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24V dc to 12V dc battery charger

Discussion in 'Power Electronics' started by energuy, Apr 6, 2016.

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  1. energuy


    Apr 5, 2016
    For the attached circuit, could someone please explain the purpose of the BC557, 1n457, as well as the 15ohm resistor at the bottom left of the circuit?
    Also, the LM338 is being used as a regulator, current controller, and circuit breaker. But im confused as to how it reduces the current as the charge of the battery is increased.

    I have wrapped my head around the functionality of all the other components, and just need a little direction.


    Attached Files:

  2. dorke


    Jun 20, 2015
    Welcome to EP.

    1.The PNP BC557 is simply the LED driver(on to indicate battery fully charged.)

    2.The diode 1n457 is there for the purpose of letting the LM301 output influence the LM338 only when the LM301 output is in the it's "-Vsupply state"(i.e close to GND).

    3.The lower 15-ohm is part of the resistor network to set the output voltage of the LM338.

    4.with relatively low battery voltage the charging current is (VoutLM338 -Vbatt)/0.2ohm
    As the battery charges it's voltage rises and thus the charging current is lower.

    5.Later on there is the point when the LM301 kicks in (at a fully charged battery).

    Can you take it from here?
    Hope this may help you in the investigation of the circuit.
    Last edited: Apr 7, 2016
    energuy likes this.
  3. energuy


    Apr 5, 2016
    Yes this helps alot. now i have added a diode with a voltage drop of 1V at the positive terminal of the battery to prevent any unnecessary draining of the battery.
    I am having trouble getting the output voltage set correctly(i currently have a 20ohm and 300ohm resistor in place of the 15 and 230 that are at Vout). Could you tell me what resistor values i am going to need in order to get the proper voltage?(around 14 Volts + 1V drop of the diode)
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