I was surprised by one of them, which should output 9VDC and 350 mA as written on the sticker, but in reality it outputs 16VDC and 600mA, nearly the double ... Before I measured it, I used it to power the Arduino, hopefully only for a few minutes, and I will never use it again for this purpose...A power supply does NOT push-out current,
and a power supply of the proper voltage and a very-high current rating will not harm your Arduino!
The current rating on a power supply is the maximum that you should take from it, and the actual current depends on what you've connected.
You can usually get more current out of a power supply than it's maximum rating... But, it might burn up.
Per Ohm's Law
, current flow depends on voltage and resistance
(or impedance). Resistance means "resistance to current flow".
In the USA household power is 120 VAC (RMS), and most power outlets are rated at 15 Amps. With nothing connected the 120 Volts is there, but there is zero current. A 100W light bulb "draws" about 1 Amp. (100W/120V = 0.83 Amps). A toaster or hair dryer might require the full 15 Amps. If you run a toaster and a hair dryer at the same time from the same circuit, you'll "pull" more than 15 Amps, 'till the circuit breaker blows.
With nothing connected, there is infinite resistance and zero current. With a short circuit (zero ohms) you theoretically
get infinite current (as long as there is some voltage). In the real world there is no such thing as zero-resistance. The power supply has an internal resistance and if you short the output, the voltage will drop to zero and you may burn-up the power supply. (Ohm's law is always true
, so the voltage must
drop if the power supply can't supply the "calculated" current .)