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LM317 Battery charger

Yusiang Leong91190
Prodigy 70 points
Other Parts Discussed in Thread: LM317

Based on figure 12 of the LM317 datasheet, it says that LM317 can be used as a battery charger (for what chemistry?). Can someone please elaborate on how this circuit works? Also, for the NPN current transistor, can i use a BC337 for current limiting? 

Thanks,

Yusiang Leong

  • 0
    TI__Guru**** 166116 points

    Yusiang,

    That application was probably created over 30 years ago (I do not know what type of battery was intended), but the operation is pretty simple.
    R1 and R2 set the output voltage. Because the LM317 tries to set the OUT - ADJ voltage to 1.25V, the current in R1 will be constant. The battery charging voltage is (1+R2/R1)*1.25V = 7V, if the current through R3 exceeds the turn on voltage of the BC337 (about 0.6V at 25C) then some of R2 current flows thorough the BC337 and the output voltage will be reduced to limit the current. However, the charging voltage can not be lowered to less than 1.25V.

  • 0 in reply to Ron Michallick
    Prodigy 70 points

    Ron Michallick said:

    The battery charging voltage is (1+R2/R1)*1.25V = 7V,

    Maybe you could update the datasheet as it is named "Figure 12. Current-Limited 6-V Charger Circuit"

    Thank you for your answer.

    Leong Yu Siang