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LM317 as battery charger

Jorge Balderrama
Prodigy 30 points
Other Parts Discussed in Thread: LM317

According to the datasheet for LM317, figure 8. can be used as a battery charger circuit. What exactly does Note A mean? How is the output impedance controlled by Rs? If I can set Zout to a value can I then get a constant current through the battery while allowing the output to vary to achieve the 1.25V, Vadj-Vout difference? 

  • 0
    TI__Mastermind 22610 points

     An alternate definition would be : Zout = (ΔVbattery / ΔIbattery)

     " ... If I can set Zout to a value can I then get a constant current through the battery ..."

    With Figure 8? No.

    The circuit of Figure 12 could probably be modified.

  • 0 in reply to Donald Jones
    Prodigy 30 points

    Thank you for the explanation.

    In regards to figure 12, could the transistor be disconnected from the circuit and controlled manually with, lets say, a microcontroller and by then choosing the proper resistor for R3, control the amount of current through the battery and the duration?

  • 0 in reply to Jorge Balderrama
    TI__Mastermind 22610 points

    The current through R3 vs the Vbe threshold of the NPN transistor sets the peak current. This is the 'sense and control'.

    " ... could the transistor be disconnected from the circuit and controlled manually ..."

    Perhaps.

    You're down to basic 'sense' and 'control'. If the transistor is really "...disconnected from the circuit ...", then there is is no more (Ibattery x R3) vs (Vbe) for the sense and control. Since R3 is not connected to the base of the transistor  "... choosing the proper resistor for R3 ..." becomes a moot point. You could use A/D in your microcontroller to sense the voltage across some R3 value (IxR) and have the software determine when to output an appropriate signal to the base (or gate) of a transistor.

    Not sure where you want to go with this circuit, but it sounds like your current 'sense and control' circutry is going to be fairly complex with current sensing and PWM control. There may already be a dedicated battery management circuit that can do what you need.

    http://www.ti.com/lsds/ti/power-management/battery-management-products-products.page

     

  • 0 in reply to Donald Jones
    Prodigy 30 points

    Thank you once again, the explanations were very helpful.