Battery charger current indicator

This circuit was inspired by a company's need for a DC current indicator small enough to be molded into a medium-sized 28V rechargeable battery assembly. They specified that the circuit should illuminate the LED when the battery charging current reaches 500mA or more. When using surface-mount components, the entire assembly should be smaller than a postage stamp. An LM4040 5V reference diode is used to provide a fixed 5V voltage to the circuit, and it also serves as the reference for the low-power voltage comparator when the voltage is divided down to only 5mV.

A 0.01-ohm shunt resistor is used to sample the flowing current. When the DC current flowing into the battery reaches approximately 500mA, the voltage is sufficient to trigger the comparator, which in turn turns on the green LED by driving the transistor. The entire circuit consumes approximately 140μA in standby mode, so it can be continuously connected to the battery. If you wish to light the LED at a lower current level, the value of the shunt resistor can be easily changed. IRC manufactures several small surface-mount shunt resistors in various resistance values.