Charging any type of chargeable battery can be critical and involves some attention to be paid. Especially the current or the rate at which the battery is being charged becomes an important factor as far as maintaining life and efficiency of the battery for a longer period of time is concerned.
In this article we are discussing one such circuit which is very easy and cheap by its design yet extremely accurate with its output voltage and current specs.
We all know how smart the IC 317 is and it’s no surprise why this device finds so many applications requiring precise power control.
The circuit diagram presented here shows how the IC LM317 ca be configured using just a couple resistors and an ordinary transformer bridge power supply for charging a 12 volt lead acid battery with utmost accuracy.
The IC is basically wired in its usual mode where R1 and R2 are included for the required voltage adjustment purpose.
The input power to the IC is fed from an ordinary transformer/diode bridge network; the voltage is around 14 volts after the filtration via C1.
The filtered 14 V DC is applied to the input pin of the IC.
The ADJ pin of the IC is fixed to the junction of the resistor R1 and the variable resistor R2. R2 can be fine set for aligning the final output voltage with the battery.
Without the inclusion of Rc, the circuit would behave like a simple LM 317 power supply where the current wouldn't be sensed and controlled.
However with Rc along with BC547 transistor placed in the circuit at the shown position makes it capable of sensing the current that’s being delivered to the battery.
As long as this current is within the desired safe range, the voltage remains at the specified level, however if the current tends to rise, the voltage is withdrawn by the IC and dropped, restricting the current rise any further and ensuring appropriate safety for the battery.
The formula for calculating Rc is:
R = 0.6/I, where I is the maximum desired output current limit.
The IC will require a heatsink for operating optimally.
The connected ammeter is used for monitoring the charge condition of the battery. Once the ammeter shows zero voltage, the battery may be detached from the charger for the intended use.
Parts List
The following parts will be required for making the above explained 12 volt constant current charger circuit using IC LM 317.
R1 = 240 Ohms,
R2 = 10k preset.
C1 = 1000uF/25V,
Diodes = 1N4007,
TR1 = 0-14V, 1Amp
