Do I need a resistor for this case?

[ruriimasu]

I am hooking up a LED, which I salvaged from a fauxton, to 4 x AAs. I noticed the LED gets quite hot quite fast so I was wondering whether a resistor is needed. I am going to solder it to an unused cheap light which originally uses a krypton bulb and runs on 4 x AAs. The LED light doesnt seem as bright as the original krypton, but I guess LED is more efficient and should be able to stretch the batts' runtime, am I correct here?

 

[LukeA]

Yes, your setup is running 100+mA through an LED rated for 20mA maximum.

Yes, you want a resistor, I believe one circa 175 Ohm.

A 5mm LED might not be as bright as the bulb you're replacing, but you certainly could find a power LED brighter than the incan bulb. Either, when driven correctly will far outlast teh bulb on a set of cells.

 

[ruriimasu]

Hi LukeA,
Thanks for your reply. May I know the color of this resistor?

 

[Norm]

Lookee here http://www.quickar.com/noqbestledcalc.htm
Norm

 

[ruriimasu]

Lookee here http://www.quickar.com/noqbestledcalc.htm
Norm

Thanks Norm!

Btw, how do I know how much mA my setup is running and how much mA my LED is rated for?