Beginner's problem - LEDs and resistors.

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Tengil

Joined Mar 11, 2007
3
Hello there!

I have recently started learning electronics and I have now successfully programmed a PIC and made a 8-flashing-LED-circuit :)

First I made it with a resistor on every LED connected pin, but then I realized it could be done with a single resistor between all the LEDs and the ground. But now with this connection setup the LEDs get very dim when several are lighten up at the same time. My thought of this is that the resistor only can let through a fixed amount of current. So what I wonder is wheter I'm right or if this has something with the PIC to do? I've read about "pull-up resistors" several times, but I haven't yet understood what that is. I would be glad if anyone could enlighten me! :)
 

hgmjr

Joined Jan 28, 2005
9,029
The first connection you employed was the correct way to power the LEDs.

The use of a common current limiting resistor creates a problem since the voltage at which each of the LEDs varies slightly from one to the other. The result is that the one LED that has the lowest forward voltage drop hogs a disproportionate share of the available current and thus starves the remaining LEDs of their fair share of the current available.

hgmjr
 
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