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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?

Kaelin Colclasure
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  • I could be wrong, but I don't think I've ever heard of an unregulated switching power supply. That said, Harddrive current depends on it's state. If it's spun down, in a sleep mode, the drive will not take any where near 2~3 Amps. And Harddrives can be in a sleep state for a long time. – Passerby Apr 17 '13 at 23:27

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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.

  • Thanks, this confirms what I'd come to suspect after learning a tiny bit more about power supply design. – Kaelin Colclasure Apr 18 '13 at 06:22
  • Worth pointing out that minimum load for proper regulation is likely to be 5ma or 1% or 5% of rated current, and that you cannot choose between a 3A or 5A PSU that way. It's often poorly documented and depends more that anything else on whether the customer asked the question... –  Apr 18 '13 at 08:27
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It is okay to replace a power supply with one that is rated at higher amperage. However, you do need to consider few things. The connector may look the same, but the polarity could be reversed, do check for that. Also, some power supplies are regulated and they will put out the voltage very close to what is stamped on them; some power supplies are not regulated, they may put out voltage that is much larger than what is stamped on them and may have large noise ripples as well. If you measure the voltage under no load and it is much higher than the stamped value, that indicates the power supply is not regulated.

Suirnder
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It is correct that you can design a supply to it's intended load. However, there are two things to consider in this particular situation:

1) Hard drives vary in load, as pointed out by @DavidKessner.

2) The guys who did the work on that backup HDD almost surely did NOT design that 3 amp supply. Almost no one designs and manufactures their own wall-wart power supplies.

Half or more of the point of using a wall-wart is that you can buy the supply on the open market as a pre-made part that already has all the relevant safety agency ratings, test certificates, etc, etc. It saves the design an enormous amount of time going through various approval processes that have very little to do with the actual thing they are trying to make money on.

Given that information, we can then guess that yes, any decent supply with the appropriate connector, the same voltage and a bit higher amps rating should work fine.

Be sure to check the polarity before plugging in, but otherwise, you should be good to go.

Michael Kohne
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