# Max Current Output [duplicate]

Looking around at a couple of power supplies that I own such as a guitar pedal adapter that says it puts out 9V and 1700mA max, how does a power supply like this work so that it doesn't put out too much current and blow up what's attached? There isn't a switch that I physically have to switch but how does the adapater "know" how much current to put out?

## marked as duplicate by PeterJ, Leon Heller, gbulmer, placeholder, JYeltonSep 19 '14 at 7:25

$$I = \dfrac{V}{R}$$
For a fixed voltage $V$, the load $R$ sets the value of $I$. An improved adapter or a power supply, can implement a current limiter, wich senses the output current and reduces the output voltage to get a current reduction.