I am trying to wire up a 7 segment LED display but when I connect the common pin to ground and the LED pins to V+ the display doesn't work. However when I use the common pin as an anode and connect it to the positive rail and use a lower voltage for the LED pins then it works, how do I make it work with the common pin as a cathode

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    \$\begingroup\$ (1) Please post the datasheet for your 7-segment character. (2) The LEDs in the character are hard-wired internally. If it's common anode, you will not be able to make a common cathode out of it. \$\endgroup\$ – Nick Alexeev Sep 5 '15 at 5:29
  • \$\begingroup\$ You throw away your common anode display and buy a common cathode display. \$\endgroup\$ – Roger Rowland Sep 5 '15 at 5:29

You don't. Multisegment LEDs with common pins are manufactured in a specific arrangement, and this cannot be changed after fabrication.

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  • \$\begingroup\$ Thanks I didn't know this was hardwired. After a Google search I thought that they were reversible... \$\endgroup\$ – pranay Sep 5 '15 at 5:55
  • \$\begingroup\$ @pranay in general diodes operate in one direction or "bias". This means they allow current to flow after a particular voltage threshold is reached. the threshold varies on the type of diode used. In reverse bias (+ attached to cathode) no current flows as you inc the voltage until you permanently damage the diode. The one exception to this rule is the zener diode which will conduct at a specific rev voltage. led are easily damaged if you provide too much current so always either put a series resistor in or use a current limited power source. \$\endgroup\$ – BenG Sep 5 '15 at 21:51

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