Why do good LED beads not light up when measured with a multimeter
LED beads, also known as light-emitting diodes, belong to a type of diode. When using a digital multimeter to measure LED beads, it is generally measured using its diode range. The red lead is connected to the positive pole of the LED bead, and the black lead is connected to the negative pole of the LED bead. The brightness (slightly bright) of the LED bead is judged by visual observation. Sometimes, why does a good LED bead not light up when measured with a multimeter? There are two main reasons.
1. The no-load voltage of the digital multimeter diode range is generally around 2.8V ± 0.2V.
Recall that we often use a multimeter beep range to locate faults, such as measuring wire continuity. When two test points are directly connected, the multimeter will beep. The buzzer level measures the resistance value between two points, and the multimeter will beep when the resistance value is less than about 70 Ω. Many multimeter diodes and beep gears are combined in the same gear, and can be switched through a button. In fact, the measurement principle is similar, with one actual voltage value and the other experiencing a beep.
The principle of the diode range of a multimeter is to have a constant current source internally. The no-load output voltage of the constant current source is about 2.8V, and the voltage value of different models of multimeter varies slightly, usually between 2.6V and 3V. The output current of the constant current source is about 1mA,
When measuring ordinary diodes, the red lead is connected to the positive pole of the diode, the black lead is connected to the negative pole, and the multimeter displays the voltage value as the voltage drop of the diode. Silicon diodes are generally around 0.7V, and germanium diodes are around 0.3V.
2. The forward voltage drop of LED beads is about 1.8V~3.5V
LED beads of different colors and models have different conduction voltage drops. Generally, LEDs used as power indicator lights have red, green, and yellow colors, and their conduction voltage drops are about 1.8V~2.2V.
The conduction voltage drop of white and blue LED beads is about 2.7V~3.3V, while household lighting LED beads are generally white LED beads with working voltages of about 3V, 2.8V, 3.3V, 3.5V, etc.
Comprehensive analysis: Understanding the principle of the multimeter diode range and the working voltage of the LED bead shows that the highest test voltage of the multimeter diode range is around 2.8V ± 0.2V. When the measured LED bead working voltage is within this voltage value, a 1mA working current will cause the LED to light up slightly. However, when the measured LED bead working voltage is high, such as above 3V, the LED bead will not light up slightly due to insufficient output voltage of the multimeter diode range. That's why LED beads are normal, but they don't light up when measured with a multimeter.
