Resistors in Light Emitting Diode (LED) Circuits. An LED (Light Emitting Diode) emits light when an electric current passes through it. The simplest circuit to power an LED is a voltage source with a resistor and an LED in series. Such a resistor is often called a ballast resistor. For example, if you are using an LED in a display or lighting application and you need to adjust the brightness to match the surrounding environment, placing the resistor after the LED can allow you to do so with more precision. Another potential benefit of placing the resistor after the LED is that it can offer some protection for the LED The Arduino is a convenient source of 5 Volts, that we will use to provide power to the LED and resistor. You do not need to do anything with your Arduino, except plug it into a USB cable. With the 270 Ω resistor in place, the LED should be quite bright. If you swap out the 270 Ω resistor for the 470 Ω resistor, then the LED will appear a Calculate the resistor value using ohm's law, which states: voltage = current times resistance. The required voltage drop is four volts, as the battery provides nine volts and five volts is needed to power the LED. Putting the voltage drop and an example LED current of 20 milliamps into the equation gives: 4 = 0.02 x R. To increase the current for the LEDs there are two options - either connect more Arduino pins in parallel or use a transistor. For Arduino pins you do not need a current limiting resistor. However you should ensure all PWM pins switch synchronously - it is not hard and there will be no doubt the pins are safe. Left on its own, an LED will get hot, which will make it conduct more, which will make it hotter, until it burns up. Transistors behave similarly, btw. Aside from biasing them, they typically need resistors to limit current. The resistor just limits the current for a given voltage. \$\begingroup\$ The LED strip is built to run on 2.5 A at 12Volts. That is equivalent to a resistor of 4,8 Ohms. Given that we are still supplying the thing with only 12Volts, then if you put a resistor in line with the LED strip, the resistance will go up. Less current, same voltage=lower power use. A resistor is required to limit the current through each LED, except in the case that LEDs are in series. The parallel connection you are describing will not work because individual LEDs have slightly different characteristics. That means that the current through the resistor would not be divided equally among them. 1vXTN.

do i need a resistor for led