power adapters, amperage, and power consumption

[mattkime]

given two power adapters that produce the same voltage, supplying the same device -

if one power adapter has a higher amperage rating, does it draw more current?

(i'm trying to figure out if its wasteful to use a power adapter rated for more than a device requires)

[freeradical]

mattkime wrote:
given two power adapters that produce the same voltage, supplying the same device -

if one power adapter has a higher amperage rating, does it draw more current?

(i'm trying to figure out if its wasteful to use a power adapter rated for more than a device requires)

No, the amperage rating is how much current it can provide to a load.

[Carm]

If the product calls for 1 amp and your adapter can provide 3 amps. You product takes the 1 amp is was designed for. It will/should allow the adapter to run cooler.

[JoeH]

Depends on the design of the power adapter. A well designed one will generally use less power and do that even when not in the most efficient range of the design. The only way to know for sure is to test both with the load you want to use it with.

[modelamac]

A device will draw the current it needs and no more, even if more is available. The light bulbs in your house have 15 amps available, but use only a fraction of that. It does not matter what the power supply is, whether a wall wart or a circuit box, or AC or DC.

mattkime wrote:
given two power adapters that produce the same voltage, supplying the same device -

if one power adapter has a higher amperage rating, does it draw more current?

(i'm trying to figure out if its wasteful to use a power adapter rated for more than a device requires)

[Filliam H. Muffman]

A good question. If you are talking about using the 65W PowerBook adapter versus the 85W one, the 85W will likely use a half watt more when the laptop is running lightly loaded. If the 85W is a full generation newer (3 or 4 years), Apple might have improved the efficiency a small amount so they use about the same amount.

Power supplies run at the lowest efficiencies (70% to 75%) at under 10% loaded. I am too lazy to type out all the equations but here is an example:
the 85W is 10% loaded and running at 73% efficiency versus
the 65W is 15% loaded and running at 78% efficiency. Which uses more power?