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Old 03-05-2011, 11:23 PM
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stoopkid stoopkid is offline
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Default How to choose NPN transistor

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

Last edited by stoopkid; 03-05-2011 at 11:40 PM. Reason: details
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Old 03-06-2011, 01:10 AM
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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.
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Old 03-06-2011, 09:40 AM
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Excellent, thank you, sir.
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