Generic LED resistance question
 

I'm playing with a couple of these 12V LEDs and need to reduce their brightness by 50% under certain conditions. They're currently installed out of reach so before I have to dismantle the dash again, I'm hoping that someone might be able to reasonably say what resistor I would need to achieve this? They're 5mm diameter lamps. Thanks.

Dougie.

You would need to know the forward voltage of the LED i.e. the voltage that it turns on and also the current rating in mA to drive it, different colour LED's has different forward voltages and current ratings.

Try this for starters... http://led.linear1.org/1led.wiz

To properly control brightness an LED driver is used with variable PWM.

Yep thanks, I have a variable PWM but it's physically big for the job.

Great link, thanks Phil. The LED is described as 0.1W operating between 6/12V which taken at face value, means an 8mA rating at 12V. If that's correct, then guessing 9V as the forward voltage on the linear1 calculator, produces a 390kΩ resistor. Am I on the right lines? Maybe I should just (re)consider a small PWM.

Dougie.

Typically the forward voltage of the led will be in the range of 2.8-3.6V and the forward current 20-30mA. These values, as already mentioned, vary according to many things, but colour is one of the factors.
So if you assume a 12V supply, from the car. A forward voltage of 3.2V and a forward current of 25mA, you would be looking at a 390 Ohm resistor.

(12V-3.2V)/0.025 Amps=352 Ohms preferred value resistor = 390 Ohm. :D

To reduce the brightness, assuming you are going to keep the forward voltage the same, the current has to be reduced, by increasing the resistance. :D

That LED will already have a resistor built-in, as it has been designed to be operated with just a simple 12v supply.

As has already been mentioned, you really need a PWM supply whereby you can alter the duty cycle to control the brightness.


Depending on what you are trying to achieve, I'd take a look at an Arduino (Mini Pro is small, but a standard Nano has a USB socket for ease of programming), they are very small and can be programmed with lots of helpful information available on the internet.

So can you build in a variable resistor. I notice that house lighting LEDs can have a variator switch now.

Thank you gentlemen, all very helpful. :) Decision made - I'm using an off-the-shelf 60x30x15mm variable PWM which will a) fit under the dash out of sight, and b) let me adjust the LED brightness to what I want before burying it. And it's about three quid.

Dougie.