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LM317: LM317 Battery Charger Circuit

mustafayilmaz
Prodigy 140 points
Part Number: LM317

Hi,

I know that the Rs resistance should be 0.8 < Rs < 120 ohms. On the first page of datasheet, Rs resistance is selected as 0.2 ohms. 

1.2V / 0.2ohm =  6A 

Is there an explanation for this?

http://www.ti.com/lit/ds/slvs044x/slvs044x.pdf?ts=1588102791960

 

  • 0
    TI__Mastermind 37065 points

    Hello,

    The LM317 is configured to regulate the output voltage.
    The resistor dividers and 1.25 typical reference voltage provide about 1.375V on the output.

    The 0.2 ohm resistor provides current limiting in the case of a short on the output, such as if the battery was very low.
    The linear regulator is trying to regulate to 1.375V but lets say the battery is charged to something less than that.
    Using a typical maximum output current of 2.2A, we can see: 1.375 - (2.2*0.2) = 0.935V.
    So with loads less than 0.935V the linear regulator will be current limited to approximately 2.2A, otherwise it will be voltage regulated at 1.375V until the battery charges to this voltage.

    Thanks,

    - Stephen

  • 0 in reply to Stephen Ziel
    Prodigy 140 points

    Thanks Stephen.

    Did you mean 13.75 volts?

    When I calculate the output voltage according to this page, it should be 13.75 volts.

    https://diyaudioprojects.com/Technical/Voltage-Regulator/

  • 0 in reply to mustafayilmaz
    TI__Mastermind 37065 points

    Yes you are correct, this is a floating topology so the reference is across the high side setpoint resistor and not the low side setpoint resistor.
    This might be a mistake in the diagram as typical NiMH and NiCd batteries are in the 1.3-1.5V range.  If you were to use this linear regulator as a battery charging circuit, you would change the setpoint resistor values.  If you need something in the 1.3-1.5V range you might flip the values in the diagram so the 2.4k resistor is from Vout to the adjust pin, and 240 ohm resistor is from the adjust pin to RTN.

    Nice catch.

    Thanks,

    - Stephen