Quote:
Originally Posted by rowan
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?
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your calculation is somewhat right, but 22 ohms of resistance is so small that it can be ignored and we assume LEDs are working on 3v each.
No doubt these values vary depending on the type of LED and strength of 9v battery. A fully charged 9v battery have potential difference of up to 9.4 volts whereas a less charged can have 8.4 to 8.8
And to answer how it works, in the chain of 3 LEDs .. other two acts as simple resistor to the third. So, actually each LED get ~3v potential difference across its terminals.
I don't know if I got what you were really asking .. may be lol ..
Hey, have a look at this .. it might help
http://www.theledlight.com/ledcircuits.html