LEDs again!

[gunpowder]

I recently bought a set of 8 LEDs that are connected up to a 9v battery connector. I took the join apart and there are no resistors, they are just connected to the battery connector. I was under the impression that each LED needed a resistor, is that correct? If so how do i know what resistors to use?!

Thanks in advance

[jwrjr]

It will probably work as-is ... but there is a 'gotcha' in there. A battery looks like just a source of voltage and current, but it isn't quite that simple A battery also has an internal resistance to it. That's why you see keychain lights with only a battery and a led. You have no control of the brightness of the leds this way. So you also can't mix red/orange/yellow/green leds with blue or white leds unless you use resistors for each led.
Of course using a 9 v. battery with 8 leds will only give you an hour or so of light anyway.

[en'til Zog]

It sounds like the people who made up that 9V/LED gadget are counting on the typical 9 volt batterie's limited ability to provide much amperage to keep the LEDs from burning out. Or: the LEDs can survive more amperage than a poor little 9 volt battery can put out.

Just do not try and use a Lithium 9 volt battery which may be able to put out a lot more amperage (relatively) than the typical alkaline 9 volt, and may cause the weakest LED in the bunch to expire. Leaving the rest to absorb even more amperage. So the next weakest LED pops - quickly. Then the rest of the surviving LEDs go like pop corn.

For your own circuits? Use resistors! (Or drive 'em with a CMOS chip which limits current anyway.)