The reason why the LED lamp bead does not light up when measured by a multimeter
LED, also known as light-emitting diode, is a type of diode, so the characteristics of diodes also have the same characteristics as light-emitting diodes.
Diode characteristics
The diode has the characteristics of forward conduction and reverse cut-off, that is, it only allows current to flow from one end of the diode to the other end; if the direction is opposite, the current will be blocked and cannot pass. In order for the diode to achieve forward conduction and reverse cut-off, a condition must be met—the voltage applied in the forward direction is greater than the conduction voltage.
What does this mean? for example:
As shown in the figure above, assume that the forward voltage of the diode is 2V. When the voltage at terminal a is 2V higher than the voltage at terminal b, the diode is turned on, and the current flows from terminal a to terminal b; if the voltage is less than 2V, the current cannot pass.
How do you understand it?
We can regard the diode as an iron door, which can only be pushed from the outside to the inside, and vice versa, it cannot be pushed from the inside to the outside. This is like the unidirectional conduction characteristic of a diode.
Similarly, because the iron door is relatively heavy, although it can be pushed open from the outside to the inside, it requires a certain amount of force to do so. This is like the turn-on voltage of a diode.
Multimeter to measure light-emitting diodes
There is a diode file in the multimeter, which is generally integrated with the buzzer file. The no-load voltage of the diode gear of the digital multimeter (that is, the voltage between the two test leads when the multimeter is not connected to the resistor) is very low, generally about 3V, and different multimeters may have slight differences.
The turn-on voltage of different colors and models of LEDs is different. Generally, the voltage drop of red, yellow and orange LEDs is about 1.8-2.2V; the voltage drop of white, green, blue and pink LEDs is 2.7- About 3.3V; household LED lamp beads are generally about 3V.
Summarize
Because the conduction voltage of different light-emitting diodes is different, this parameter range is very large, so it is normal that the LED lamp bead does not light up when measured by a multimeter. Even if the same multimeter measures LED lamp beads of different colors, their brightness is also different. Generally, red is the brightest, blue is the darkest, and purple cannot even be lit.
It can be seen from this that the LED lamp bead does not light up when the multimeter measures it. The possible reason is that the positive and negative poles of the test pen and the diode are reversed.
For the pointer multimeter, the black test lead is connected to the positive pole of the diode, and the red test lead is connected to the negative pole of the diode; for the digital multimeter, the red test lead is connected to the positive pole of the diode, and the black test lead is connected to the negative pole of the diode.
If the multimeter does not light up when the test leads are connected correctly, the main reason is that the output voltage of the multimeter is insufficient and the conduction voltage of the diode is high. It is recommended to change to ×100 or ×1K for measurement.
