Feb 182020
 

Auto Turn-off 12V Battery Charger circuit using op-amp & voltage regulator

This Auto Turn-off 12V Battery Charger circuit allows you to charge a 12-volt car battery (lead acid battery) using a voltage regulator and an operational amplifier. This circuit is very useful when the battery was discharged, and we cannot start the car.

12V Battery charger circuit operation

The battery charger charges the battery at maximum current (limited to 2 amps) when it is discharged and gradually reduces it as the battery voltage approaches its maximum voltage. When this happens the current is limited to about 150mA.

When the “Start” button is pressed (to start the charging process) the voltage delivered by the charger is 14 volts.

Auto Turn-off 12V Battery Charger circuit (LM301A op-amp, LM350 voltage regulator)

At that time the operational amplifier, which functions as a comparator, has the current battery voltage at its inverting input and the reference voltage at the non-inverting input. In this way, when the battery does not have full charge, the output keeps transistor Q1 in its cutting region.

As the battery charges and demands less power from the charger, the voltage is reduced to 12.5 volts, ending the charging process.

When the battery is fully charged, the comparator has a low voltage output and transistor Q1 goes to its saturation region and the LED lights up warning that the battery charge is complete.

List of components for the battery charger

  • U1: LM350 voltage regulator
  • U2: LM301A Operational Amplifier (op-amp)
  • C1: 0.1uF capacitor
  • C2: 0.1uF capacitor
  • C3: 1000pF capacitor
  • R: 500 ohm resistor
  • R1: 3K, 1/4 watt resistor
  • R2: 15 ohm, 1/4 watt resistor
  • R3: 230 ohm, 1/4 watt resistor
  • R4: 15K, 1/4 watt resistor
  • R5: 1K, 1/4 watt resistor
  • R6: 0.2 ohm resistor, 2 watts or more
  • D1: 1N467 rectifier diode
  • LED: 1 green
  • Q1: 2N2905 PNP bipolar transistor
  • Others: 1 momentary contact switch, normally open
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