@kat Different current (colloquially called amperage); the voltage is always the same 5 V (except for things using Qualcomm quickcharge or USB-PD)
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@Felthry so you can just plug any current into a smartphone and it's fine??

@kat it's a voltage source, not a current source--the voltage is decided by the power supply but (up to the supply's limits) the current is decided by the phone

if it's a 2 amp power supply and your phone only needs 1 amp it'll only draw 1 amp
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@Felthry but if amperage is decided partly by voltage, isn't a higher voltage power source going to create higher current?

@kat yes, but all USB power supplies are 5 V (except for quickcharge and USB-PD, which start at 5 V and can increase that but only if the phone sends a special signal asking for higher voltage)
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@Felthry but like it doesn't matter how much current there is because it's always going to get limited by the phone which doesn't have to do anything to do that?? i'm kind of lost

@Felthry except you need different resistors to run different currents through a particular thing, so i guess i'm not sure how a device "just draws as much as it needs" without an active system, unless i'm totally lost on how electricity works, which is probably what's happening

@kat this is in fact a large part of the job of transistors. a transistor, to gloss over a lot of very complicated details, is a device that can adjust its resistance in response to other signals
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@Felthry ah okay, so the more current the transistor gets, the more it resists to balance it out, okay

@kat not exactly, but sort of? in this application, that's how it's used, but the signal that adjusts the resistance can be separate from the current, which enables things like amplification and stuff--here, though, it's being used to sense current and control its resistance to keep the current constant
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