That is not accurate. All LEDs are not equal - some require higher voltage
differential than others to achieve similar output.
Check out the history of white LEDs for illumination ;-)
/daytripper
LEDs draw current and have a maximum current rating. No matter what
voltage source you use inside the PC, there must be a current limiting
resistor. For LEDs which connect to a header on the motherboard, Asus
has already put a current limiting resistor on the motherboard. If you
want to do illumination with LEDs, then you must provide the resistor
yourself. Here is an example of a circuit:
resistor |\ |
+5 or +12V -----------/\/\/\-----------| \|------------- GND
(must be greater | /|
than Vfb to work) |/ |
^ ^
/ \
Anode / \ Cathode
(longer lead?) (shorter lead?)
The LED has a "forward bias voltage" rating Vfb. In fact, this voltage
is a function of the color of the LED, and for the old style colored
LEDs, the slope of the voltage versus wavelength plot is Planck's
constant (blue needs the most Vfb). Some of the newer LEDs (high
power illumination types) have forward bias voltages of 5 or 6 volts,
and I don't know if they are LEDs in series or a radically different
internal structure. A white LED is sometimes an ultraviolet LED with
a phosphor painted over the front of it to make visible light. So, it
has the high forward bias voltage of a device in the blue end of the
spectrum.
So, let us say you are using an old red LED. You download the datasheet
and it says "50 ma max" and "Vfb = 1.7 volts". Further, you intend to
use the +5 supply pin from a drive connector as the power source. The
smallest value of resistor that can be used is (+5 - 1.7)/0.050 or
66 ohms. Using a resistor a little higher than this, will be less hard
on the LED, and won't be running it at maximum current all the time.
So, say a 100 ohm or 150 ohm resistor could be used. The resistor has
to be able to handle the power, where the power in the resistor is
P = V*V/R = 3.3*3.3/100 = 0.1 watts for the 100 ohm resistor choice.
In that case, a 1/8 or 1/4 watt resistor will be fine for the application.
Other factors in LED selection, are whether the LED has a lense or not,
and how wide an angle it disperses the light over. A narrow beam will be
blindingly bright, but only looking straight into it. A Lambertian output
will spread over a wide angle, like 40 to 60 degrees off axis.
LEDs also have a maximum reverse voltage rating. For many LEDs, this is
5V applied in reverse across the LED. Motherboards use no more than +5V
to power LEDs, so when connecting case LEDs to the motherboard, this is
why connecting the LED backwards to the motherboard does no harm -
because the reverse voltage spec hasn't been exceeded. If you use a
+12V source to power the LED, and the maximum reverse voltage spec is
less than 12V, be careful to connect the LED the correct way the first
time. I don't know what the failure mode of a reversed LED is, so
I can't tell you if they fail shorted or open, or whether the resistor
helps to protect them when the power is reversed.
To shop for LEDs, you can download a catalog, like the Digikey catalog.
http://dkc3.digikey.com/pdf/T032/DigiKey.pdf (40MB !!) You can also
view selected page numbers, if you don't want to browse the catalog.
The following device is pretty tame and ordinary.
Pg.1135 has blue 60 degree lense LEDs (10mm diameter $3.05)
There are much higher power devices available, but the price is also
higher. If you go to lumileds.com, you can read about their 1W and
5W devices, which are suitable for use as sources of illumination.
These are serious LEDs, and at 5 Watts, you can expect to supply
somewhere around 0.7 amps of current at a Vfb of 7 volts to these
things. The 1 watt and 5 watt devices require a heatsink on the LED,
to keep it from overheating. They should also be treated with respect
(don't point them into people's eyes - use them for indirect lighting
etc.)
One source is here (but this supplier wasn't found in a Google search
or in Resellerratings.com, so seek confirmation that this company is
for real):
http://www.quickar.com/lux.php
Star V 5 Watt, Blue (460-490nm) Lambertian (60 degrees off axis spread)
LXHL-LB5C $43.20 . You'll definitely need +12V source to feed this thing,
as the source voltage must be greater than Vfb of 7 volts. A 10 ohm
resistor would have to be rated for 2.5 watts, so buying a 10 ohm (5 watt
or greater) resistor will help dissipate the heat. The resistor will
get hot. Insulate all leads so no accidental contact between +12V and
the chassis or motherboard can happen ! Smoke/fire if you aren't
careful ! Read up on how to heat sink these devices as well - I think
even the heatsink should not be in electrical contact with your case,
so perhaps it should be held in place with nylon tywraps or something.
http://www.lumileds.com/luxeon/products/star_index.html (Star V datasheet)
Luxeon (lumileds.com) devices seem to be hard to find from legitimate
distributers, which is kind of strange. Due to the high prices, I expect
there are look-alike ripoffs galore out there.
There is also a forum for people who like LEDs. As I said earlier,
do some research before dealing with any suppliers they happen to mention.
http://www.candlepowerforums.com
Have fun and don't blind yourself,
Paul
