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Limiting current into an LED is very important. An LED behaves very differently to a resistor in circuit. Resistors behave linearly according to Ohm's law: V = IR. For example, increase the voltage across a resistor, the current will increase proportionally, as long as the resistor's value stays the same. Simple enough. LEDs do not behave in this way. They behave as a diode with a characteristic I-V curve that is different than a resistor.
For example, there is a specification for diodes called the characteristic (or recommended) forward voltage (usually between 1.5-4V for LEDs). You must reach the characteristic forward voltage to turn 'on' the diode or LED, but as you exceed the characteristic forward voltage, the LED's resistance quickly drops off. Therefore, the LED will begin to draw a bunch of current and in some cases, burn out. A resistor is used in series with the LED to keep the current at a specific level called the characteristic (or recommended) forward current.
Using the circuit above, you will need to know three values in order to determine the current limiting resistor value.
i = LED forward current in Amps (found in the LED datasheet)
Vf = LED forward voltage drop in Volts (found in the LED datasheet)
Vs = supply voltage
Once you have obtained these three values, plug them into this equation to determine the current limiting resistor:
Also, keep in mind these two concepts when referring to the circuit above.
What current limiting resistor value should you use if you have one LED and want to power it with a supply voltage of Vs = 3.8V?
To calculate the current limiting resistor, you first need to look in the datasheet (always RTFM first!) for the LED's recommended forward voltage and forward current specifications. In this example, they are 3.1V and 30mA respectively. Don't forget to convert all of your units to Volts, Amps, or Ohms! e.g. 1mA = 0.001Amps
If you plug the values into the above equation, you get:
23.3 Ohms might be an odd value to find, so round up to the next highest common value.
What if you wanted to power a high power LED? What should the power rating for the resistor be?
The resistor's purpose is to limit current and thus uses some amount of power. You need to be sure the wattage (power) rating for your resistor is sufficient for the power being used. The equation for power is:
Let's say you are using the LED above with a supply voltage of 12V, an LED forward voltage of 3.9V, and a total forward current of 1400mA. What power rating should you choose for your resistor?
The resistor has a voltage drop and so does the LED. So, according to Kirchoff's Voltage Law:
If you solve for the voltage drop of the resistor, you get 8.1V. Now we have enough information to plug the numbers into the power equation (be sure to convert all units to Amps and Volts, e.g. 1400mA = 1.4A):
The calculated value is approximately 12 Watts. Generally, you should get a resistor wattage rating close to twice the calculated value. So a resistor in the neighborhood of 20-25 Watts would be sufficient. Also, keep in mind, a 20-25 Watt resistor will be pretty darn big!