LED Constant Current Tester

LED-wikipediaAs we saw in the LED Calculations article, determining Vf acoss the LED is helpful to calculate the needed current-limiting resistor. 

This simple constant current tester will allow rapid determination of Vf for an unknown LED.  Power is supplied by a 9 volt battery and converted to a constant current by a 7805 voltage regulator configured as a current source. This circuit supplies a constant current to the LED; the voltage across the LED will be Vf.  An external DVM is used to measure the resulting voltage.  Disconnect the LED when not in use.

Four test currents are supplied by the tester: 5 mA, 10 mA, 20 mA and 25 mA.  Meausure the forward voltage at the current closest to the desired operating point.  Use linear interpolation between ranges if needed for optimal results.

Switch Current
none 5 mA
S1 10 mA
S2 20 mA
S1 & S2 25 mA


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