Basics of LEDs

LEDs are available in red, orange, amber, yellow, green, blue, white, UV, IR and even more colors. In general the Blue and white LEDs are much more expensive than other colors.

 

The actual color of an LED is determined by the semiconductor material and not by the coloring of the plastic body. LEDs of all colors are available in uncolored packages which may be diffused (milky) or clear. The colored packages are also available as diffused (the standard type) or transparent. They also make multi colored LEDs. The most popular are tri colored LEDs.

 

The most popular type of tri-color LED has a red and a green LED combined in one package with three leads. They are called tri-color because mixed red and green light appears to be yellow and this is produced when both the red and green LEDs are on. There are also, what are called Bi-Color LEDS but these tend to be less popular because you can only have one color on at a time.

 

Sizes, Shapes and Viewing angles of LEDs

LEDs are available in a wide variety of sizes and shapes. The 'standard' LED has a round cross-section of 5mm diameter and this is probably the best type for general use, but 3mm round LEDs are also popular. As well as a variety of colors, sizes and shapes, LEDs also vary in their viewing angle. This tells you how much the beam of light spreads out. Standard LEDs have a viewing angle of 60° but others have a narrow beam of 30° or less (great for spotlights.)

 

An LED contains an anode and a cathode. Generally you can determine the anode and cathode by the length of the wire coming out of the base of the LED. The anode (+) is the longer of the wires, while the cathode (-) is the shorter. This holds true even for multi-color LEDs. The only difference between the wiring of a single Color LED and a Multi-color LED is the number of anodes.  If the wires appear to be the same in length coming out of the LED Base then there is another way to tell which wire is the anode and which is the cathode. If you look at the sides of the led from the top, along the base you will notice a flat spot. This flat spot corresponds to the cathode side of the led.

 

Now most LEDs operate at about 2V (white and blue can be 4V.) One thing you do need to keep in mind is that even if you plan on running your circuit at 5V and have 2 LEDs in series that are 2.5V each you will still want to put a resistor on the cathode side of the circuit. In my experience, when a resistor is not used the LEDs have a tendency to blow after a period of use or after an initial quick power on. The resistor can be 1k ohms if the power voltage is going to be 12V or less. I normally opt to calculate the resistor that is needed which is pretty easy and thee are tools online that makes this even easier with diagrams, and color codes of the resistor needed.

 

An LED must have a resistor connected in series to limit the current through the LED, otherwise it will burn out almost instantly.

 

The resistor value, R is given by:

S = supply voltage

V_L = LED voltage (usually 2V, but 4V for blue and white LEDs)

I = LED current (e.g. 20mA), this must be less than the maximum permitted If the calculated value is not available choose the nearest standard resistor value which is greater, so that the current will be a little less than you chose.

 

In fact you may wish to choose a greater resistor value to reduce the current (to increase battery life for example) but this will make the LED less bright.

 

An example If the supply voltage V_S = 9V, and you have a red LED (V_L = 2V), requiring a current I = 20mA = 0.020A,

R = (9V - 2V) / 0.02A = 350, so choose 390 (the nearest standard value which is greater).

 

Working out the LED resistor formula using Ohm's law Ohm's law says that the resistance of the resistor, R = V/I, where:
  V = voltage across the resistor (=V_S V_L in this case)
  I = the current through the resistor So   R = (V_S - V_L) / I

 

If you wish to have several LEDs on at the same time it is possible to connect them in series. This prolongs battery life by lighting several LEDs with the same current as just one LED. All the LEDs connected in series pass the same current so it is best if they are all the same type. The power supply must have sufficient voltage to provide about 2V for each LED (4V for blue and white) plus at least another 2V for the resistor. To work out a value for the resistor you must add up all the LED voltages and use this for V_L. Example calculations:

A red, a yellow and a green LED in series need a supply voltage of at least 3 × 2V + 2V = 8V, so a 9V battery would be ideal.
V_L = 2V + 2V + 2V = 6V (the three LED voltages added up).
If the supply voltage V_S is 9V and the current I must be 15mA = 0.015A,
Resistor R = (V_S - V_L) / I = (9 - 6) / 0.015 = 3 / 0.015 = 200,
so choose R = 220 (the nearest standard value which is greater).

 

Avoid connecting LEDs in parallel!  Connecting several LEDs in parallel with just one resistor shared between them is generally not a good idea. If the LEDs require slightly different voltages only the lowest voltage LED will light and it may be destroyed by the larger current flowing through it. Although identical LEDs can be successfully connected in parallel with one resistor this rarely offers any useful benefit because resistors are very cheap and the current used is the same as connecting the LEDs individually. If LEDs are in parallel each one should have its own resistor.