(1) Detection with a multimeter. Using a pointer multimeter with a ×10kΩ stop, you can roughly judge the quality of the light-emitting diode. Under normal conditions, the forward resistance of the diode is tens to 200kΩ, and the value of the reverse resistance is ∝. If the forward resistance value is 0 or ∞, and the reverse resistance value is small or 0, it is easily damaged. This detection method cannot see the luminescence of the light-emitting tube in the field, because × 10kΩ block cannot provide a large forward current to the LED.

If there are two pointer multimeters (preferably the same model), it is better to check the luminescence of the light-emitting diode. Connect the "+" terminal post of one multimeter to the "-" terminal post of the other meter with a wire. The remaining "-" pen is connected to the positive pole (P region) of the light-emitting tube under test, and the remaining "+" pen is connected to the negative electrode (N region) of the light-emitting tube under test. Both multimeters are set × 10Ω stop. Under normal circumstances, it can emit light normally after turning on. If the brightness is very low, or even does not emit light, both multimeters can be dialed to ×1Ω If, if it is still very dark, or even does not emit light, it means that the light-emitting diode performance is poor or damaged. It should be noted that two multimeters cannot be placed at × 1Ω at the beginning of the measurement, so as not to damage the light-emitting diode by excessive current.

(2) External power supply measurement. The optical and electrical characteristics of light-emitting diodes can be measured more accurately with a 3V regulated voltage source or two dry batteries in series and a multimeter (either analog or digital). To do this, simply connect the circuit as shown in Figure 10. If the measured VF is between 1.4~3V and the luminous brightness is normal, it can indicate that the luminescence is normal. If VF=0 or VF≈3V is measured and does not emit light, the light-emitting tube is broken.