78Xx Monitor

006 POWER SUPPLIES When a voltage regulator is supplied from a mains adapter, it sometimes happens that its output is too low (because the output from the adapter is too low, or because the voltage has dropped to a low value owing to an overload). It is useful if a warning of that situation is indicated. The proper operation of the 78xx series of regulators depends on the difference between input and voltage voltage, which must be not less than 3 V (worst case; many regulators are much better). ELEKTOR ELECTRONICS JULY 1989 SUPPLEMENT 78XX MONITOR The voltage drop across the regulator is monitored by IC1. The input and out- put voltage of the regulator are supplied to IC1 via potential dividers. If the input voltage to the regulator is too low, IC1 goes high, which causes Cl to charge and this turns on T1, so that D2 lights. You may, of course, use a buzzer instead of D2 and R7. The charge on Cl ensures that the LED lights for at least 10 ms. This means that the circuit will rea...