Figure 1

Bipolar Power Supplies

by Lewis Loflin

In figure 1 we have an example of a bi-polar power supply using two 9-volt batteries. They share a common ground and the output can be any combination of polarities or voltages. For example the ATX power supplies used in most PCs today have multiple voltage outputs, but share one common. In the ATX supply we have plus and minus 5 volts, plus and minus 12, and three volts.

Besides their use in home PCs, bipolar supplies are used in many OP-AMP circuits and high power audio amplifiers.

Figure 2

In figure 2 we have constructed a bi-polar power supply using a 12.6 volt transformer. D2 charges up C3 on the positive half-cycle, while D3 charges up C4 during the negative half-cycle. In both cases the output voltage would be 17.8 volts. (12.6 * 1.414.) The polarities would be opposite in regards to ground.

The same rules for half-wave rectification for both polarities still holds true. Because capacitors C3 and C4 would have to large (at least 3300 uF), R1 limits the inrush of current when power is applied. The values is usually 1-10 ohms at 5 watts.

Figure 3

Figure 3 illustrates a bi-polar supply using a diode bridge and the transformer center tap as common. The same rules apply as before with the voltage at half while the current is doubled. This is full-wave rectification and can use smaller capacitors than those in figure 7 for the same load.

If we use a 25.5 volt 3 amp transformer each output voltage will be 25.2/2 * 1.414 or 17.8 volts.

Figure 4

Figure 4 illustrates a regulated bi-polar power supply for use with OP-AMP circuits.

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