This circuit generates 3 source regulated voltages applying a minimum of electronic parts. The output DC voltage are +12V ; +5V and -5V. Diodes D2 and D3 conduct full-wave rectification, at the same time charging capacitor C2 on each halves of the alternating current cycle. At the same time, diode D1 with capacitor C1, and diode D4 with capacitor C3 each conduct half-wave rectification. The full- and half-wave rectification arrangement is satisfactory for modest supply currents drawn from -5 and +12-V regulators IC3 and IC2.
You are able to use this circuit as an auxiliary supply in an up-based instrument, as an example, and prevent the less attractive alternatives of purchasing a custom-wound transformer, building a extra complicated supply, or applying a secondary winding, say 18 Vac, and wasting power inside the 5-V regulators.