The usual method of using a 9V battery for backup in digital clocks, though common, is not very desirable for many reasons. The cost of the battery and the very short life are some of them.
Circuit Description of Power Back-up Circuit for Digital Clock
The circuit described here is a Power backup Circuit for a digital clock and it enables us to get the required 9V from just two penlight cells. Initially one has to invest in this circuit power backup for a digital clock, in the long run, the lower cost of the cells and their longer life more than compensate for the cost of the circuit.
The circuit of the power backup circuit for the digital clock is a simple DC to DC converter formed by the astable multivibrator (transistor T1 and T2) and the step-up transformer X1. When the supply due to mains is present, T1 and T2 remain off because of reverse bias due to positive supply. During the condition of power failure T1 and T2 together function as astable multivibrators for 15kHz. The stepped-up voltage is rectified by high-speed diodes D6 through D9. Diode D1 and D2 serve to isolate the base circuits of T1 and T2 while D3 and D4 protect T1 and T2 against reverse voltages and at the same time help to get a better wave shape.
The whole circuit of power backup circuit for the digital clock can be made into a compact unit and placed inside the clock cabinet. The 9V battery holder can be removed and a 2-cell holder used in its place.
PARTS LIST OF POWER BACK-UP CIRCUIT FOR DIGITAL CLOCK
|
Resistor (all ¼-watt, ± 5% Carbon) |
|
R1, R2 = 2.3 KΩ R3, R4 = 1.5 KΩ |
|
Capacitors |
|
C1 = 100 µF/16V (electrolytic capacitor) C2, C3 = 0.33 µF (ceramic capacitor) C4 = 0.1 µF (ceramic capacitor) C5 = 470 µF/16V (electrolytic capacitor) |
|
Semiconductors |
|
T1, T2 = 2N2907A D1 – D4, D6 – D9 = 1N4148 D5 = 1N4001 |
|
Miscellaneous |
|
X1 = Primary: 10 turns + 10 turns 36 SWG, Secondary: 40 turns 36 SWG, Core: POT-CORE. HE, 14MM/8MM (Hilversum Electronics) Battery = 2 X 1.5V Batery |