First off, and LED is a very versatile, low power, low voltage light source. the average Life expectancy of an LED far exceeds the life of an incandescent lamp. I have used LED's since I was about 10 (im 23 now) and I have never seen an LED burn out unless it was abused. Most standard LEDS have similar specs, and ratings. ill explain, but before that, we need to look into how an LED works.
An LED is a Light Emitting Diode. A diode is an electronic device that allows electrical current to pass through one way but not the other. A light emitting diode does the same thing, but emits light as current passes through the junction.
Junction you ask? A diode is comprimised of 2 different mixtures of Silicone, this allows a "P" segment and a "N" segment. A diode is a chunk of each, side by side. The silicone mixtures determine the specs of the diode, as well as how big the chunks are. The mixtures also determine which way the electrons flow through the junction as well.
When you order LEDs, you should have a spec sheet with all the information about the said LED. Lets take a look at some of the specs and their functions and features.
LED Specs:
Forward Voltage and Forward Current
These are the most important specs on the sheet. Forward voltage is usually a volt or two or three. This is the target operational voltage of the LED. Forward current is...you guessed it...the target current for the LED. The only thing is tha the forward current has to be less or equal to what the LED is rated for. Passing more current than is indicated by this spec may burn out the LED instantly or gradually over time. Passing WAY too much current all at once may induce intense heat, explosion (well, a small pop anyways), or generally just make the LED unusable.
By looking at the Spec sheet, say we have these specs in front of us:
FV: 1.8V
FI: 20mA (.020A)
You can say that if you put EXACTLY 1.8V across the diode, it will draw EXACTLY 20mA and be happy. But this is dangerous because the slightest fluctuation of voltage across the diode will rapidly increase the current. After the LED current spec is met, and excessive current turns the LED into a direct Short, burning out the LED, resistors, power supplies, or batteries or whatever yo u have going on that the time. Ill explain in a bit how to engineer your way around this to make LEDs work safely.
Reverse Voltage and Reverse Current
LED's are used in the direction that allows current to flow, so not much attention is payed to the reverse specs. We know that Diodes allow current to flow one way and not the other, so, hooking them up backwards wont burn them out. That is, unless the Values for Voltage and Current exceed the Reverse specs.
A LED that is "reverse biased" (hooked up backwards) will not glow, beacuse current will not flow. If your LED's dont light up, make sure the Anode of the LED is more positive than the Cathode of the LED.
Anode -> +
Cathode -> -
LEDs are sometimes sold with resistors. Resistors Drop the Voltage and Current down to a suitable level for the LED. to figure out what Resistor value you need, you need a little math; But first, i will introduce to you the resistor Color Code:
By looking at this diagram, you can determine the value of your resistor. Lets start a few examples:
Band 1 : Brown
Band 2 : Black
Band 3 : Red
Band 4 : Gold
We see that the Brown means 1, so the first number is 1, and black is 0, so so far we have 10. The third band is the multiplier. the multiplier means you add that number of zeros. Red is "2" so you add 2 zeros. that gives us
1 0 00
1000 Ohms (or 1K Ohms)
Next example, will be yellow, violet, brown.
Yellow is 4
Violet is 7
Brown is 1 (but being the thrid band, means you add "1" zero to the end) (0)
so the value is 470 Ohms
Ok, now ill show you the math involved and you can try this if you'd like.
Say we have a Fresh 12V battery, an LED, and some resistors.
to light an led, we need some specs:
1.7V, 20mA
if you connect the LED across the battery, it will burn out because obvously 12V > 1.7V on the spec. so we need a resistor to drop the voltage and limit the current to the LED.
Ohms Law is used to determine the correct value of the resistor.
The ohms law formula looks like this:
V
--------
R | I
V is Volts
R is Resistance (in Ohms)
I is Current (in Amps)
we know the value for V is
12V (from the source)
and we know the value that we want for I is 20mA (which is .020 Amps)
.020
the "V" is over the "I" in the formula, this means we divide the two. It is read from the top down, so we have Volts divided by Amps
or
12 / .020 = 600
the value we recieve is the remaining piece of the formula, R or Ohms.
600 Ohms is the resistor value we need to safely light the LED. any resistor value less than 600 Ohms allows too much current to pass through the LED.
to do this, the circuit would look like this:
Now, people ask if Each LED requires it's own resistor. The answer to that is yes and no. if the LED's are wired in paralell, then each LED needs its own resistor. If they are in series, then the current is the same throughout the entire series branch, so only one resistor is needed.
Here is an example of Paralell LED's:
***Notice that each LED has it's own current limiting resistor.
Advantages of Paralell LED's
1) There is no limit to how many LED's you can add
2) If one LED burns out, the rest will stay lit, easily indicating which LED has a problem.
Disadvantages of Paralell LED's
1) More Parts are required in circuit
2) Less efficient because more power is dissipated with more resistors.
3) Takes more time to assemble with added parts.
This is an example of LED's in series
***Notice that there is only one Resistor
Advantages of Series LEDs:
1) Less Parts, Simpler Hookups
2) More efficient
Disadvantages:
1) You are Limited to how many LEd's you can have in a series branch
2) Resistor that's required is a lesser value, meaning that it may get warm, or may need to be of a higher wattage rating.
3) If one LED goes out, they all go out.
The reason why you are limited to how many LED's are in series is because of the forward Voltage of the LEDs. For now we'll say that ours are 1.6V. Each LED you add to a series circuit divides the Voltage up among the LEDs. Whatever Voltage is Left over is comprimised by the resistor.
So we have 12V Total from the battery and our LED's have a forward voltage of 1.6
1.6 goes into 12 7 Times, with a little left over, so we can have 7 of our LEDs in series.
1.6 Times 7 = 11.2 Volts
12V - 11.2 = .8V
We now need a resistor to decrease the Voltage by .8V
By using Ohms Law, we know the current we need is 20 mA (.020) so we can divide the Volts by the amps to get a resistance.
.8V Divided by .020 Amps = 40 Ohms.
we need a 40 Ohm resistor to give each LED the 20mA it needs to light up safely.
(and by looking at the color code, a 40 Ohm resistor will have the color bands yellow, black, black.)
Yellow: 4
Black: 0
black: (zero zero's) = .0
40.0
CLOSURE!!!
In closing, i would like to remind you that for Auto applications, beware that the Voltage Fluctuates between 11 to 14.5V, so be sure that your LEd's and resistors can work in that range. If you choose a resistor and LED combination that works with 12V, it will work until you start your car up, then when the alternator gives it 14.1V, the LED and resistor my burn out, or get hot and burn whatever they are mounted in.
Also be sure to FUSE ALL YOUR WORK!!!
Only you can prevent car fires!!! (lol)
I hope you have an better idea now of how LED's are used in electronics.
Josh Nolan
Electronics Technician by Trade
Any comments, ideas, Questions or whatever about anything is welcome!
im on AIM:
Metalfan036
I have a myspace for auto electronics. check my sig.
email: DigitalDomination@live.com
