Testing Techniques for LEDs with a Multimeter
① A Light-Emitting Diode (LED) is a current-injection light-emitting device. Light emission occurs when excited electrons inside the semiconductor crystal transition from high energy levels back to low energy levels and emit photons; this process is commonly known as spontaneous emission transition.
② When forward bias voltage is applied to the PN junction of an LED, the injected minority carriers recombine with majority carriers (electrons and holes) to produce light. It should be noted that numerous high-energy particles spontaneously emit light waves with angular frequency ν = Eg/h. These wave trains have no fixed phase relationship and may have different polarization directions. Furthermore, light emitted by each particle propagates in all possible directions. This process is spontaneous emission, and the emission wavelength can be calculated by the formula: λ (μm) = 1.2396 / Eg (eV).
③ Light-emitting diodes (LEDs) are generally made of materials such as gallium phosphide arsenide and gallium phosphide. A PN junction exists inside the LED with unidirectional conductivity. When an LED conducts in the forward direction, it emits light. Brightness increases with rising forward current, and light color depends on wavelength.
④ Test Method for Ordinary LEDs with a Multimeter Test using the R×10k range An analog multimeter with the R×10k range can preliminarily judge whether an LED is intact. For a good LED, forward resistance is tens to 200 kΩ, and reverse resistance is infinity (∞). If forward resistance reads 0 or ∞, or reverse resistance is very low or zero, the LED is defective. This method cannot visually verify LED illumination because the R×10k range cannot supply sufficient forward current to the LED.
⑤ Dual-Multimeter Combined Test If two analog multimeters (preferably of the same model) are available, the luminous performance of the LED can be inspected more effectively. Use a wire to connect the "+" terminal of one multimeter to the "−" terminal of the other. Use the remaining "−" test lead to connect to the anode (P-region) of the LED under test, and the remaining "+" test lead to the cathode (N-region). Set both multimeters to the R×10 range. A good LED will light up once the circuit is connected. If brightness is very low or there is no light, switch both meters to the R×1 range. If it remains dim or fails to light up, the LED has poor performance or is damaged. Note: Do not start testing directly in the R×1 range, as excessive current may destroy the LED.
⑥ External Auxiliary Power Supply Test A 3V regulated power supply or two series dry cells together with a multimeter (analog or digital) can be used to accurately measure the optoelectronic characteristics of the LED. Wire the circuit as shown in Figure 10. If the measured forward voltage VF ranges from 1.4 V to 3 V and the LED lights normally, the device works well. If VF equals 0 or is close to 3 V and no light is emitted, the LED is damaged.
