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#245168 - 02/01/2005 08:01 Voltage of a HDD LED?
tfabris
carpal tunnel

Registered: 20/12/1999
Posts: 31563
Loc: Seattle, WA
If I was making a PC case, and I wanted to put my own LED in for the hard disk drive indicator, would that LED need to be a specific voltage LED? At rat shack I can buy LEDs in 2.1v, 5v, and 12v.
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Tony Fabris

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#245169 - 02/01/2005 08:28 Re: Voltage of a HDD LED? [Re: tfabris]
tfabris
carpal tunnel

Registered: 20/12/1999
Posts: 31563
Loc: Seattle, WA
<emily litella> Never Mind. </emily litella>

(2.1v works fine)
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Tony Fabris

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#245170 - 02/01/2005 12:48 Re: Voltage of a HDD LED? [Re: tfabris]
genixia
Carpal Tunnel

Registered: 08/02/2002
Posts: 3411
For reference...

The forward voltage of an LED is an attribute of the semiconductor used to create it, as is the color of the LED. So most LEDs of the same color will have the same forward voltage, as they will mostly be using the same (cheapest to manufacture) semiconductor material.

This voltage is typically 1.7V for red, 2.1V for yellow and 2.2V for green. (All 'standard' LEDs not 'high efficiency' or 'super-bright'). These 'traditional' LEDs have forward voltages so close together that in the vast majority of applications where the output brightness is not required to be calibrated, engineers would simply use 2V as the forward voltage when calculating the series resistance. ( R=(Vsupply-Vforward)/ Iforward ). For a typical Iforward of 10mA this worked perfectly fine.

Newer developments have created LEDs with a wider range of Vforward. Blue and white LEDs are typically 4V or more, so that rule of thumb can't be used for all LEDs anymore.

There are no 12V LEDs. When you see something labelled as such, it is really a standard voltage LED with a built in series resistor calculated for a supply voltage of 12V. Such items are convenient for indicators in automotive and marine environments, which is exactly why they are made.

A PC motherboard has the series resistance built in. I'm not sure whether the ATX specs defined the colors used for the power and HDD LEDs or not. But I'd bet that most of the engineers used the 2V value anyway.
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Mk2a 60GB Blue. Serial 030102962 sig.mp3: File Format not Valid.

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#245171 - 02/01/2005 15:33 Re: Voltage of a HDD LED? [Re: genixia]
tfabris
carpal tunnel

Registered: 20/12/1999
Posts: 31563
Loc: Seattle, WA
Very interesting. Thanks for that information, I didn't know any of that.

I'm using a green LED now, I wonder if a blue one would work?
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Tony Fabris

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#245172 - 02/01/2005 15:56 Re: Voltage of a HDD LED? [Re: tfabris]
SE_Sport_Driver
carpal tunnel

Registered: 05/01/2001
Posts: 4903
Loc: Detroit, MI USA
Good thread.

I'm thinking of having a monitor and keyboard/mouse in the garage of my next home. The case would be on the other side of the wall (and away from the humidy and extreme temperatures of Midwest winters). In addition to the monitor and input devices, I want to have a remote power switch and power LEDs (hopefully as easy as running longer wires and remounting the parts on something). But if I have to wire my own LEDs, this info will come in handy. I just love the idea of being able to research a project while in progress. (What was the torque spec for a 2002 Impreza again?)
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Brad B.

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#245173 - 02/01/2005 16:02 Re: Voltage of a HDD LED? [Re: genixia]
peter
carpal tunnel

Registered: 13/07/2000
Posts: 4172
Loc: Cambridge, England
Quote:
The forward voltage of an LED is an attribute of the semiconductor used to create it, as is the color of the LED.

Specifically, each photon output gains its energy from precisely one electron input. The colour and the forward voltage are thus related by eV = hf, where e is the electron charge (1.6 x 10^-19C), V is the voltage, h is Planck's constant (6.6 x 10^-34 Js) and f is the frequency of the emitted light.

Peter

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#245174 - 02/01/2005 16:07 Re: Voltage of a HDD LED? [Re: tfabris]
peter
carpal tunnel

Registered: 13/07/2000
Posts: 4172
Loc: Cambridge, England
Quote:
I'm using a green LED now, I wonder if a blue one would work?

It's not like a higher frequency wouldn't work, it's just that it probably wouldn't be operating at its maximum brightness. I've replaced red LEDs with white ones and they look fine. It's possible that if you replaced a high-voltage (high-frequency) LED with a lower-voltage one you'd run the risk of exceeding its rated current, but in general it's very hard to blow up LEDs.

Peter

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#245175 - 02/01/2005 19:59 Re: Voltage of a HDD LED? [Re: tfabris]
StigOE
addict

Registered: 27/10/2002
Posts: 568
Most likely. I replaced the green leds in my case with blue ones and they work without any problems...

Stig

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