UPS Battery: Uninterruptible Power Supply battery backup system

UPS stands for Uninterruptible Power supply. It is generally used in emergency purposes. It has a battery backup system. The UPS battery is a rechargeable general battery which continuously charges from the power provided from the main supply. They are used in situations where there are frequent main power cuts so that the backup systems can be used to drive the electronic peripherals.


  • To design and implement a battery backup circuit which gets activated automatically in case of power failures.

Circuit Diagram of UPS: Battery Backup


Circuit Diagram: Battery Backup
Circuit Diagram



uninterruptible power supply: UPS Battery
uninterruptible power supply


  • The main power supply is a 230V AC Supply
  • A step down transformer is used to step down the voltage from 230V to 20-30V AC.
  • We are using bridge rectifier circuit to make it DC Supply.
  • Now a 7815 Voltage regulator IC is used to give 15V Constant output voltage.
  • The capacitors are filter capacitors. Check out: How DC Votage regulator works.


  • It is connected to main power supply through the switch. We can disconnect it easily if we don’t want to use.
  • Diode is connected so that the there is no flow of current in reverse direction and also to make battery backup system activate. Check out: PN Junction Diode Characteristics.
  • S6065J Thyristor is used which has a major role in working of battery backup system. It will be explained in the Working section below.
  • A Rechargeable battery is connected which gets charged continuously from the main power supply.

LOAD Circuit:

  • A Load circuit contains a 7805 for constant output voltage.
  • A LED Connected as a load.

Working of UPS:


Uninterruptible power supply system-2
Uninterruptible power supply system-phase 1
  • The mains power supply is ‘ON’ and the output at the first 7815 is 15V.
  • The load circuit is connected to this 15V supply.
  • Hence, the LED starts glowing.
  • The diode in the backup system also conducts which is connected to the 12V Battery.
  • This battery is continuously charging as long as the main power supply is ON.


Uninterruptible power supply system-3
Uninterruptible power supply system- Phase 2
  • The main power is switched OFF.
  • Hence the Diode stops conducting as it is now reversed biased. The cathode voltage is around 12V while anode is approximately around 0 volts.
  • The thyristors gate is always supplied 12V from the battery. Hence, to make it ON we need to make it forward bias.
  • In this configuration the thyristor turns out in forward bias. Its cathode is approximately at 0V. Hence. it turns ON.
  • As the thyristor turns out ON, the 12V battery is coupled to the load circuit via thyristor.
  • Hence the load circuit gets the 12V supply from the battery and the LED remains ON.
  • This is how battery backup system works.


UPS: Battery Backup System
Uninterruptible power supply system-Phase 3
  • The battery backup block is connected to the mains power supply.
  • The diode D3 is ON but the thyristor is OFF as its cathode voltage is at 15V[from 7815] while anode voltage is at 12V[from battery]. Hence reverse bias.
  • Therefore, the load circuit consumes power from the main supply not from the Battery backup unit.
  • Hence, the led remains in ON state and always glowing.

This is how the above system works.

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