Why do good LED lamp beads not light up when measured with a multimeter?
LED lamp beads are also called light-emitting diodes, which are a type of diode. When using a digital multimeter to measure LED lamp beads, the diode setting is usually used for measurement. The red test lead is connected to the positive electrode of the LED lamp bead, and the black test lead is connected to the negative electrode of the LED lamp bead. Judge by observing the brightness (slightly bright) of the LED lamp beads with the naked eye. Sometimes, why do good LED lamp beads not light up when measured with a multimeter? There are two main reasons.
1. The no-load voltage of the diode position of a digital multimeter is generally about 2.8V±0.2V.
Recall that we often use the buzzer setting of a multimeter to locate faults, such as measuring the continuity of wires. When two test points are connected, the multimeter will buzz. The beep mode measures the resistance value between two points. 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 can be switched by pressing a button. In fact, the measurement principle is similar. One actual voltage value and the other a buzzer.
The principle of the multimeter's diode gear 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 an ordinary diode, the red test lead is connected to the positive electrode of the diode, and the black test lead is connected to the negative electrode. The voltage value displayed by the multimeter is the voltage drop of the diode. Silicon material diodes are generally about 0.7V, and germanium material diodes are 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, the colors of light-emitting diodes used as power indicator lights are red, green and yellow, and their conduction voltage drop is 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's diode range and the working voltage of the LED lamp bead. It can be seen that the maximum test voltage of the multimeter's diode range is about 2.8V±0.2V. When the measured working voltage of the LED lamp bead is within this voltage value, the operating current of 1mA The LED will be slightly bright, but when the measured working voltage of the LED lamp bead is relatively high, such as above 3V, the LED lamp bead will not be slightly bright because the multimeter diode output voltage is not enough. This is why the LED lamp beads are obviously normal, but they do not light up when measured with a multimeter.
