besides being very difficult (if not impossible) to find this battery currently, its cost is very high. An alternative is the use of a source that is just that exemplified in the figure. This source can also be used in bias circuits that require a voltage at this value (22.5 V) under very low current. The transformer has primary winding according to the power grid, and secondary winding of 12 V with at least 50 mA current. The zener diode is 400 mW and the capacitors must have a working voltage of 50 V or more. The same circuit, with modifications, such as the inversion of all diodes and capacitors, can be used as a source of negative voltage for circuit polarization.
