P5LD2-LED voltage?

T.O. wrote:
> Does anyone know what the voltage is that's being supplied to the HDD
> activity LED pins?
>
> Trying to do a little case customizing. Radio Shack has a couple of LEDs
> there,
> but their voltages are 3.7 and 5.0. Don't want to smoke it first thing!
>
> Thanks
>
> T.O.
>

It would have helped, if you'd posted the URL to the LED information,
as then I wouldn't have to explain anything

LEDs have three important parameters. VF @ IF in the forward direction,
and something like PIV in the reverse (non-lighting) direction.

LEDs have a curve, where as you increase the voltage, the current flow
increases. This curve is sharp enough, to easily get the LED in trouble
and cause it to overheat and blow in seconds. To prevent this, most usages
of a LED, will include a current limiting resistor. The current limiting
resistor prevents the circuit from "going too far up the curve".

In an automotive application, the current limiting resistor and LED may
be packaged together. This allows a product to replace an incandescent
bulb, without the installer having to worry about the resistor.

In other situations, the resistor and LED will be separate. That is the
way it is on a motherboard. The motherboard has the resistor. The LED is
just a LED.

The VF (forward voltage drop) varies with color. The Radio Shack reference
to 5V is probably not a reference to VF. The 3.7V reference would be, and a
3.7V LED would be blue in color. This table shows some LEDs, and how their
VF in volts, varies with the color of the LED. (The slope of the line,
in fact, is Planck's constant in disguise.)

HLMP-K101 AlGaAs Red 637nm 1.8volts
HLMP-1321 GaP Red 626nm 1.9volts
HLMP-1401-E0000 GaP Yellow 585nm 2.0volts
HLMP-1521 GaP Green 569nm 2.1volts
HLMP-K640 GaP Green 560nm 2.2volts
HLMP-DS25-R0000 InGaN Blue 470nm 3.6volts
HLMP-KB45-N0000 GaN Blue 462nm 4.0volts

To work some examples, and identify a potential problem, install a
red LED in the following circuit. The motherboard uses the 5V supply
to light LEDs, because in some cases, this is compatible with the integrated
circuit driving the LED. I selected a resistor value here, to make the
numbers work out. The 160 ohms might not be the exact value on the
motherboard.

current If --> anode |\ | cathode
| \ |
+5V ------- resistor ----------| \|-------- Ground
160 ohms | /|
+ | / | -
|/ |

Vf 1.8V @ 20ma (red LED)

The equation to work out current is If = (5.0 - Vf) / Resistor.
In this example (5 - 1. / 160 = 0.020 amps or 20 milliamps.

If we substitute a 3.7V Vf blue LED in the circuit, the current will be

(5 - 3.7) / 160 = 0.008 amps

Notice that the current has dropped to only 8 milliamps. The blue
case LED will be dimmer than the red one.

So as the color changes, the resistor value that should be used changes
as well. The resistor used on the motherboard may be good for a red or green
LED, but the blue one could benefit from a lower valued resistor on the
motherboard.

The other item of interest, is the PIV or peak inverse volts rating.
That happens when you install a LED backwards, connecting the anode to
a more negative voltage than the cathode. Most general purpose LEDs
(i.e. the small ones) can take 5V in reverse on their leads, without
damage. That is why LEDs can be installed backwards, without any
damage. A situation where this might not apply, is in an automotive
situation - there you could be applying 12V in reverse to a general purpose
LED, to a LED which is rated for 5V PIV.

High power LEDs are a different animal altogether, and have other
concerns. The brightest of them, have power limitations in the forward
direction, and to drive them close to their light output limits,
you almost want temperature measurement circuits right next to the
LED, to reduce the current if the LED is about to overheat. That
extra protection helps extend the limited life of the high power
LEDs. But for building computers, people would seldom be using those,
because they are blindingly bright. The high power LEDs are also sometimes
not tolerant to being reversed, and have a poor PIV rating - reversing the
high power LED could ruin it. So for the high power ones, you definitely
want to get a copy of the datasheet for the LED, because all the rules
are different.

For cases where the motherboard is not cooperating with your efforts, the
easiest way to fix it, is with a buffering IC or transistor circuit. That
way, you can correct whatever deficiencies the motherboard driving circuit
might have. Soldering stuff to the motherboard is a warranty violating
procedure, and should only be attempted with the motherboard owner's consent
and knowledge, so they won't be disappointed if their RMA request is
rejected by the motherboard manufacturer.

So, there is no problem connecting the Radio Shack LED. If the LED is
blue, don't expect it to "knock out an eyeball". It'll be pretty dim.

Paul
 >> Stay informed about: P5LD2-LED voltage? 

Thanks so much for spending the time to write out all of that info.
I was trying to do some calculations on current through the LED, but didn't
take into account that the color would matter.

Well, just to let you know what I did.....I W.A.G.ed it !!

I dismantled an old non functional LED that was used to give some fancy
light to a handle I have installed. It had a 330 ohm resistor in series with
the positive lead. It was a very small blue LED. I think it was called a T1
or 3mm in size.

All I could get my hands on was a bit larger, 5mm.
The specs on the back of the card:
Forward supply voltage: 3.7 typ--4.5 max
Forward current: 20ma typ.--30ma max
Luminous intensity 2600mcd (it's pretty bright)
468 nm wavelength
That's all the info there is.

