Never Ever Connect an LED Without a Resistor, Mostly. When hooking up an LED, you are always supposed to use a current-limiting resistor to protect the LED from the full voltage. If you hook the LED up directly to the 5 volts without a resistor, the LED will be over-driven, it will be very bright for a while, and then it will burn out. LEDs do not obey that law but resistors do. If the supply is 5V and the led forward voltage is 2V, then the resistor will drop the other 3V. If you want 15mA to flow through the led & resistor, the resistor value needs to be R = V / I in this case R = 3 / 0.015 = 200 Ohms. We’re using two resistors, R1 a 1K Ohm resistor (top) and R2 a 2.2K Ohm resistor (bottom) to create a voltage divider. The legs of R1 and R2 go into the same row of the breadboard. Into R1 we thats not really great advice, not all arduinos have a resistor on pin 13 (mine doesnt) yes you need one. Actually it's worst then that, it's very poor advice and false and misleading. Even boards that have an internal resistor/LED wired to pin 13, does not protect pin 13 at the connector, which wires directly to the chips pin. Resistor Voltage = Input Voltage – LED V f. Voltage Across Resistor = 4.5V – 3.4V. So about 1.1V will be dropped across the resistor. Now that we have this, we can use Ohm’s Law to calculate the needed resistance! Resistance = Voltage / Current (in Amps) Resistance = 1.1 / .02 (20mA) Resistance = 55 Ohms. Depending on the LED, the Blue LEDs typically have a higher voltage drop, which means a lower resistor value. If you assume 5 volts applied and 3V across the LED, that leaves 2V across the resistor. If you want 20mA, that's 2V/0.02A = 100 Ohms. Thomas499 January 6, 2015, 5:05pm 3. I bought two starter packs so I have plenty of led's. The LED load resistor creates a voltage drop so the LED turn signals blink at a proper speed. If the LED load resistor was not installed, the LED turn signal would blink too rapidly, and would eventually be destroyed by the high voltage. Need to upgrade? Check out this post to learn how to make the switch to LEDs. Common Resistor Applications 5. vin17285. • 7 yr. ago. Resistor restrict the flow of electricity. They are really useful when trying to prevent sensitive electronics from breaking. We can use resistors to lower the voltage and also the current if we need to. There's are loads of videos that explain the concept and the math. NeoMarxismIsEvil. xwwM.