Battery Circuit for Backup and Standby Operation

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.

Stand by battery

Stand-by battery

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.

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