What do i need to know about case LEDs

[TeeJay88]

I plan on replaceing my blue case LEDs. Ive picked these.
http://feeds2.feedburner.com/bit-tech/all
I have an ASUS P6T, and SilverStone TJ09.
I plan on ording 2 with 67ohm resistors for 3.3v, 2 with 100 ohm 5v resistors.
Now as its been explained to me.The mobos come with an automatic variable resistor but are also powered at 5v. Now I want to know if i would be safe to use the 5v LEDs. Or is there something im missing.

 

[Format _C:]

I don't think you will need a resistor as the Motherboard already has a current limiting resistor on the board. I remember back in the day (InWin Q-500 Abit KT7A, ETC) I changed the LEDS on the Q500 and I did not use a resistor and the LEDS did not burn up (Unloaded the Power LED Header was 4.9 Volts and The Hard Disk Drive LED was constantly varying as that LED is never solidly lit up like the power LED is. When you put an led on the Power LED Header the voltage drops to about 3.3-3.0 Volts I think it really depends on the LED you will be using though.
I think if you use the 5Volt LEDS they will be very dim or not light up at all
So a normal led would be fine for the Power/Hard Disk LEDS However if you connect directly to a Molex connector (Yellow is 12Volts Black is Ground Red is 5 Volts) You will then need the appropriate resistor

Hope I helped you with your LED Quest

 

[E.T]

Unloaded the Power LED Header was 4.9 Volts

Forget unloaded readings, they don't tell anything useful. (maybe except that circuit is powered by 5V)
I noticed that when building circuitry for getting HDD access LED controlled by both motherboard and 3ware RAID card.

Readings with normal red LED as load told lot more.
Drives at idle anode voltage was ~5V and cathode ~3.1V.
During access anode dropped to ~2.8V and cathode to ~1V.
So it uses cathode control which leaves cathode floating to roughly source voltage minus forward voltage during idle and desptie of switch being on cathode side there's major resistance between source voltage and anode because of voltage drop during access.

As for keeping current safe it could use current limiting regulator but that's lot more complex and hence unprobable compared to plain serial resistor sized for nomimal current and forward voltage of typical LED, which would still work also with little higher forward voltage LEDs of common colours.

My case uses blue LED as power indicator so at least 2.5V forward voltage isn't too much but I wouldn't count on using any "high voltage" LEDs.
http://en.wikipedia.org/wiki/LED#Colors_and_materials