I am specifically looking to replace a 12V 3A power supply that seems to have disappeared from a backup HDD. I found a 12V 5A power supply with the right connector on Amazon, and ordered it without hesitation as I've always "understood" that as long as the power supply was rated for at least the peak current the device might draw, it was fine.
But now that I've had time to consider it a bit, I'm less sure… Having tinkered a bit with TI's designer tool it seems there is considerable room for optimizing the supply circuitry with the expectation of a particular range of loads. Will a "typical" off-the-shelf, switching power supply brick rated for 5A deal gracefully with all loads from 0-5A?
A little knowledge is a dangerous thing, as they say… Is my "understanding" that the higher-rated supply is safe / reasonable actually correct?
Best Answer
Is it always safe? No. It is usually safe? Yes.
There are some characteristics of a power supply that will effect how you use it.
Minimum Load: A regulator, even a linear regulator, will have a minimum load. Below this load the power supply will not regulate the output as well. Sometimes the output will be +/-10% instead of +/-2%. Other times the output will be way out of whack. Generally speaking (but there are always exceptions), a power supply rated for a higher max output will also have a higher minimum load. You might get into trouble if the new supply has a higher minimum load than the old supply.
Quiescent Power: This is the power that just the supply is consuming, separate from the load and separate from the supply efficiency. Bigger supplies generally have higher quiescent power, and that means that the supply will generate slightly more heat.
Efficiency: Switching supplies generally have the highest efficiency at about 80-90% of the rated max current. If you get a bigger supply and now you are somewhere near 50-60% of the max current then you are likely running at a lower overall efficiency-- which means more heat. If you are putting this supply into a chassis that is designed for less heat, you could have problems.
Usually, these issues are not major and you will be safe using a larger supply. But, you must consider these other issues, just in case.