A load and a supply are different....
This is a basic bit of knowledge that needs to be well understood - but it is not hard.
- A power supply that is rated at 12V, 5A will provide a fixed 12 volts and be capable of providing up to 5 amps of current at that voltage.
- A load that is rated at 12V, 5A will require a fixed 12 volts and 5 amps of current at that voltage.
- In all cases, the voltages must match and the current capabilities of the supply must always be equal to or greater than the demand of the load.
Examples. Let's continue to use the 12V 5A supply for these examples.....
- A circuit (load) requiring 12V 1A will work happily. (In fact, you could run 5 such circuits.)
- A circuit (load) requiring 12V 200mA will work happily.
(In fact, you could run 25 such circuits.)
- A circuit (load) requiring 12V 5.5A
(it's just a little bit over) continuously
** would be a problem. It
might work, but the supply is overloaded. Depending on the supply, this could cause overheating, a drop in voltage or activation of protection mechanisms and your load may not function correctly. If you have a supply that can still deliver the power, then it will be under a greater stress than it was designed to handle - and it may suffer a shortened life.
- A circuit (load) requiring 12V 20A continuously
** will not work. The supply will just cave in with either overheating and a drop in voltage or activation of protection mechanisms. Depending on the supply, this could be a serious fire risk.
** Often there are circuits that perform different operations at different times and their current requirements can fluctuate.
One example is a temperature controller with a heating element. When monitoring, it may draw very little current which is well within the capabilities of the supply - and during these periods, all will be well. But when the element is switched on, the current draw will go up dramatically. If the total current required by the load now exceeds the capabilities of the supply - problems will arise.