I need an npn transistor to switch the cathodes of a 4 digit 7 segment led display. I don't want to get something meant for too high of a rating and have it mean I'm not providing enough power to actually operate it, or something. How do I choose the one I need? Do I just need the rating to exceed my use, or do I need to be close in terms of Voltage, Power Dissipation, and Collector Current?

Thanks

 

You only need something that can handle the applied voltage and about 10 ma current. This is assuming your transistors have their emitters to ground, and a current limiting resistor in the cathode circuit. That value of resistor will vary according to the voltage supply to the displays.

There's hardly a transistor not able to handle that low voltage and current, so let cheap be your guide. Find something on the order of 2N2222 or 2N3904, preferably for less than $.10 each.

 

Excellent, thank you, sir.