The Random Thoughts of a Geek heading for Banbury

# LED Resistors

Like the bearing size code, resister values for LEDs is something that I keep needing to work out, or just go back to my nominal somewhere between 200 Ω and 1 kΩ rule of thumb.

While I could work it out every time I need it using

`Resistor = (Supply Voltage – LED voltage(Vf)) / desired LED current(If)`

and then chose the nearest common resister value. I have just assembled the following table:

If 20 mA If 10 mA If 5 mA Colour Vf Red Orange Yellow Green Blue 3V3 5V 12V 3V3 5V 12V 3V3 5V 12V 1.7V 82Ω 180Ω 560Ω 180Ω 330Ω 1k2Ω 330Ω 680Ω 2k2Ω 2V 68Ω 150Ω 560Ω 150Ω 330Ω 1kΩ 270Ω 680Ω 2k2Ω 2.1V 68Ω 150Ω 560Ω 120Ω 330Ω 1kΩ 270Ω 680Ω 2k2Ω 2.2V 56Ω 150Ω 560Ω 120Ω 330Ω 1kΩ 220Ω 560Ω 2k2Ω 3.2V 5.6Ω 100Ω 470Ω 10Ω 180Ω 1kΩ 22Ω 390Ω 1k8Ω 3.2V 5.6Ω 100Ω 470Ω 10Ω 180Ω 1kΩ 22Ω 390Ω 1k8Ω

All of the above resistors can be 1/4 W resistors, most values will work on 1/8 W resistors however I only stock 1/4 W.

The values for series resistors for Blue and While LEDs when running off of a 3.3V supply need checked, since the Vf of these LEDs is typically around 3.2V, this leaves only 100mV across the resistor. As the Vf can vary quite a bit from device to device, as well as by temperature – there will be a LOT of variation in diode current…

If you are doing anything more than prototyping, I would recommend getting the data sheet for the LEDs you are using and test it out on a breadboard before you commit to PCB layout, and take that time to work out the best value of resister for your needs.

I made these Breadboard LEDs Previously with all the same 560 Ω value resistors, which as you can see from the above chart is set for 20 mA If at 12V and between 5 and 10 mA at 5 V so that one value is able to be used on a very wide band without causing long term damage to the LED.

If I want to make them dimmer I could have used a higher value resistor.