The voltage on the MOBO pins measured 4.8v during hard drive activity and
strangely enough, it's around 1.6 volts with no activity--go figure.
Anyway----So with my scientific W.A.G., and the fact all I had handy was a
1500 ohm resistor I temporarily held all of that together and gave it
a try--worked fine!

Thanks for the info--I'll see if I can make it fit to what is running and
working.



T.O.


"Paul" <nospam.DeleteThis@needed.com> wrote in message news:fdutk4$7p0$1@aioe.org...
> T.O. wrote:
>> Does anyone know what the voltage is that's being supplied to the HDD
>> activity LED pins?
>>
>> Trying to do a little case customizing. Radio Shack has a couple of LEDs
>> there,
>> but their voltages are 3.7 and 5.0. Don't want to smoke it first thing!
>>
>> Thanks
>>
>> T.O.
>>
>
> It would have helped, if you'd posted the URL to the LED information,
> as then I wouldn't have to explain anything
>
> LEDs have three important parameters. VF @ IF in the forward direction,
> and something like PIV in the reverse (non-lighting) direction.
>
> LEDs have a curve, where as you increase the voltage, the current flow
> increases. This curve is sharp enough, to easily get the LED in trouble
> and cause it to overheat and blow in seconds. To prevent this, most usages
> of a LED, will include a current limiting resistor. The current limiting
> resistor prevents the circuit from "going too far up the curve".
>
> In an automotive application, the current limiting resistor and LED may
> be packaged together. This allows a product to replace an incandescent
> bulb, without the installer having to worry about the resistor.
>
> In other situations, the resistor and LED will be separate. That is the
> way it is on a motherboard. The motherboard has the resistor. The LED is
> just a LED.
>
> The VF (forward voltage drop) varies with color. The Radio Shack reference
> to 5V is probably not a reference to VF. The 3.7V reference would be, and
> a
> 3.7V LED would be blue in color. This table shows some LEDs, and how their
> VF in volts, varies with the color of the LED. (The slope of the line,
> in fact, is Planck's constant in disguise.)
>
> HLMP-K101 AlGaAs Red 637nm 1.8volts
> HLMP-1321 GaP Red 626nm 1.9volts
> HLMP-1401-E0000 GaP Yellow 585nm 2.0volts
> HLMP-1521 GaP Green 569nm 2.1volts
> HLMP-K640 GaP Green 560nm 2.2volts
> HLMP-DS25-R0000 InGaN Blue 470nm 3.6volts
> HLMP-KB45-N0000 GaN Blue 462nm 4.0volts
>
> To work some examples, and identify a potential problem, install a
> red LED in the following circuit. The motherboard uses the 5V supply
> to light LEDs, because in some cases, this is compatible with the
> integrated
> circuit driving the LED. I selected a resistor value here, to make the
> numbers work out. The 160 ohms might not be the exact value on the
> motherboard.
>
> current If --> anode |\ | cathode
> | \ |
> +5V ------- resistor ----------| \|-------- Ground
> 160 ohms | /|
> + | / | -
> |/ |
>
> Vf 1.8V @ 20ma (red LED)
>
> The equation to work out current is If = (5.0 - Vf) / Resistor.
> In this example (5 - 1. / 160 = 0.020 amps or 20 milliamps.
>
> If we substitute a 3.7V Vf blue LED in the circuit, the current will be
>
> (5 - 3.7) / 160 = 0.008 amps
>
> Notice that the current has dropped to only 8 milliamps. The blue
> case LED will be dimmer than the red one.
>
> So as the color changes, the resistor value that should be used changes
> as well. The resistor used on the motherboard may be good for a red or
> green
> LED, but the blue one could benefit from a lower valued resistor on the
> motherboard.
>
> The other item of interest, is the PIV or peak inverse volts rating.
> That happens when you install a LED backwards, connecting the anode to
> a more negative voltage than the cathode. Most general purpose LEDs
> (i.e. the small ones) can take 5V in reverse on their leads, without
> damage. That is why LEDs can be installed backwards, without any
> damage. A situation where this might not apply, is in an automotive
> situation - there you could be applying 12V in reverse to a general
> purpose
> LED, to a LED which is rated for 5V PIV.
>
> High power LEDs are a different animal altogether, and have other
> concerns. The brightest of them, have power limitations in the forward
> direction, and to drive them close to their light output limits,
> you almost want temperature measurement circuits right next to the
> LED, to reduce the current if the LED is about to overheat. That
> extra protection helps extend the limited life of the high power
> LEDs. But for building computers, people would seldom be using those,
> because they are blindingly bright. The high power LEDs are also sometimes
> not tolerant to being reversed, and have a poor PIV rating - reversing the
> high power LED could ruin it. So for the high power ones, you definitely
> want to get a copy of the datasheet for the LED, because all the rules
> are different.
>
> For cases where the motherboard is not cooperating with your efforts, the
> easiest way to fix it, is with a buffering IC or transistor circuit. That
> way, you can correct whatever deficiencies the motherboard driving circuit
> might have. Soldering stuff to the motherboard is a warranty violating
> procedure, and should only be attempted with the motherboard owner's
> consent
> and knowledge, so they won't be disappointed if their RMA request is
> rejected by the motherboard manufacturer.
>
> So, there is no problem connecting the Radio Shack LED. If the LED is
> blue, don't expect it to "knock out an eyeball". It'll be pretty dim.
>
> Paul
 >> Stay informed about: P5LD2-LED voltage?