Circuits designed by David Johnson, P.E.
Last Updated on: Monday, October 09, 2017 03:38 PM

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More Current Indicator Circuits

Battery Charge Current Indicator 7

This circuit was inspired by a company needing a DC current indicator light which was small enough to be molded into a medium size 28v rechargeable battery assembly.  They specified that they wanted the circuit to turn on the LED indicator light with 500ma or more of battery charge current.  When surface mounted components are used, the complete assembly should be smaller than a postage stamp.  A LM4040 5 volt reference diode used both to provide a fixed 5v for the circuit and when divided down to just 5mv, it also serves as a reference to a low power voltage comparator.

A 0.01 ohm shunt resistor is used to sample the flowing current.  When the DC current into the battery reaches about 500ma, the voltage is sufficient to toggle the comparator, which turns on the green LED using a driver transistor.  The whole circuit draws about 140ua in standby mode, so it can be left connected to the battery. If you want to turn on the LED at a lower current level, you can easily change the value of the shunt resistor.  IRC makes some small surface mounted shunt resistors of various resistances.

Click on Drawing Below to view PDF version of Schematic

Battery Charger Indicator Circuit designed by David Johnson

Current Sense Resistor

Current Sense Resistor

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