Battery Charging Regulator

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Battery Charging Regulator

Q. Describe with neat diagram battery charging using SCR.

Circuit diagram of Battery Charging Regulator :

  • In the simple battery charging circuit the battery voltage is not being sensed.
  • However in the battery charging regulator circuit shown in Figure, the battery voltage is being sensed continuously by using a potentiometer R3 and capacitor C.

Circuit Diagram for a Battery Charging Regulator
  • VR is the voltage proportional to the actual battery voltage. (see Figure).
  • A single phase supply is stepped down using the transformer T1 and rectified by the full wave rectifier circuit consisting of diodes D3 and D4.
  • The 12 Volts battery which is to be charged is connected in series with the main thyristor SCR1 shown in Figure.

Operation of Battery Charging Regulator :

  • The battery voltage is sensed by connecting a potentiometer (or preset) R3 across it The voltage VR at its variable contact is proportional to the battery voltage.
  • When the battery voltage is low VR is small, less than VZ1 therefore VZ remains off, there is no gate current for SCR2 and SCR2 remains off. SCR , will get the gate current through R1 and D1 and it will be turned on in each half cycle of the rectified voltage.
  • This will apply the rectified voltage across the battery and the battery will start charging through SCR1. This will continue till the battery voltage is lower than its rated voltage.
  • Due to charging the terminal voltage of battery rises, the magnitude of VR equals VZ1
    + Vgk of SCR2   and SCR2 of and starts to turn on in each half cycle of rectified voltage.
  • At first SCR2 triggers at 90° in each half cycle of the rectified voltage. But as the battery voltage increases the firing angle α for SCR2 decreases in each half cycle until eventually SCR2  is triggered before SCR1 comes into conduction
  • Then the voltage at the centre point of resistors R1 and R2 is half the instantaneous supply voltage (R1 = R2).
  • This will reverse bias diode D2 and SCR1 does not get the gate triggering current. Heavy charging of battery.
  • The diode D2 and resistor R4 may be added in the circuit for “trickle charging” in order to compensate for the loss of charge during the off period of the battery due to leakage.
  • Heavy charging will recommence automatically when VR drops below VZ1 as then thyristor stops tuming on in each half cycle.

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