# Led voltage

Discussion in 'Abit' started by Bell C, Jul 7, 2004.

1. ### Bell CGuest

Could anyone enlighten me, I am in the process of reversing a case so that
the front becomes the back so that the pci output cards are at the front.
But I would like to replace the LED,s that come off the MOBO, but what
voltage are they 5 volt or 12 volt. Or should I put it a different way what
is the voltage output sent to the LED,s on an ATX case?

Bell C, Jul 7, 2004

2. ### PaulGuest

In article <KNGGc.194\$>, "Bell C"
<> wrote:

> Could anyone enlighten me, I am in the process of reversing a case so that
> the front becomes the back so that the pci output cards are at the front.
> But I would like to replace the LED,s that come off the MOBO, but what
> voltage are they 5 volt or 12 volt. Or should I put it a different way what
> is the voltage output sent to the LED,s on an ATX case?

Here is a typical LED driver circuit:

+5V
|
|
Resistor
|
|
+ Plus terminal
|
-----
\ / LED
---
| Minus terminal
|
| / C
Logic ---|/
Drive |\
v E
|
|
GND

To make a LED work, you must apply a minimum voltage across
it, called the forward bias voltage. The voltage needed is a
function of the color the LED emits - the closer to UV, the
more voltage required. The slope of the line is actually
related to Planck's constant.

Once the minimum voltage is met, current begins to flow. The
LED has a low internal resistance, so applying just a little
more voltage, makes the current grow quite large. That is
why the current limiting resistor, in the top of the circuit,
is there. You should always use at least one resistor, between
the power supply, and the LED.

The circuit above has three components. I have shown an NPN
bipolar transistor, like a 2N2222a for example. On a motherboard,
for example, there is a LED for the HD activity, and the transistor
shown above, switches to ground any time it is required to light
the LED. If the LED is dumb, like the green standby LED on the
motherboard, then the transistor is replaced with a straight piece
of wire to ground.

This is your most basic, "dumb", LED circuit.

+5V
|
|
Resistor
|
|
+ Plus terminal
|
-----
\ / LED
---
| Minus terminal
|
|
GND

Here is some data for a typical LED (this one is from Agilent)

HLMP-K101 AlGaAs Red wavelength=637nm output=45mcd Vf=1.8v If=20ma

This LED has a minimum voltage of 1.8 volts. In the circuit
above, +5V is applied to the circuit. Since 1.8V is "dropped"
across the LED, there is 3.2V left across the resistor.

To work out the resistor value, R=V/I, R = 3.2V/0.020amps = 160 ohms
To work out the required power rating of the resistor, P=V*V/R
P=3.2*3.2/160 = 0.064 watts, a heat dissipation easily met by a
1/8th watt resistor (i.e. 0.125 watts). So, you go to Radio
Shack and ask for a 160 ohm 1/8th watt or higher resistor, say 5%
tolerance, to limit the current in the circuit.

That is all there is to it. The motherboard already has the +5V,
and the resistor is there as well. You don't have to add anything,
just extend the wires on your LED, so the LED can be connected to

If you just want to make LEDs glow (i.e. case lighting), then
you will need to find a +5V connection, a ground connection,
a LED, and a resistor. With the LED, you should know the Vf
(forward bias voltage) and the If LED max current. Running less
than the max current into the LED makes it less intense, so a
larger value of resistance makes it dimmer.

There is a reason that +5V is being used to run this circuit.
Most common LEDs have a reverse rating of 5V. That means if
you put the LED in the circuit backwards, it won't be harmed.
If you use +12V to power the circuit, and reverse the LED
leads by accident, it might not survive having +12V applied to it
backwards. That is why +5V is being used, so a user can connect
the LED backwards without the LED being damaged. If just won't
glow if it is backwards.

If you modify the circuit above, you can use any DC voltage
you want. If you use +12V, for example, and then do the math,
you'll find both the resistance value required will be higher,
but so will the power rating of the LED as well. The power resistor
could physically get pretty huge if you start with a high enough
DC voltage. That is why +5V is a pretty convenient value.

There are also some weird LEDs around, the exotic ones used to make
car headlights or car lighting systems. Some of these run with
3.5V or 7V, and aren't as tolerant of being reversed by accident.
You'll know these ones by the high price (>\$20 ea). The ordinary
status LEDs on a computer case should cost less than a dollar in
small quantities.

I don't know exactly what you want to do, so I hope the info
above helps.

So, to reiterate, a LED isn't powered by +5V or +12V, all it
needs is more than 1.8V or whatever the Vf happens to be,
plus the right current limiting resistor for whatever voltage
greater than 1.8V that you happen to have handy. You can
run my circuit above with either +5V or +12V if you want,
as long as you work out V/I to make the resistor the right
value. And, in the case of using +12V, if you put the LED in
backwards, I cannot guarantee it will still be working when
you reverse it and connect it properly.

Paul

Paul, Jul 7, 2004

3. ### - HAL9000Guest

All LEDs are about 1.6 - 1.9 volts. Some LEDs have a built in voltage
dropping resistor where you can apply 5 or 12 volts to the LED
terminals.

Last I checked, the motherboards I've seen have dropping resistors on
the mobo (instead of inside the LED) so standard (without internal
dropping resistors) would be what you need.

Does that make sense? Do you need a Radio Shack part number?

Forrest

Motherboard Help By HAL web site:
http://home.comcast.net/~hal-9000/

On Tue, 06 Jul 2004 23:42:34 GMT, "Bell C" <>
wrote:

>Could anyone enlighten me, I am in the process of reversing a case so that
>the front becomes the back so that the pci output cards are at the front.
>But I would like to replace the LED,s that come off the MOBO, but what
>voltage are they 5 volt or 12 volt. Or should I put it a different way what
>is the voltage output sent to the LED,s on an ATX case?
>

- HAL9000, Jul 7, 2004