Battery Circuit for Backup and Standby Operation Project

The circuit was designed to create a spare and reserve power source with the use of batteries during the absence of power from the mains.

• BD139 – NPN power transistor used for driver stages in hi-fi amplifiers and television circuits because of its low voltage at 80 V maximum and high current at 1.5 A maximum.
• Bridge Rectifier – also known as bridge rectifier which has four diodes arranged in a bridge configuration where the output voltage has the same polarity with either polarity of the input voltage.    
• 1N4002 – a 1.0 Amp Silicon rectifier with voltage range of 50 to 1000 Volts and possessing features such as guaranteed high temperature soldering, high current capability, diffused junction, low reverse leakage, utilizes void-free molded plastic technique for low cost construction, and carries Underwriters Laboratory Flammability Classification of 94V-0 by its plastic package.

One of the main components in the construction of this circuit is the transformer which is rated with 230V/10V with a given current of 0.5A. It will handle the transfer of electrical energy from one circuit to another via inductive conductors or windings. Other components include a bridge rectifier and an electrolytic capacitor. This will be followed by an 8V2 Zener diode rated at 0.4W, which provides the regulation of voltage in the transistor as it is connected in series with the base to produce a constant output voltage of 7.5 V.

 To produce the desired 7.5 V, there are five batteries used in this circuit with 1.5 V each, in series with diode D7. This voltage is hanging across the output terminals, which takes over in case of failure in the main supply. The power supply will be reduced to 7 V due to the voltage drop across D7. The presence of resistor R3 performs an additional operation where it drops the charging of dry cells or storage battery, when it stops working with the main supply. To obtain the actual resistance of R3, the voltage potential difference between the battery and the Zener diode D6 is divided by the reliable current drop which measures around 0.7 mA.

The circuit of 7V backup battery is widely applied in several uninterruptible power supplies that efficiently function in times of power outage. Some models are incorporating the use of Lithium Ion and NiCd types of batteries. The most advantage feature that the circuit brings is providing backup power for emergency and important loads. Other models are created with high power capability to support high power applications.