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[1 - Overview] [2 - My wreck!] [7 - Crazy Robin!] [9 - Candy's Ranger] [10 - Keypad entry system]

Meets and Outings
[11 - Centralia adventure Part I] [11 - Centralia adventure Part II]

How-To Articles
[3 - Overhead Console Install] [4 - Overhead Console Wiring] [5 - MAF mod] [6 - LED switch mod] [8 - Homebrew Remote Bass Control] [12 - Headlight/4x4 Switch LED Mod] [13 - General LED tutorial] [14 - EATC install how-to (pages 14-19)] [20 - IAT Resistor Mod (older engines only)] [22 - Automatic Power Windows] [23 - Cruise Control Pod LED's]

LED Tutorial

I've been telling some people about how to put LED's in specific places. But in this how-to I'm going to tell you how to "engineer" LED's in automotive applications. This way you can do various things with LED's on your own.

Now, to do this, you're going to have to learn to use the "specs" that come with LED's, or get a meter so you can measure some things about them. I'll cover all that here so you can do some simple engineering so that what you end up with WORKS as you'd like it to, with no guesswork involved -- and hopefully no parts destroyed in the process!

This tutorial may not be for everyone, and it may stink with my little hand drawn diagrams -- but hey, maybe it'll help you understand.

General background information

First, lets talk a little about just what an LED is. LED stands for "Light Emitting Diode" and that means it is a semiconductor component called a diode that emits light. Hey, I bet you had that one all figured out already! ;-)

Anyway, diodes have the characteristic of passing current in only one direction, and blocking it in the other direction. But unlike a "power" diode, LED's have a very weak blocking ability, and they break down at fairly low voltages. So the very first thing you need to know about LED's is that you MUST get them connected properly, or you can kill them dead. Kaput.

Connected properly means you need to have the "polarity" correct. Polarity refers to where you put the + voltage from the battery, and where you put the - voltage.

Diodes (and by inference, LED's) have two terminals called the "anode" and the "cathode". The anode is always the positive terminal, and the cathode is always the negative or "ground" terminal in an automotive application. To the right you see the symbols used to represent diodes, LED's and resistors in wiring diagrams. Note that you can clearly tell which side is which from the shape. The resistor does not care about polarity, and so it's shape doesn't show any hint of requiring polarity. In actual schematics, the (+) and (-) and the words anode and cathode are omitted.

The photo to the right shows an actual resistor (left) and an LED (right). Note that the LED has two leads and that one is longer than the other. This is the anode lead. The cathode lead, besides being shorter, can also be identified by looking closely at the base of the LED next to the lead. The little flange around the base of the LED has a flat molded or ground in it right next to the cathode lead. So, if you get the leads cut and don't know which is which, you can tell by looking at the base. You can't really see the flat in this picture, though.

Another thing about LED's is they try to keep a near constant voltage across their terminals. The voltage they try to maintain is different for different color and type LED's, but it is ALWAYS lower than the 12 volts in vehicles. This voltage characteristic in LED's is called the "forward voltage".

What this means is that if you connect 12 volts directly across a standard LED, it will try to pull the battery voltage down to its forward voltage -- basically a tug of war. Guess who will win? The battery will send so much current through the LED that it will burn out. Not good if you want more than a few seconds or minutes of light -- although they are very bright before they finally start to go out.

So we will combine a resistor and an LED together in vehicle applications to prevent the LED from drawing excessive current. The diagrams to the right show two different ways this can be done. It makes no difference if you put the resistor on the anode or cathode side of the LED -- it works the same. The diagram shows the resistor and LED symbols we just learned, and the lines between them represent a "connection" -- either by connecting the leads directly, or by using wires between them.

When you go to use an LED in a circuit, you need to know what resistor value to use. This you can calculate once you know (or assume) the specifications of the LED you're using. Once you have some pieces of information, you can use simple math to figure out the resistor value.

Now, I'm not going to teach you Ohm's Law, or teach you electrical theory in general, just give you some basic ways to figure out what you need.

