Multimeter Test Method for Standard Light-Emitting Diodes (LEDs)
Measurement with a Fluke Digital Multimeter (×10kΩ Range)
You can roughly test whether an LED is good or bad using an analog multimeter with a ×10kΩ resistance range.Under normal conditions, the forward resistance of the diode ranges from several tens of kΩ to 200 kΩ, and the reverse resistance is infinite (∞).If the forward resistance is 0 or ∞, or the reverse resistance is very small or 0, the diode is likely damaged.
This method cannot show the actual lighting of the LED, because the ×10kΩ range cannot supply enough forward current to the LED.
Measurement Using Two Multimeters Together
If you have two analog multimeters (preferably the same model), you can properly check whether the LED lights up.Use a wire to connect the "+" terminal of one multimeter to the "–" terminal of the other.Connect the remaining "–" probe to the anode (P-region) of the LED, and the remaining "+" probe to the cathode (N-region).Set both multimeters to the ×10Ω range.Under normal conditions, the LED will light up when connected.If the brightness is very low or it does not light up at all, switch both meters to the ×1Ω range.If it is still dim or unlit, the LED has poor performance or is damaged.
Note: Do not start with the ×1Ω range to avoid excessive current that may damage the LED.
Measurement with an External Auxiliary Power Supply
You can measure the optical and electrical characteristics of the LED more accurately using a 3V regulated power supply (or two series-connected dry batteries) plus a multimeter (analog or digital).Connect the circuit as shown in Figure 10.
If the measured forward voltage VF is between 1.4 V and 3 V and the brightness is normal, the LED is working properly.If VF = 0 or VF ≈ 3 V and the LED does not light up, the device is defective.
