Quote:
Originally Posted by Vick!
Well, If we connect 3 LEDs of 3v each in series with a 9v battery. Do we need resistor?
|
I understand what you're saying, I just don't understand how it works. With every fraction of a volt above the combined forward voltage of the LED string, surely the dissipation will increase remarkably?
I know that those keychain torch LEDs don't usually have a resistor, but that's because the button cell batteries cannot supply more current than the LEDs can tolerate. A short circuited 9V battery would probably be able to push out more than 30mA...
If the forward voltage of each LED was (say) 2.8V and the battery puts out exactly 9V then the difference is 0.6V, and to stay at 30mA you'd need 22 ohms of series resistance. Surely a battery does not have such a high internal resistance?