Resistor values for white LED circuits

02/03/2008 Resistor values for white LED circuits
The question is often asked - "How big a resistor do I need to protect an LED ?"

I run all my LEDs with a 12.2 Volt DC power supply (from a PC power supply)

When wiring up a building for interior lights I sometimes also include a diode to protect the circuit from being connected with the incorrect polarity. A diode will consume about 0.7V Volts so it can affect the size of the resistor.

White LEDs typically have a forward voltage of 3V - I find that the one I have are about 2.8 V so this is the table I use for my white LED circuits. Note: red, green, yellow LEDs have a forward voltage of about 2V and may need slightly higher resistor values.

I also find that a current of 20mA makes the LEDs too bright so I run mine at about 15 or 16 mA

This is the table that I use:

Number of white LEDs	Without Diode	With a diode
1	634 Ω	590 Ω
2	470 Ω	392 Ω
3	261 Ω	220 Ω
4	68 Ω	20 Ω