Question about circuit and datasheet

Discussion in 'The Projects Forum' started by Macabra, Jun 3, 2009.

  1. Macabra

    Thread Starter Active Member

    May 31, 2008

    I'm trying to make a small circuit using this chip (ICL7673). The circuit that is attached is suppose to switch to the secondary source when the primary source is out, in essence, it is a battery backup circuit. However, the part that I don't understand is the Rc that needs to be added in order to be able to recharge the backup battery. How can I determine what resistor to use if I plan on using a regular wall wart (12V Dc) as Primary and a rechargeable 9V battery as secondary? I do have the data sheet for this but it does not say what recommended value for resistors to use. All it states is "A trickle charge system can be implemented by placing a resistor and a diode."

    Thank for any help on this matter
  2. balisong


    Feb 26, 2008
    Rc limits the current going to the battery so it isn't overcharged.
    Assuming a voltage drop of 0.7V across the diode, there's 2.3V across Rc.
    Select a resistor value of Rc = 2.3/Ic, where Ic is the trickle charge for the battery.
    The charge rate will depend on the capacity and chemistry of the battery.