Okay, here's the first configuration I showed you, with some things written in to show what we need to know. "Vf" and "If" are something you get from the manufacturer of the LED, or in the case of "Vf" you can measure it. "If" is usually in milliamps, which are thousandths of an amp (0.001 amp). Most LED's are rated for 20 mA which means 20 milliamps which for any math you do will be written as 0.020. "Vf" will be in volts and will usually be in the range of 1.5 to 4 volts. Typical values are between 2 and 3 volts. If you are not able to find the values for "If" and "Vf", you can assume 20 mA (0.020 amps) for "If" and 2.5 volts for "Vf". I'll also show you how to measure "Vf" in a later part of this tutorial. "Vdrop" and "R" I will explain shortly.

So what are "Vf" and "If"? Well, "Vf" is that voltage the LED tries to hold at. "If" is the current the LED is designed to run at. If you want the LED to be as bright as it should be, and to last a long time, you must pay attention to these things.

How to calculate what resistor to use

The resistor and LED form what's called a "series" circuit. In a series circuit, if you add up all the voltages across the different pieces, it should add up to the voltage being applied to the circuit (we'll call that 12 volts in this case).

So, let's say we have an LED, and the manufacturer says that "Vf" is 3 volts. That means that the voltage across the LED will be 3 volts. So, the rest of the voltage from the 12 volts being applied will be "dropped" by the resistor.

To figure out what "Vdrop" is, subtract "Vf" from 12 volts:

12-3=9 (so "Vdrop" is 9 volts)

Resistors are rated in ohms for how much they resist electrical current, and a power rating in watts for how much heat they can stand. They resist current by turning it into heat. The ohms value is represented by the "R" in our diagram.

We need to know "Vdrop" (which we just found) and "If" to compute the value "R". Let's assume the manufacturer says "If" is 20 mA (0.020 amps). To find "R", divide "Vdrop" by "If" like this:

R=9/0.020 (so R=450 ohms)

The nearest "standard" resistor value is 470 ohms, so that's what we'll use.

But resistors come in various power ratings, so will this resistor be okay? To find out how much power the resistor will have to be rated for, multiply "Vdrop" by "If" like this:

P=9 x 0.020 (so P is 0.18)

From this we can see a tiny 1/4 watt resistor will be adequate (since 1/4 watt is 0.25 watts, and 0.18 is less)

And from this, we now know a 470 ohm, 1/4 watt resistor will be fine for the application.

One final note on system voltage

Some of you no doubt wonder why I use 12 volts as the voltage for calculation. The nominal system voltage in automotive applications is more like 12.6 volts with the engine off, and 13.2 to 14.4 or so with the engine running. Many systems are rated at 13.8.

You may use any voltage in that range that suits you. I've found that in my truck the dimmer line runs a volt or more below the battery voltage, even at full brightness.

But another reason to use 12 volts for this tutorial (and pick 3 volts for the LED forward voltage) is that is makes the math simple and obvious right away for puposes of teaching.

Connecting LED's in series

This is actually pretty simple. Just a few "gotcha's" to watch out for.

Remember that I said that in a "series" type circuit (where things are just strung in a line), the current is the same in all parts of the circuit? Well, that's just as true when we connect LED's together. The diagram at the right shows a 3 LED series connection. The "If" specification will be the same as for a SINGLE LED. But now, when we figure out what resistor we're going to use, we will add up the individual "Vf" of each LED to make a "Vftotal" and use THAT to calculate the resistor value. You can see that other than putting 3 LED's in series, our diagram is really the same as before. If you've different types of LED's connected in series, make sure you use the right "Vf" for each one when you add them up. Obviously it's easier when they're all the same.

So, let's do the math. In our example, let's use our imaginary 3 volt "Vf" blue LED. There are 3 of them, so "Vftotal" is 9 volts. Let's say also the we're designing for the same nominal "If" of 20 mA (0.020 amps).

We need to find "Vdrop" and it will be the system voltage (12 volts in our example) minus "Vftotal". Doing the difficult math we get:

12-9=3 (so "Vdrop" is 3 volts)

We divide "Vdrop" by "If" to compute the resistance value "R" and get:

3/0.020=150 (so we need a 150 ohm resistor)

If you do the wattage calculation of "Vdrop" times "If" (3 x 0.020) you get 0.060 watts so a 1/4 watt resistor is overkill! But it's cheap and available and 1/8 watt resistors are tiny and not as available. The reason the wattage went down is that the LED's are using more of the energy in this circuit, leaving less to be gotten rid of as heat by the resistor.

