Universal Battery Charger

When power is applied to the circuit, SCR1 is off so there is no bias-current path to ground;

thus, LM317 acts as a current regulator. The LM317 is connected to the battery through steering

 diode D1, limiting resistor R1, and bias resistor R2. The steering  diode prevents the battery

from discharging through the LED and the SCR when power is removed from the circuit. As the

battery charges, the voltage across trip-point potentiometer

R5 rises, and at some point, turns on the SCR.

Then, current from the regulator can flow to

ground, so the regulator now functions in the volt

age mode. When the SCR turns on, it also provides

LED1 with a path to ground through R3. So. when

LED1 is on, the circuit is in the voltage-regulating

mode; when LED1 is off, the circuit is in the cur

rent-regulating mode.