## LED Constant Current Tester

- Details
- Published: Saturday, 16 April 2011
- Written by Jon Chandler
- Hits: 7672

As we saw in the LED Calculations article, determining V_{f} acoss the LED is helpful to calculate the needed current-limiting resistor.

This simple constant current tester will allow rapid determination of V_{f} 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 V_{f}. 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 |