Calculating LED Resistor Values

Calculating LED Resistor Values

If you want to use a standard LED (as opposed to an automotive 12V LED) in a vehicle, you'll need to use a series resistor to prevent it burning out immediately. Here's the information you need to help you work out what value to use.

"12V" LEDs come with the resistor moulded into the case.

LED specifications

LEDs have just two really important parameters for this calculation:

• Forward voltage (Vf), typically around 2.5V
• Rated current (If), typically around 20mA

Often, you'll see the maximum current specified, with the rated brightness at a lower current. For example:

• Maximum current: 25mA
• Typical brightness: 1000mcd (at 20mA)

It's the figure in brackets you need.

The current drawn by an LED varies wildly over a small range of voltages, as it is a semiconductor. By using a series resistor, we "damp out" this sensitivity to voltage.

The calculation

We want our series resistor to drop the excess voltage (supply minus the LED forward voltage) and pass the LED's rated current (horribly over-simplified, but it works). Simple Ohm's law stuff - here's an example for the 2.5V, 20mA resistor above, running off a 12V supply:

Resistance = (Voltage / Current) = ((12V - 2.5V) / 0.02A) = 475 ohms

Choose the closest available standard resistor value: here, 470 ohms. Don't go much smaller than the value you calculate, but you can go significantly larger to be safe, although that'll cost you a bit of brightness.

Complications

Vehicle voltage

Vehicle voltage is only nominally 12V, and typically runs higher than that. It'd be wise to use 14V or 15V in the above equation, giving 575 or 625 ohms respectively. Err on the side of caution (larger values) unless you need to wring every last lumen out of the LED.

Resistor power rating

Because of the relatively large voltage drop across the resistor, it'll actually dissipate quite a large amount of power. At 14V, the 575 ohms resistor will be dropping 11.5V at 20mA, which is 230mW (voltage times current). A standard 1/4W resistor will get quite hot, and if it's wrapped in heatshrink or tape, or put in an enclosed space, may fail or burn stuff.

Get around this by using multiple resistors - they add up in series, or divide in parallel (if they're all the same). For example, to make 600 ohms, you could use three 200 ohms resistors in series or three 1.8k ohm resistors in parallel. Space them out to improve heat dissipation.

Alternative

Alternatively, you can do it properly with a constant current source. See here.