If I have an LED with 3V forward voltage. A 12V power supply, and 4 LEDS that I want to put in series.

What size resistor do I use? If you do the normal calculations you wouldn't need any resistors, but is that right? How would you get the 20mA current needed?

 

The forward voltage rating is an average, not a precise figure and the effective series resistance of an LED reduces as the temperature increases causing overcurrent. Just wire them in two series strings of 2 and waste a little energy in 300Ω resistors.

 

Entirely correct. The most common difficulty with people and LEDs is that they can not understand that the LED has no helpful resistance. It is a "breakover" device. Once the voltage breaks over the threshold, the LED can not protect itself from excessive current. You absolutely must "waste" some voltage on a current limiting resistor or the only thing you will accomplish is smoke.

 

All of which is correct, so if you have 3.0V Vf and 12V you should only have X3 LEDs, which adds up to 9V, and the other 3V is used by a resistor to set the current. It drives beginners crazy, but there it is.

 

To be sure of good results, you need to know the possible range of the "3V" LED forward voltage, and that of the "12V" supply. The LEDs will vary a bit between individuals, and with temperature, and the supply voltage may vary a good deal, especially with a battery supply.

Suppose you choose a resistor to set the LED current to a safe value with maximum battery voltage and minimum LED voltages.

Now, check the LED current with that same resistor, but minimum battery voltage and maximum LED voltages (a little over-pessimistic, as the current will be less): is the current too low? Then you have to use fewer LEDs, or possibly consider a more complex active drive circuit.

 

3V is a pretty good margin. Lower voltage, such as 1V, are pretty finicky. I suspect the OP isn't going to have too much problems with 3v.

One last point from me, the current limiting resistor will tell you how much current is flowing through the LEDs. It is basic Ohm's Law, measure the current across it, divide by the resistance, and you have actual measure current. If there is a significant difference between what you are wanting (such as 20ma) and what you are measuring (say 30ma) then you need to go back and look at your basic assumptions.

Here is a simple circuit you can use to measure Vf of an individual LED.

The diodes are to prevent giving the LEDs too much back voltage if you connect it backwards (this can blow more sensitive LEDs).

A tutorial on LEDs and more....

LEDs, 555s, Flashers, and Light Chasers

 

Speaking of fluctuating voltage. I want to use these in an automotive application. Would it be best to use a voltage regulator or something?

I know that once the vehicle is running, voltage supply is 13v+.

I could design the circuit for the higher voltage and just have the LEDs dimmer when the car isn't running. or I could use a regulator. Unless someone else has a better idea.

 

Design for the higher voltage. I have noticed over the years that I really don't notice that the lights are dim when the engine is not running because I almost never drive the car without the engine running.

 

We no longer support ANY automotive modification topics.

LED lighting in automotive applications is an "automotive modification" subject.

The automotive environment is just too environmentally harsh for hobbyists to experiment in; and faults can have disastrous consequences. Even though you might think that the vehicle voltage will remain within a fairly narrow range of 10v-14.5v, there are "load dump" events where the system voltage can exceed 60v. Hobbyists circuits just won't stand up to those kinds of transients.

So, please don't attempt them.

 

Hello,

I am closing this thread as it violates AAC policy and/or safety issues.

Quote:
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