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How it works? Battery Charging Ckt.

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heberjm8

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Hello! I've had a lot of fun in this experience. The project is all complete and everything is working within acceptable parameters. However, there is just one aspect of this project I am still unsatisfied with. That is, my understanding of the basic charging circuit which I borrowed from here:

Lead acid battery charger circuit

I'm uncertain how the amplifier and transistor work together to achieve the trickle charging effect. The amplifier in conjunction with the NPN transistor, I believe, is what makes the trickle charging feature possible. As I understand it, which is probably incorrect, the current through the transistor (base) and out the transistor (emitter) decreases as the output voltage from the common of the amplifier increases. The information contained on the website linked above is sufficient only to reconstruct the circuit, but it doesn't really explain exactly how everything works. I'm wondering if I could request one last favor on this topic and have someone explain further.

Completing a working battery charger was only half of the objective. The other half, and in many ways the primary incentive, was to learn more about circuit design. Please be as elaborate as possible in your explanation, if you would be so kind as to help. I can't thank you enough for your assistance. As always, I very much appreciate your time and generosity.
 
Battery Charger circuit

This circuit is a reasonable Lead-Acid battery charger. If you will notice that the base of the BC548 transistor has a 100Ω resistor connected to it. The other end of this resistor is connected to a 0.5Ω resistor. The emitter of the same transistor is connected to the other lead of the 0.5Ω resistor. The BC548 and the 2 resistors provide current limiting for the charger, in this case it is very close to Ilimit=0.7/0.5 or 1.4 amps.
The adjustment proceedure is incorrect. Potentiometer(R5) should be adjusted for a 12V battery to one of two values 13.5-13.8V for float charging and 14.3 to 14.7 for cyclic charge. I would recommend adjusting to the float charging as then you won't over charge the battery.
The LM317 with resistors R2,R3 and R5 are the voltage regulator.
 

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The transistor creates a (sort of) constant current source by reducing the voltage output of the LM317, which in-turn limits the initial charging current into battery to ~1/2A. Initially, the battery voltage will be less than 13.7V, so that is when the current limiter comes into play. As the battery voltage rises to near the LM317's set point of 13.7V, the charging current into the battery naturally begins decreasing, which turns off the NPN transistor, allowing the LM317's output to rise 13.7V, which is the recommended float voltage for most SLAs.
 
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