Connecting LEDs to your Power Supply
By Bones (Ed T. Toton III)
September 24, 2002
Some people like blinking LEDs. Some people like them a lot.
Others just want to rig up a new power light in a different color.
Whatever your reason is, you may find yourself wanting to hook up
new LEDs in your computer, and obviously the only place you're going
to easily draw power from is the power supply.
If you've bought a fairly high-voltage blue LED, for instance, you
may find that simply connecting it to the same pin-outs as your old
power LED might not work, because your motherboard is assuming a
lower-voltage LED to be present. Connecting it directly to the power
supply might be your only option.
However, if you connect an LED without a resistor, you're most likely
going to burn-out or even explode your LED. Let's take a look at what
"LED" means. It stands for "Light Emitting Diode". The key word is the
last one: Diode. Diodes are designed to allow electricity to pass through
them in one direction, but not the other. They're not meant to absorb or
use power, just to control the direction of current. Light-Emitting Diodes
are a slightly different case, in that they do use some power and put it
to work, but they still allow most of the power they're given to pass
through. That means that connected by themselves, they will draw an enormous
amount of current, overheat, and possible burn up.
Many standard diodes, rectifier diodes, etc, tend to cause a voltage drop
on the line of about 0.5 to 0.7 Volts. LED's are all different, depending
on their designed brightness and color. Low-power red LEDs tend to start
with a voltage drop of about 1.5 to 1.7. Many of the blue LED's require
3.7 to 5 volts. And nearly all LEDs use between 20 and 30 milliamps (0.02 to 0.03 Amps).
Luckily Radio Shack has started printing these specs on the LED packages
that they sell, and if you order from a larger electronic component distributor,
the specs are often in the catalog.
So what do we do with these numbers? Well, the equation is very simple:
Rs = (Vin - Vled) / Iled
- "Rs" = Resistance needed
- "Vin" = Input voltage (either 12v or 5v, depending on which wire you used from the power supply)
- "Vled" = Voltage of the LED (typically 1.7v to 5v)
- "Iled" = Current of the LED (usually 0.02A to 0.03A)
So for instance, if you're using the 5v power lead, with a blue LED that
uses 3.7v and 20mA, the equation would be:
Rs = (5 - 3.7) / 0.02
Rs = 65 Ohms.
And there's your answer. For a plain red LED, it might be more like this:
Rs = (5 - 1.7) / 0.02 = 165 Ohms
Once you have that figured out, all you need to do is connect a resistor of
approximately the correct resistance in series with the LED in question. You
can connect resistors in series with each other to add them up. Or if you can't
find any resistors exactly fitting your need, you can often approximate. In the
case of this latter red LED example, you can probably get away with a 110 Ohm
resistor, for instance.
It should be noted, however, that self-blinking LEDs are a completely different
story. They usually have the appropriate resistor built into them, and can often
be connected directly to a 5v line safely.
Now you can safely connect LEDs without them melting or popping or slowly burning
[PS- I found a mod-site that has a handy
web-based LED calculator you can use, and a site with
cheap bulk LEDs]