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DC Current Indicator 6 --
February 7, 2009
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| 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 I show a circuit for 12v operation, the
circuit can be used over a wide range from 3v to perhaps 24v. |
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| 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. |
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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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Click on Circuit Below to view PDF of Schematic |
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eMail David A.
Johnson, P.E. about this circuit |
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