Why does a good LED lamp bead not light up when measured with a multimeter?
LED lamp beads are also called light-emitting diodes, which are one of the diodes. When using a digital multimeter to measure LED lamp beads, it is generally measured with its diode gear. The red test pen is connected to the positive pole of the LED lamp bead, and the black test pen is connected to the negative pole of the LED lamp bead. Judging by visually observing the brightness (slightly bright) of the LED lamp bead. Sometimes, why do good LED lamp beads not light up when measured with a multimeter? There are mainly two reasons.
1. The no-load voltage of the diode gear of the digital multimeter is generally about 2.8V±0.2V.
Recall that we often use the multimeter beep to locate faults, such as measuring the continuity of wires, etc. When two test points are connected, the multimeter will beep. The beep mode measures the resistance value between two points, and the multimeter will beep when the resistance value is less than about 70Ω. In many multimeters, the diode gear and the buzzer gear are combined in the same gear, and are switched by a button. In fact, the measurement principle is similar, one is a real specific voltage value, and the other is a buzzer.
The principle of the diode gear of the multimeter is that there is a constant current source inside. The no-load output voltage of the constant current source is about 2.8V. The voltage value of different types of multimeters is slightly different, generally between 2.6V and 3V. The constant current source The output current is about 1mA,
When measuring ordinary diodes, the red test lead is connected to the positive pole of the diode, and the black test lead is connected to the negative pole. The voltage value displayed by the multimeter is the voltage drop of the diode. The silicon material diode is generally about 0.7V, and the germanium material diode is about 0.3V.
2. The forward voltage drop of LED lamp beads is about 1.8V~3.5V
Different colors and types of LED lamp beads have different conduction voltage drops. Generally, light-emitting diodes used as power indicator lights are red, green and yellow, and their conduction voltage drops are about 1.8V~2.2V.
The conduction voltage drop of white and blue lamp beads is about 2.7V~3.3V. Household lighting LED lamp beads are generally white LED lamp beads, and their working voltage is about 3V, 2.8V, 3.3V, 3.5V, etc.
Comprehensive analysis: understand the principle of the multimeter diode and the working voltage of the LED lamp bead. The highest test voltage of the multimeter diode is about 2.8V±0.2V. When the measured working voltage of the LED lamp is within this voltage value, the working current of 1mA It will make the LED light up slightly, but when the measured LED lamp bead has a high working voltage, such as above 3V, the LED lamp bead will not be dimly lit due to the insufficient output voltage of the diode block of the multimeter. This is why the LED lamp bead is normal, but it does not light up when measured with a multimeter.
