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I've read a bit about this but there seems to be much ambiguity. When using common cathode seven segment display LEDs would you use NPN or PNP? Or can you use either?

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    \$\begingroup\$ NPN when between LEDs and GND, PNP when between LEDs and VCC. \$\endgroup\$ – Vicente Cunha Apr 7 '18 at 7:43
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    \$\begingroup\$ You might explain (in your question) what an SSD LED is. \$\endgroup\$ – Transistor Apr 7 '18 at 7:55
  • \$\begingroup\$ Depends if the transistor is driving the cathode or the anodes. \$\endgroup\$ – Brian Drummond Apr 7 '18 at 10:13
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There are 2 possible combinations :

schematic

simulate this circuit – Schematic created using CircuitLab

The PNP circuit must be driven with inverted signal and it is a bit less efficient than the other.

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  • \$\begingroup\$ Your PNP version circuit won't work the LED will be always OFF. And the NPN version is also a poor example. \$\endgroup\$ – G36 Apr 7 '18 at 9:40
  • \$\begingroup\$ About the PNP version, I forgot a base resistor. About the NPN version, it works fine to me and it has the least component \$\endgroup\$ – Long Pham Apr 7 '18 at 9:50
  • \$\begingroup\$ The NPN (left circuit) does not need a base resistor in this configuration. \$\endgroup\$ – Stefan Wyss Apr 7 '18 at 12:02
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    \$\begingroup\$ Put the PNP resistor in the collector rather than the emitter, then the transistor can be made to saturate and will thus run cooler and will generally provide a better defined LED current. If a relatively low power supply voltage is used, for example 3.3V, with a blue or white or even green display, the NPN version should not be used. \$\endgroup\$ – Spehro Pefhany Oct 16 '18 at 6:33
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    \$\begingroup\$ @SpehroPefhany Thanks for your suggestion, I've edited. \$\endgroup\$ – Long Pham Oct 19 '18 at 10:47
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When using common cathode 7 segment leds, you need a NPN type to drive the common cathode to GND. The P base must be driven P(ositive) with a series resistor.

Edit: I misunderstood your question. Please see Long Phams answer and drawings. Although he did not mention the common cathode. This must be connected to GND.

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  • \$\begingroup\$ Thank you. Are there consequences to using the wrong type of transistor? Will it cause damage or at worst just do nothing? \$\endgroup\$ – William Apr 7 '18 at 9:22

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