The above is a general classification of the LED lights that are usually seen. I just said that low-power LED lights can be used for power indicator lights and running indicators, so let's talk about the tips of using this low-power LED light. Let's take a direct plug 3mm red LED light as an example.

    First of all, if you want to use this LED light, you must know its characteristics. This is a data manual about red in-line 3mm light-emitting diodes found on the Internet. We can see from the voltage-current transmission characteristic curve of the manual that the cut-off voltage is 1.6V, and when it is less than 1.6V, the light-emitting diode does not conduct. . When the turn-on voltage is 1.8V, the current is 10mA, and then as the turn-on voltage increases, the current also increases proportionally. At this time, a question arises, as the voltage increases indefinitely, will the current increase indefinitely? By continuing to check this manual, it can be seen that the limit forward continuous current is 25mA. At this time, returning to the previous voltage-current transmission characteristic curve can be seen, when the forward continuous current is 25mA, the forward voltage at this time is 2.1 V. The voltage and current parameters of the diode can be obtained in turn: 0.01mA≤IF≤25mA, 1.6V≤UF≤2.1V.

    We know its features just now, and we will start using it now. In our weak current system, the power supply is mostly 5V or 3.3V. Here we take the 5V power supply as an example. It can be seen from the above parameters that when the 5V power supply is directly applied to the two ends of the LED light, a large forward continuous current will be generated. We also know that when the forward continuous current exceeds 25mA, the LED light will be damaged. At this time, a resistor needs to be connected in series to limit the current and share a part of the current to the resistor to ensure that the LED lamp is used normally without damage. This resistor is the legendary "current limiting resistor", so what we really want to say today Circuit.

 This is the most classic and commonly used LED light circuit.

    So now the problem comes again. In the above circuit, the power supply is determined to be 5V, and the LED light is also determined. Then, how big should I choose the current limiting resistor? At this time, you need to determine the resistance value according to the two parameters mentioned above: IF and UF. The calculation is as follows:

The voltage across the resistor: 5V-2.1V≤UR≤5V-1.6V;

Resistance size: (5V-2.1V)/25mA≤R≤(5V-1.6V)/0.01mA;

You can get: 116Ω≤R≤340kΩ.