How to choose the resistance value of the current limiting resistor?

The selection of the resistance value for current limiting resistors is usually based on the following steps and principles:

1. Determine the parameters of the LED:
-Need to know the forward voltage (VF) and forward current (IF) of LED. These parameters can usually be found in the LED data manual.

2. Calculate the resistance value:
Using Ohm's law to calculate the resistance of the current limiting resistor, the formula is: \ [R=\ frac {V_ {\ text {supply}} - V_F} {I2} \], where \ (V_ {\ text {supply}} \) is the power supply voltage, \ (V_F \) is the forward voltage of the LED, and \ (I2 \) is the operating current of the LED.

3. Round to standard value:
The calculated resistance value may not be the standard resistance value and needs to be rounded to the nearest standard resistance value. For example, if the calculation result is 133 Ω, you can choose 130 Ω or 150 Ω.

4. Consider resistor power:
Calculate the power dissipation of the resistor to ensure that the selected resistor can withstand the power loss in the circuit. The power calculation formula is: \ [P=I2 \ times R \] or \ [P=\ frac {V ^ 2} {R} \], and ensure that the rated power of the selected resistor is higher than the calculated power.

5. Temperature changes and reliability:
Consider the temperature changes and temperature rise of the resistor during operation, select a resistor that meets the operating temperature requirements, and ensure that the resistor has sufficient reliability.

6. Price factor:
On the premise of meeting other requirements, choose resistors with lower prices as much as possible to reduce costs.

7. Color coding of resistors:
Understand the color coding of resistors to ensure that the obtained resistance values are consistent with the calculated results.

8. Actual value verification:
Use the same formula, but insert the actual resistance value instead of the calculated value, to verify the measurement results of the resistance at the highest and lowest tolerance limits.

By following the above steps, the resistance value of the current limiting resistor can be correctly selected to ensure the safe and stable operation of the circuit.