Triple Power Supply

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Inexpensive miniature
transformers normally provide one or two secondary voltages, which is
sufficient for generating a set of positive and negative supply
voltages, such as are needed for operational amplifier circuits. But
what can you do if you need an additional voltage that is higher than
either of the supply voltages (such as a tuning voltage for a
receiver?). This circuit shows a simple solution to this problem, and it
certainly can be extended to suit other applications. Using a 2×15-V
transformer, it generates positive 24-V and 12-V supply voltages and a
negative 12-V supply voltage. The little trick for generating the +24-V
output consists of using IC1 to create a virtual ground.

This is based on a well-known circuit with a voltage divider formed
by two equal-valued resistors, which divide the voltage Ub across the
rectifier from approximately 40 V down to 20 V. This Ub/2 potential is
buffered by an opamp, which allows this virtual ground to drive a load.
The present circuit uses the same principle, but instead of being
divided by a factor of 2, the voltage across the rectifier
(approximately 40 V) is divided unequally by R1 and R2. The resulting
potential, which is buffered by the opamp and the subsequent transistor
output stage, lies approximately 15 V above the lower potential, and
thus around 25 V below the upper potential.

Circuit diagram:

Triple Power Supply Circuit

Triple Power Supply Circuit Diagram

The three voltages are stabilised using standard 100-mA voltage
regulators, as shown in the schematic. The supply voltages for the opamp
are also asymmetric. Thanks to the low current consumption, this can be
managed using two Zener diodes. You must bear in mind that the
secondary voltage generated by an unloaded miniature transformer is
significantly higher than its rated secondary voltage. The following
results were obtained in a test circuit using a 1.6-VA transformer with
two 15-V secondary windings: the positive and negative 12-V outputs
could be loaded at around 10 mA each, and the 24-V output could be
loaded with approximately 20 mA, all without any drop in any of the
output voltages. For small circuits such as a 0(4)–20-mA instrumentation
loop, this is fully adequate. For more complex circuits or switched
loads, additional compensation may be necessary.

Author: Bernd Schädler – Copyright: Elektor Electronics