Why is a good LED lamp not bright when measured by a multimeter?
LED beads, also known as light-emitting diodes, belong to a type of diode. When measuring LED beads with a digital multimeter, the diode range is generally used. The red probe is connected to the positive pole of the LED bead, and the black probe is connected to the negative pole of the LED bead. The brightness (slight brightness) of the LED bead is observed with the naked eye to determine. Sometimes, why do good LED chips not light up when measured with a multimeter? There are mainly two reasons.
1. The no-load voltage of the diode range of a digital multimeter is generally around 2.8V ± 0.2V.
Recalling that we often use the multimeter beep mode to locate faults, such as measuring wire continuity. When two test points are connected directly, the multimeter will beep. The buzzing mode measures the resistance value between two points, and the multimeter will beep when the resistance value is less than about 70 Ω. Many multimeters have their diode and buzzer modes combined in one position, which can be switched with a button. In fact, the measurement principle is similar, with one specific voltage value and the other triggering a buzzer.
The principle of the diode mode of a multimeter is that it has a constant current source inside, and the no-load output voltage of this constant current source is about 2.8V. The voltage value of different models of multimeters varies slightly, generally between 2.6V~3V. The output current of this constant current source is about 1mA,
When measuring a regular diode, the red probe is connected to the positive terminal of the diode and the black probe is connected to the negative terminal. The multimeter displays the voltage drop of the diode, which is generally around 0.7V for silicon diodes and 0.3V for germanium diodes.
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, the light-emitting diodes used as power indicator lights are red, green, and yellow in color, with a conduction voltage drop of about 1.8V~2.2V.
The conduction voltage drop of white and blue LED beads is about 2.7V~3.3V. Household lighting LED beads are generally white LED beads, with a working voltage of about 3V, 2.8V, 3.3V, 3.5V, etc.
Comprehensive analysis: Understanding the principle of the multimeter diode mode and the working voltage of the LED bulb, it can be known that the highest test voltage of the multimeter diode mode is around 2.8V ± 0.2V. When the measured working voltage of the LED bulb is within this voltage value, a working current of 1mA will cause the LED to light up slightly. However, when the measured working voltage of the LED bulb is high, such as above 3V, due to insufficient output voltage of the multimeter diode mode, the LED bulb will not light up slightly. That's why even though the LED beads are normal, they don't light up when measured with a multimeter.
