You can connect the LED directly, without a resistor, only to a power source with a large internal resistance. In all other cases, current limiting is necessary to prevent diode failure. Determination of the value of the current-limiting resistor is made by calculation.
Instructions
Step 1
It is impossible to determine the nominal LED current empirically. This parameter of the device must be asked from the seller when purchasing it. If you know the type of diode, enter it into any search engine - there is a chance that there will be reference data for it, including the rated current.
Step 2
If there is no information about the LED, we can assume that the device in the SMD type case has a nominal current of 3 mA, for a round 3 mm diameter - 5 mA, for a rectangular cross-section of 3 by 5 mm - 10 mA, for a round one with a diameter of 5 or 10 mm - 20 mA.
Step 3
The voltage drop across an LED depends on its color. For infrared, it is 1, 2 V, for red - 1, 8, for green - 2, 2, for blue, white and violet - from 3 to 4.
Step 4
Determine the voltage drop across the current-limiting resistor using the following formula: Ur = Up-Ud, where:
Ur is the voltage drop across the resistor, V;
Up - power supply voltage, V;
Ud - voltage drop across the LED, V.
Step 5
Use Ohm's Law to calculate the resistance of the resistor. Before calculating, convert the nominal current of the LED into amperes, for which its value, expressed in milliamperes, divide by 1000. For example, 20 mA = 0.02 A. Then determine the value of the resistor using the following formula: R = Ur / Inom, where:
R - the required resistance of the resistor, Ohm, Ur - voltage drop across the resistor, calculated according to the previous formula, V;
Inom - LED rated current, A.
Step 6
The last step is to calculate the power of the resistor. To do this, multiply the voltage drop across the resistor by the current flowing through it (also previously converted to amperes): P = Ur * Inom, where:
Ur is the voltage drop across the resistor, V;
Inom - the rated current of the LED, A. It is possible to use a higher power resistor, but not to use a lower one.
Step 7
To increase the overall efficiency of the system when the supply voltage is too high, LEDs with the same rated current can be connected in series in such a way that this chain has about 2/3 of the supply voltage, and the resistor - about 1/3. In this case, the nominal diode voltages must be added together before performing the calculation.