The calculation method and example of the current-limiting resistor in LED lamp circuit design

The calculation method and example of the current-limiting resistor in LED lamp circuit design
In electronic circuit design, the use of LED (Light Emitting Diode) is becoming more and more widespread, and the current limit of LED is an indispensable part of the design. In order to ensure the service life and luminous effect of the LED, we often need to add a current-limiting resistor between the LED and the power supply to control the current passing through the LED. This article will explain in detail the calculation method of the current-limiting resistor in LED lamp circuit design and illustrate it with examples.
1. Working principle and current limitation of LED
The LED is a diode that requires a certain forward voltage and forward current during operation. The forward voltage of the LED increases with its brightness, while the forward current determines the brightness of the LED. If the current passing through the LED exceeds its maximum allowable value, the LED may overheat or even be damaged. Therefore, to protect the LED, we need to add a current-limiting resistor between the LED and the power supply.
2. Calculation method of current-limiting resistor
The calculation of current-limiting resistors is usually based on the forward voltage and maximum allowable current of the LED. Assuming the forward voltage of the LED is Vf and the maximum allowable current is If, then the current-limiting resistor R can be calculated by the following formula:
[ R = \frac{V_{cc} - V_f}{I_f} ]
Where, Vcc is the power supply voltage.
Example 1: Assuming the power supply voltage is 5V, the forward voltage of the LED is 2V, and the maximum allowable current is 20mA, the calculation of the current-limiting resistor is as follows:
[ R = \frac{5V - 2V}{20mA} = \frac{3V}{0.02A} = 150\Omega ]
Therefore, a current-limiting resistor of about 150Ω needs to be selected.
3. Example design
To better understand the above calculation method, we design a simple LED circuit. Suppose we have a 5V power supply and need to drive an LED with a forward voltage of 2V and a maximum allowable current of 20mA.
Circuit design

Power supply: 5V
LED: forward voltage 2V, maximum allowable current 20mA
Current-limiting resistor: calculated 150Ω

Circuit connection as follows:

Connect the positive pole of the power supply to one end of the current-limiting resistor, and the other end of the current-limiting resistor to the positive pole of the LED.
The negative pole of the LED is connected to the negative pole of the power supply.
The negative pole of the LED is also connected to the other end of the current-limiting resistor to ensure that the current flows from the power supply to the LED.

4. Matters needing attention

In practical applications, in addition to calculating the resistance value, it is also necessary to consider the voltage withstand value of the LED to ensure that the power supply voltage does not exceed the voltage withstand range of the LED.
When selecting resistors, in addition to meeting the current limitation, it is also necessary to consider the power of the resistor to ensure that it can withstand the maximum current in the circuit.
For high-brightness LEDs, it may be necessary to use multiple current-limiting resistors to limit the current to avoid overheating of a single resistor due to excessive current.

Through the above introduction, we can see the importance of current-limiting resistors in LED circuit design. Proper selection and calculation of current-limiting resistors can not only ensure the normal operation of the LED but also extend its service life, improve the safety and reliability of the circuit.