This is formula from Ohm's Law that we will use to calculate the resistor value: Operating Voltage(Vs) (Volts) = 12 Volts.LED Forward Voltage (Vf) (Volts) = 2.0 Volts.LED Forward Current (If) (mA) = 20mA (0.020A).If you choose to use only 1 resistor, size it for the smallest voltage and current value, but be sure to increase the size of the resistor so that it can dissipate enough heat for all 3 elements (multiply the power dissipation by 3).įor this example, we are going to calculate the resistor value required for the red LED segment. Does this mean that we need 3 separate resistors? Yes - interesting things will happen if only one resistor is used on the cathode, for example, the Green and Blue will have slightly less light output and the Red will appear to be brighter than if we used 3 individual resistors. Notice that 3 separate voltage values are listed. The forward current is listed as 20mA for all 3 segments. The datasheet for the RGB Common Cathode LED shows that the forward voltage is 2.0 volts for the Red segment, 3.0 volts for the Green segment and 3.0 volts for the Blue segment. To calculate the resistor value (in ohms) and size (in watts), we need to know the following: There are 2 steps to selecting the correct resistor:Ģ) Calculate Power dissipated by the Resistor (in watts) Step 1: Calculate Resistor Value (Ohms)
COMMON CATHODE LED STRIP HOW TO
How to calculate the resistor value and size: Reducing the current to just below the maximum forward current gives the LED slightly less light output, but a much longer life. Running an LED at its maximum specified forward current could result in a shorter life with reducing light output over time. It's a good practice to build in a safety margin when it comes to limiting current. A properly sized current limiting resistor will dissipate the extra current (in the form of heat), that would normally be flowing through the LED when running higher than the rated forward voltage. If you draw more current than the maximum listed "forward current” in the datasheet, the LED will self-destruct in a less than spectacular flash of light with more smoke than you care to inhale. Why a resistor is needed:Īn LED draws more current as the voltage being used to run it is increased.Ĭurrent causes the LED to heat up high current results in too much heat. The steps below will work for Common Cathode or Common Anode LEDs. – It comes in a package of 3 LEDs for a very affordable price. This may not seem so easy if you are new to electronics or have limited experience with LEDs but don't worry – the math required could be done by your 5 th grade son or daughter.įor this example, I'll be using Vetco's VUPN6563 Ultra-Bright RGB LED (Common Cathode). Running an RGB LED on 12 Volts DC is just like running any other LED, except you need 3 current limiting resistors instead of 1.Įven though there are 4 leads on an RGB LED, It is still a "Simple" matter of using Ohm's law and some information from the LED's datasheet to calculate the correct value and size for the current limiting resistors. This post was originally written on November 8, 2012.
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