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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2\$\begingroup\$ NPN when between LEDs and GND, PNP when between LEDs and VCC. \$\endgroup\$– Vicente CunhaCommented Apr 7, 2018 at 7:43
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1\$\begingroup\$ You might explain (in your question) what an SSD LED is. \$\endgroup\$– TransistorCommented Apr 7, 2018 at 7:55
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\$\begingroup\$ Depends if the transistor is driving the cathode or the anodes. \$\endgroup\$– user16324Commented Apr 7, 2018 at 10:13
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3 Answers
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There are 2 possible combinations :
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\$– G36Commented Apr 7, 2018 at 9:40
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\$\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\$ Commented Apr 7, 2018 at 9:50
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\$\begingroup\$ The NPN (left circuit) does not need a base resistor in this configuration. \$\endgroup\$ Commented Apr 7, 2018 at 12:02
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2\$\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\$ Commented Oct 16, 2018 at 6:33
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1\$\begingroup\$ @SpehroPefhany Thanks for your suggestion, I've edited. \$\endgroup\$ Commented Oct 19, 2018 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.
answered Apr 7, 2018 at 9:12
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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\$– WilliamCommented Apr 7, 2018 at 9:22
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Actually, it depends on what you want to do. If you want to switch between segments, you should use PNP transistors and if you want to switch between digits then you should use NPN transistors.
In other words, NPN should be at the low side and PNP should be at the high side of the circuit.
For more information about transistors, take a look at this link: https://learn.sparkfun.com/tutorials/transistors/applications-i-switches
