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When using a common cathode 7-segment display, is there a reason that two resistors can't be used to connect the two cathode pins to GND instead of using seven resistors to connect the seven anode pins to VDD?

I'm assuming there's a good reason, because I haven't been able to find any schematics that do it this way, but I'm unsure of why this would not work?

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The purpose of the resistor is to limit current to a segment LED, by dropping the surplus voltage (over the LED Vf) across itself.

When a resistor is used on a common cathode, the current through this resistor would vary by the number of segments lit at any time. Thus, the voltage dropped would change as well. This will lead to intensity of digits changing with number of lit segments.

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  • \$\begingroup\$ Please correct the answer: The voltage across the segments in parallel is Vf, therefore the voltage and current across the resistor stays the same. But the current gets distributed between the several LED segments, making them dimmer. In the current form of the answer, you imply the forward voltage of a LED changes if you put another led in parallel. \$\endgroup\$ – posipiet Jan 10 '13 at 8:44
  • \$\begingroup\$ @posipiet Vf is not a sharp knee in the graph. \$\endgroup\$ – Anindo Ghosh Jan 10 '13 at 9:12
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    \$\begingroup\$ In your typical red LED segment, Vf varies between 1.7 and 1.9 V with If in the range from 5mA to 45mA. Compared to the other effects, this change in voltage is negligible. See de.rs-online.com/web/p/led-displays/0589092 \$\endgroup\$ – posipiet Jan 10 '13 at 12:16
  • \$\begingroup\$ @posipiet The forward voltage of a LED will change slightly, when you have two LEDs in parallel and a resistor in series with that, compared to having one LED in series with the same resistor, but much more importantly the forward current through each LED will change significantly. \$\endgroup\$ – immibis Jun 30 '16 at 9:31
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You can, but you get uneven lighting in good situations, and in worst case, since LEDS (or any given diode) are not perfect systems, variations in them will cause one led to conduct more current then the rest. Eventually it will burn out before the rest, which would lead to the same current to be divided by a smaller amount of imperfect diodes, which would lead to (rinse, lather, repeat).

See Why exactly can't a single resistor be used for many parallel LEDs? for more info.

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Each resistor should only be used to control the current through one LED at a time. If on each display one only wanted to drive one segment at a time, one could simply use one resistor to limit current to the display's common wire. This could be a reasonable approach if e.g. one had a ten-digit display and wished to drive it with 1/7 duty cycle. Most of the time, though, one will wish to energize one entire digit at a time, and would thus need one resistor per segment wire.

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