![]() This diode will have a voltage drop across it, but it doesn’t limit current once you exceed that drop. The base connects to the middle of the NPN/PNP triplet, so the base-emitter path is essentially a diode. Relying on this mechanism runs the risk of burning out your I/O pin as well as damaging your transistor, so I recommend you use a base resistor.Īs far as not thinking a resistor would be required, remeber that a transistor is a PNP or NPN while a diode is just NP (or PN, if you look at it the other way). It sometimes works, but it’s a terrible practice and it’s just asking for trouble. It’s the same reason why people can power an LED via an I/O line with no resistor in series. You probably haven’t encountered problems yet because something else is limiting current, such as the source capability of whatever is connected to your base (I’m assuming an I/O line?). Without limiting base current you run the risk of damaging your transistor. A resistor is often used in series with the base of a transistor to limit base current. ![]()
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