Electronics Replacing power/hdd activity lights

Im a totl noob at electricity and was wondering if i need a resistor for this. my motherboard gives of 5 volts to the two and i want to put a blue led in for the current green power led, and a red led in for the current orange hdd activity led.
-thnx for any help

 

If you've tested the motherboard and it gives off 5v, then you would need a resistor unless you can find 5v LEDs.

 

you don't need anything... just replace the original ones with the new, blue ones

 

the mobo have a built in resistor, so when you measure you won't have the right load, manking the multimeter show full voltage...

try measuring when you have a LED on there, and you get something quite different

 

To avoid confusing you with the conflicting advice, the motherboard led headers already have a suitable resistor fitted, so Smilodon is correct - just swap the existing leds for new ones.

The reason you measure 5V at the header is because a digital multimeter has a very high resistance, generally over 10 Megohm.

5V-----330R resistor------10M meter------0V

By Ohm's Law the meter measures 5 x 10,000,000/10,000,330 = 4.9998V

 

Oh, well that would explain it
You'd probably have to test it somehow with the existing LED still attached.

Hmm, well don't I feel silly. I'd really hate to see my ultrabright Sea Green power led without that resistor then.

/edit: wait, what size LED would be "intended" for a 330ohm resistor? Because a lot of the ultrabright ones at least have totally different power requirements. I've seen from 1.7v to 3.6+v at varying power draws (20mA to 80mA, see LSDiodes new THC3 LEDs which are like 22000MCD )

 

Just about any, though you won't get the full rated brightness on a Luxeon Star. A 2V led will get 9mA, a 3.5V gets 4.5mA, plenty bright enough for an indicator.

 

I thought the idea behind amperage was they take however much they want and you need to be careful not to draw more than the circuit can handle... which is why some people don't want to use ultrabright LEDs for those parallel port LED meters (for example)

 

With a current-limiting resistor fitted, they take whatever current they're allowed to take, not whatever they want. That's what current limiting means.

The resistor decides the current, based on how many volts are spare after the led's forward voltage has been subtracted from the supply voltage and Ohm's Law.

So current = (Vs-Vf)/Resistance

 

thnx guys, so if im hearing you correctly i need no resistor even if the led isnt a 5 volt or the same amount as my current LEDs becuase of a resistor built into my mobo. Kickin'