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  • Battery Charger using SCR

  • Created: Oct 26, 2019

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Summary

A battery can be charged with just a small amount of AC voltage or DC voltage. This is a simple circuit that can charge a battery by using an SCR and a LM311. Here, the SCR acts as a rectifier as well as a switch to allow rectified DC voltage to be fed to the battery while the comparator is used to detect the battery charge voltage with respect to a reference voltage so as to control the switching of the SCR.


When the circuit is powered using a 230V AC supply and the battery level of the battery to be charged is below the threshold voltage, the circuit performs the task of charging the battery. The principle behind the circuit lies in the controlling of an SCR based on the charging and discharging of the battery. Initially, the SCR gets triggered with a voltage at its gate terminal through the resistor R1 and the diode D1. It then rectifies the AC voltage from the transformer( 230/12V transformer). As the DC current starts flowing to the battery through resistor R2, the battery gets charged. The voltage across the potential divider (consist of 22k,47k and 20k pot) depends upon the voltage across the battery. This voltage is applied to the non-inverting terminal in the OPAMP LM311 while the inverting terminal is given a reference voltage using a Zener diode. Now when the battery is fully charged, the voltage across the potential divider reaches a value above the reference voltage. This implies that output of the comparator is more than the threshold base emitter voltage for the transistor. This causes the transistor to conduct and at the same time, the diode D3 is forward biased, it starts conducting and this blocks the triggering the SCR gate voltage as it is now connected to low potential or ground. The SCR thus gets switched off. Again, when the battery charge drops below threshold level, the charging operation resumes in the manner described above.


The circuit can used to charge batteries used for toys. It can also be used as an automatic battery charger, specially in driving because it is a portable circuit and can be carried anywhere. The drawback of the circuit is that, it only uses a half wave rectifier which makes the charging and discharging quite slow.

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