This puzzles me.

I have a couple of 9V power supplies at home. One has marked on it 300mA and the other 600mA. How can they know what type of circuit I will be connecting to, for them to arrive at a definite current rating? I mean isn't the total current relative to the total resistance in the circuit? (I=V/R)

So what is the difference between a 9V 300mA power source and a 9V 600mA power source?

Thanks for any help in understanding this!

 

The mA rating is the maximum current you can take from the transformer without it overheating. You can draw what current you like, but only up to that maximum. So if you want to draw 400mA then chose the 600mA one.

 

Thank you very much. So, the rating doesn't mean that the power source will necessarily pump out xxxmA but that it has the capacity to handle that amount of current without overheating.

That makes perfect sense. Thanks again.