Some old multimeters have a high resistance scale that has an internal circuit powered by a 22.5 V battery. Besides being very difficult (if not impossible) to find this battery today, its cost is very high. An alternative is to use a font that is exactly the one shown in the figure. This power supply can also be used in polarization circuits that require a voltage of this value (22.5 V) under very low current. The transformer has a primary winding according to the power grid, and a 12 V secondary winding with at least 50 mA of 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, for example, the inversion of all diodes and capacitors, can be used as a negative voltage source for circuit polarization.
