06-03-2017, 11:10 AM
Most power supply failure indicator circuits require a separate power supply for themselves. But the alarm circuit presented here does not need an additional power supply. It uses an electrolytic capacitor to store a suitable charge, to feed the power of the alarm circuit that sounds an alarm for a reasonable time when the power supply fails. During the presence of the mains, the rectified mains voltage is reduced to a required low level. A zener is used to limit filtered voltage to the 15 volt level. The network presence is indicated by an LED. The low level DC is used to charge the capacitor C3 and the reverse bias switching transistor T1. Therefore, the transistor T1 remains cut off while the mains supply is present. As soon as the mains supply fails, the load stored in the capacitor acts as a power supply for the transistor T1. Since, in the absence of mains supply, the base of the transistor is drawn 'low' through resistor R8, it conducts and the buzzer (alarm) sounds to give a warning of power failure.
With the C3 value as shown, a good quality buzzer sounds for about one minute. By increasing or decreasing the value of the capacitor C3, this time can be altered to satisfy the need. The Assembly is quite easy. The values of the components are not critical. If the alarm circuit is powered from any external DC power source, the power supply section N to points "P" and "M" can be omitted from the circuit. The following points can be pointed out:
1. At a higher DC voltage level, the transistor T1 (BC558) may pass a certain collector-to-emitter leakage current, causing a continuous humming sound from the buzzer. In that case, replace it with some low gain transistor.
2. The piézo buzzer must be a continuous tone version, with a built-in oscillator. To save space, five small 1000μF capacitors (in parallel) can be used instead of the large capacitor C3.
With the C3 value as shown, a good quality buzzer sounds for about one minute. By increasing or decreasing the value of the capacitor C3, this time can be altered to satisfy the need. The Assembly is quite easy. The values of the components are not critical. If the alarm circuit is powered from any external DC power source, the power supply section N to points "P" and "M" can be omitted from the circuit. The following points can be pointed out:
1. At a higher DC voltage level, the transistor T1 (BC558) may pass a certain collector-to-emitter leakage current, causing a continuous humming sound from the buzzer. In that case, replace it with some low gain transistor.
2. The piézo buzzer must be a continuous tone version, with a built-in oscillator. To save space, five small 1000μF capacitors (in parallel) can be used instead of the large capacitor C3.