Measure the voltage drop across the LED in the forward direction, the so-called forward voltage. For this you need to place the LED in the forward direction so that she lights up. The forward voltage is about 2.0 Volt.
Circuit measuring the forward voltage at the red LED. (Enlarge) |
Circuit diagram measuring the forward voltage of a red LED. |
A normal silicon diode requires a forward voltage of 0.7 to let current pass. For a LED, the value is much higher.
One more feature the LED and silicon diode have in common. They have a very low internal resistance. Once the required forward voltage is met, the current through the diode rises rapidly with rising voltage.
A typical red LED however is damaged if currents exceed 20 mA. You therefore need to limit the current through the LED with the resistor R1...
