Doubler Charger project

Doubler/Charger Project


    This post describes a project to trickle charge NiMH batteries as part of an UPS circuit.

   The supply voltage for the UPS comes from a 12 V wall adapter.  The battery pack of 8 NiMH batteries provides roughly 10.5 volts as the backup power.  There's not enough voltage "headroom" to connect a constant current circuit to charge the batteries.  Rather than use a separate supply, I decided to build a circuit to increase the voltage to allow the increased value to run the charger circuit.

   A well-known circuit uses a LM555 timer IC as an astable oscillator feeding a diode-capacitor network to double the input voltage to roughly 22 V.  This supplies an LM317 wired as a constant current source applied to the NiMH batteries.

   The oscillator function is controlled by a 3.3 volt logic signal from the UPS3 control board.  A logic low input from the UPS board allows the LM555 oscillator to run and the batteries to charge.







Click on the link to see a pdf of the Schematic.



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