When using a 2222 or 3904 NPN-type transistor to switch a simple LED, is it better to place it inline with the negative or positive rail of the load (LED)?
Also, which rail is best when working with darlington transistors or MOSFETs for high powered loads?
Answer
N-type devices (NPN BJTs, NMOSFETs) are for low-side (negative supply) switching. P-type devices (PNP BJTs, PMOSFETs) are for high-side (positive supply) switching. That's not to say that they couldn't be used for the other, but there are additional voltage level concerns to take into consideration when doing so.
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