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| Symbol
of LED
LEDs emit light when an electric
current passes through them. |
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Testing
an LED
Never
connect an LED directly to a battery or power
supply!
It will be destroyed almost instantly because too
much current will pass through and burn it out.
LEDs must have a resistor in
series to limit the current to a safe value, for
quick testing purposes a 1kΩ resistor is
suitable for most LEDs if your supply voltage is
12V or less.
Remember to connect the LED
the correct way round! |
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Colors
of LEDs
| LEDs are available in
red, orange, amber, yellow, green, blue and white.
Blue and white LEDs are much more expensive than
the other colors.
The color of an LED is
determined by the semiconductor material, not by
the coloring of the 'package' (the plastic body).
LEDs of all colors are available in uncolored
packages which may be diffused (milky) or clear
(often described as 'water clear'). The colored
packages are also available as diffused (the
standard type) or transparent. |
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Tri-color
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.
The diagram shows the
construction of a tri-color LED. Note the
different lengths of the three leads. The centre
lead (k) is the common cathode for both LEDs, the
outer leads (a1 and a2) are the anodes to the LEDs
allowing each one to be lit separately, or both
together to give the third color. |
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Avoid
connecting LEDs in parallel!
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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. |
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| Type |
Color |
IF
max. |
VF
typ. |
VF
max. |
VR
max. |
Luminous
intensity |
Viewing
angle |
Wavelength |
| Standard |
Red |
30mA |
1.7V |
2.1V |
5V |
5mcd @ 10mA |
60° |
660nm |
| Standard |
Bright red |
30mA |
2.0V |
2.5V |
5V |
80mcd @ 10mA |
60° |
625nm |
| Standard |
Yellow |
30mA |
2.1V |
2.5V |
5V |
32mcd @ 10mA |
60° |
590nm |
| Standard |
Green |
25mA |
2.2V |
2.5V |
5V |
32mcd @ 10mA |
60° |
565nm |
| High intensity |
Blue |
30mA |
4.5V |
5.5V |
5V |
60mcd @ 20mA |
50° |
430nm |
| Super bright |
Red |
30mA |
1.85V |
2.5V |
5V |
500mcd @ 20mA |
60° |
660nm |
| Low current |
Red |
30mA |
1.7V |
2.0V |
5V |
5mcd @ 2mA |
60° |
625nm |
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| IF max. |
Maximum forward
current, forward just means with the LED connected
correctly. |
| VF typ. |
Typical forward
voltage, VL in the LED resistor
calculation.
This is about 2V, except for blue and white LEDs
for which it is about 4V. |
| VF max. |
Maximum forward
voltage. |
| VR max. |
Maximum reverse
voltage
You can ignore this for LEDs connected the correct
way round. |
| Luminous intensity |
Brightness of the LED
at the given current, mcd = millicandela. |
| Viewing angle |
Standard LEDs have a
viewing angle of 60°, others emit a narrower beam
of about 30°. |
| Wavelength |
The
peak wavelength of the light emitted, this
determines the color of the LED.
nm = nanometer. |
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Pin
connections of LED displays
|
There are many
types of LED display and a supplier's
catalogue should be consulted for the pin
connections. Like many 7-segment displays,
this example is available in two versions:
Common Anode (SA) with all the LED anodes
connected together and Common Cathode (SC)
with all the cathodes connected together.
Letters a-g refer to the 7 segments, A/C is
the common anode or cathode as appropriate
(on 2 pins). Note that some pins are not
present (NP) but their position is still
numbered.
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