It’s often handy to have an LED indicate “Power OK” status in electronic equipment. Single-supply designs need only a resistor and an LED to perform this function.
A personal computer, on the other hand, has four supply voltages (+5 V, −5 V, +12 V, and −12 V), requiring a more sophisticated circuit.
A typical approach would use a comparator for each supply voltage to be monitored. If the comparator selected has open-collector outputs, all comparator outputs can then be wire-OR’ed to create the desired OK signal. Performance with this type of arrangement is good but the circuit is relatively costly.
If accurate voltage monitoring isn’t needed, then the simple and inexpensive circuit shown can be employed (see the figure). Each supply must be operational for LED D1 to be illuminated.
In normal operation, Q1 and R1 create a current source of about 6.3 mA (good enough for most LEDs). Similarly, Q3 and R3 create a 6.3-mA current sink. Q2 acts as a commonbase amplifier to pass the current. If the current sink is greater than the source, then Q2 will saturate with the base current making up the difference. If source is greater than sink, Q1 will saturate. In either case, the Power OK LED D1 will be lit.
As the +12-V supply drops, D1 will gradually get dimmer and finally extinguishes when the +12-V supply voltage goes below +6 V. If the +5-V supply drops below about 1 V, then D1 will be illuminated.
Operation of the negative side is similar except that the threshold for the −5-V supply is at about −2 V. D2 can be replaced with a diode if desired. For higher thresholds, use two diodes in series for D1 and D2.