8 segment LED display - why not have 1 single current limiting resistor on gnd

Other Hardware LEDs and Multiplexing
1

I'm probably missing something very basic but I was following the ShiftOut tutorial (http://arduino.cc/en/Tutorial/ShiftOut) where 8 LEDs are controlled using a shift register and had a question on the resistors.

I didn't have 8 LEDs but I did have a 8 segment LED display so I hooked that one up. My questions revolves around understanding why we need 8 resistors (one for each LED). I understand why we need a current limiting resistor when hooking up a LED but I don't understand why we need 8 of them.

I didn't put a current limiting resistor on each LED but I did put a resistor on the common ground pin of the 8 segment LED display. ( If I was working with 8 LEDs I guess it would be the same as removing the 8 resistors and replacing them with a single resistor between their common ground and the arduino ground connection.

My understanding was :

As current is flowing through the LED(s) it will pass through the resistor and current will be limited.
So instead of having 8 resistors on each LED, a single resistor on that ground connection would be the same.

If my understanding is correct, why don't we use a single resistor ?

2

Because a single resistor will limit the current to all the LEDs.

Light one LED, it gets all that current.

Light eight LEDs, each one gets an eighth of the current, so is one eighth of the brightness.

As you change the number of LEDs that are lit, the brightness (of each) changes. Is that what you want to happen?

3

If you are only lighting 1 segment at a time, 1 resistor is fine.
However, it is much faster to light all at 1 time, and when multiplexing the display will be on more and appear brighter.

4

I didn't notice the difference in brightness because it was very subtle. I was using a 560 ohm resistor resulting in about 5 mAmp current being drawn from the arduino. (wanted to use as little current as possible) so the segment was never very bright, but still very much usable.

Still learning about all of this. Rebuilt the circuit with 4 standalone green LEDs in parallel and without the 595 chip.
Was puzzled about what I saw on the multimeter, but will post that in the general section.