"Gotcha's" in series LED connection

Now, clever person that you are, you say: "Hey, I can connector FOUR in series and that's exactly 12 volts drop! I don't need a resistor!"

Try this and fry your LED's! The reason is that the system voltage is actually going to peak at more than 12 volts, and the LED's are going to try to pull the battery voltage down to "Vftotal" and they are once again going to lose that tug-of-war. YOU MUST HAVE A RESISTOR!

Well, what about putting 10 LED's in series? That would mean an even smaller resistor, right? Nope. Ten 3 volt LED's in series is a "Vftotal" of 30 volts! Unless you're running a 42 volt electrical system (as some new cars will be), you can't do that!

System voltage when using series LED's

Beware coming too close to your system voltage. If you do, the current will vary widely with the small changes in battery voltage and you will stress your LED's. I'll post more on why this is later.

Generally, choose a number of LED's which gives your a "Vftotal" AT LEAST 2 or 3 volts LESS than the "true" battery voltage. In fact, I'm in favor of 3 to 4 volts, and maybe using 13.8 as the system voltage for this. Here's why:

Let's say you use 12 volts as your calculation above, and you get 150 ohms for the resistor. That's a standard value, you put it in, and all seems well. Then you start the vehicle.

When the alternator kicks in, the voltage in the vehicle will rise to something approximating 13.8 volts, plus or minus. This increases the current through the circuit. With the constant-voltage LED's in the circuit, the current increases based ONLY on the change in "Vdrop". Let's see what happens to our "Vdrop" using 13.8 volts

Remember "Vdrop" is the system voltage minus "Vftotal", so:

13.8-9=4.8 (so "Vdrop" is now 4.8 volts with the engine running)

We can figure the current in the circuit that results, by dividing "Vdrop" by "R":

4.8/150=0.032 (the current in the circuit has risen to 32 mA!)

This could be higher than the LED's are designed for, and could shorten their life substantially, especially in hot weather, or if they are tightly packaged.

So, you should probably design series LED circuits for the "nominal" voltage of 13.8 volts to BEGIN with to be safe. As an excersize, do the 3 LED configuration I did, but use 13.8 volts instead of 12 and find the value of "R" you should use.

Is it a good idea to design single LED circuits for the higher voltage? Perhaps, but it matters a lot less. The variations in current with the small changes in system voltage don't add up to much.

Good practice:

1) Design for the "nominal" system voltage

2) Don't "overdrive" your LED's

3) Leave a "cushion" of 2 to 4 volts between "Vftotal" and the system voltage

Have fun!

Example of using LED's

I modified a friends Ranger (Buckgnarly - see pictures here on Cardomain) to make the cruise control pods blue using blue LED's to replace the stock lamps. Here's what the mod looks like on the circuit board. It uses 276-316 blue LED's from Radio Shack, and 1/4 watt 680 ohm resistors. The positive terminal goes to the resistor. You must hook the LED up with the proper polarity. The lower pad where the resistor is soldered is positive. I've since done mine in blue as well, though I originally did them in red -- the blue just looks so nice. I'm also using 470 ohm resistors now to get a little more brightness.

I currently get all my LED's at http://www.allelectronics.com and they have some great surplus deals as well. There are cheaper LED's available, but these have been consistent and reliable.

How to find the "Vf" of a bargain LED

Let's look at this diagram again. Basically, all you need to do is to use a higher value resistor that will ensure your LED does not draw too much current when you test it. Just take a 1/4 watt, 1K ohm (1000 ohm) resistor along with the unknown LED and make some kind of temporary connection like this diagram. Hook up a 9 volt battery or 12 volts to the one end of the resistor, and the other end of the chain at the LED (observing polarity). Take a voltmeter and read the voltage across the LED -- that is, put one probe on one lead of the LED, and the other probe on the other lead of the LED. The voltage you read is approximate Vf. Vf does change with current, but the change is quite small, so this method works fine to get a Vf (or Vforward) for design purposes. You can use either configuration from the diagram to the right.