Triple Power Supply
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 ...