The article explains how to make a simple dual power supply circuit without using a center tap transformer rather any ordinary two wire transformer.

This circuit is of interest not merely because it uses a bell transformer with a single secondary winding to provide symmetrical voltages for low-current applications but also because the final output voltages are greater than the normal bell transformer (220 V/8 V) output. In fact the final output can be as much as twice this value.

This multiplication is achieved using two voltage doublers each consisting of two diodes and two capacitors, connected head to tail.

Each diode/capacitor couple takes every alternate half cycle of the sinusoidal voltage such that the output voltage U is (theoretically) equal to 2V2 Ueff where Ueff is the effective output voltage of the transformer.

A current of 150 . . . 200 mA and 1 V of ripple can be expected using the capacitor values shown here. ln order to increase this current without a similar increase in ripple the values of the capacitors may be made greater but C1 must be approximately the same as C2, and C3 about the same as C4.

To get a stable symmetrical output of 15 V two voltage regulators, a 7815 and a 7915, should be used. This enhanced simple dual power supply circuit will then allow any two wire transformer to be used for any small circuits with operational amplifiers requiring a symmetrical supply of 14 or 15 V and a current of 0.1 .. . 0.2 A.

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