An Electronics question...

This is a question I've been wanting to ask for some time. I have a powered USB hub that requires an input of 5V @ 3A. The original power supply has stopped working and I'm trying to find an aftermarket replacement. Now obviously, I will need to find one which outputs 5V, but if I get one that is more than 3A, will that be ok? For some reason, I have always held the belief that the supplied amperage only had to be equal or greater than figure required by the device demanding power. Ie, while the voltage is fixed, the device will draw as much current(amps) as it requires as long as it is within the rating of the power supply. Am I correct? I'm looking to get one of those variable voltage power supplies with a current rating of 5 amps.

You are right with your assumptions/reasoning. If the voltage is fixed the unit will draw as much current as it needs. You may find that the unit will draw only 1-2 A when tested. This is however not true for a single LED that is not mounted in a circuit.

Your belief is partially true. You need to make sure you use a regulated power supply, and then the current rating can be greater than or equal to the maximum current required by the device being powered. There do exist unregulated supplies, and the voltage of these tends to be higher if you draw less current from them, sometimes much higher.

Something that everyone forgot, correct polarity if it's a DC adapter. The polarity of the new adapter needs to match the hubs polarity. Sometimes the manufacturer of a device will use a reverse polarity adapter (+ on the outside of the connector and - on the inside) so that you have to buy one of their adapters which are usally more expensive. I smoked a several thousand dollar piece of electronic equipment at work because I didn't check to see if the polarity of the used replacement adapter from the AC adapter scrap pile matched that of the unit.

I will be sure to check that the power supply I get is regulated and has adjustable polarity. Thank you all!