The circuit below is a simple way to indicate when DC current is flowing through a wire. The circuit is designed so it will turn on a LED indicator light, whenever the voltage drop across a shunt resistor exceeds about 0.1 volts. The value of the resistor can be selected based on the desired current flow.
Although the author has shown a circuit for 12v operation, the circuit can be used over a wide range from 3v to perhaps 24v.
A single NPN transistor detects the voltage drop across the shunt resistor. A silicon diode is used to bias the transistor base terminal at about 0.5v below the supply voltage. With such a bias voltage, only an additional 100mv voltage drop is needed to start turning on the transistor. Since both the transistor and the diode are nearly at the same temperature, their voltage drop with temperature should track. The circuit should therefore operate over a fairly wide temperature range.
Don’t expect precision from this circuit. With marginal current, the LED may only turn on partly. The component values shown are set for a 4 amp indication, but the LED may actually start turning on with only 3 amps.
Select the shunt resistor size as needed. As an example, if you want to detect a current flow of 50 amps, the shunt resistor should be about 0.002 ohms and should be able to dissipate 5 watts.
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