Overpowering

[Мастер]

The power supply for my portable Apple computer needed replacing. Probably nothing defective about the original power supply, it just wasn't designed to handle the level of punishment (which included an animal gnawing on it) that it received.

The computer is just about three years old, and came with a power supply that looks like this.



And it's a 60W supply. This type of supply is now considered old-fashioned, Apple having switched over to something with a different type of connector sometime in the last few years.

Being highly dependent on my portable computer, I headed over to the huge shopping mall a few metro stops away, even though today is a public holiday.

Shop #1 - I stood around for about ten minutes, and none of the workers showed the slightest interest in talking to me, so I left.

Shop #2 - they had a power supply with the right type of connector, but which was 85W, not 60W. They suggested that I should not use it, because it would damage my battery.

Shop #3 - they had the exact same supply, but said I would be just fine using it, as long as I don't "overcharge" my battery.

I'm making this post right now, using the 85W power supply plugged into the computer. (Well, not any more, I just unplugged it a few seconds ago, it having reached 100% charge.) I bought it at Shop #3.

Is there any merit to the suggestions of Shops #2 and #3 that using this higher power supply might damage the computer's battery? I know what I think, but I'm wondering if anyone has any wisdom to share on this.

[Lance]

No, it's not a problem at all. The rating is how much power the supply CAN deliver, not what it forces. It's like plugging into a wall socket. In the US, a typical wall socket can deliver 1800 watts but it doesn't blow out your 100w light bulb.

[Lance]

I typically use a 90w power supply on a PC designed for a 60w one. Never had a problem.

The battery knows when it's charged and won't take more than it needs.

[Мастер]

No, it's not a problem at all. The rating is how much power the supply CAN deliver, not what it forces. It's like plugging into a wall socket. In the US, a typical wall socket can deliver 1800 watts but it doesn't blow out your 100w light bulb.

That's what I figured - the thing would maintain a (nearly) constant voltage level, regardless of the load, and the amount of current (and therefore power) delivered would depend on the electrical characteristics of the load (in this case, the computer). If the voltage of the 60W and the 85W supplies are the same, then the same amount of current will be delivered when the same load is connected. Unless the load overwhelms the 60W supply, and it is unable to maintain nominal voltage at the desired level of current.

So I figured these people didn't know what they were talking about. But I also figured, it doesn't hurt to ask here

[Мастер]

In the US, a typical wall socket can deliver 1800 watts but it doesn't blow out your 100w light bulb.

So I guess your 1800W is 15 amps, which causes me to think of something completely different - our voltage (and the voltage in much of the world) is much higher than yours. So I'm guessing most appliances draw about half the amperage here, than where you are? Presumably, their power requirements would be similar.

[Lance]

In the US, a typical wall socket can deliver 1800 watts but it doesn't blow out your 100w light bulb.

So I guess your 1800W is 15 amps, which causes me to think of something completely different - our voltage (and the voltage in much of the world) is much higher than yours. So I'm guessing most appliances draw about half the amperage here, than where you are? Presumably, their power requirements would be similar.

Yes. Higher voltage and lower current equals the same power.

[Мастер]

Yes. Higher voltage and lower current equals the same power.

That much I remember from electrical engineering school

But I just wonder if the power requirements are the same - and I can't think why they shouldn't be, why would our refrigerators use more or less power than yours? Or if the efficiency of the power supply is similar.

[Lance]

Yes. Higher voltage and lower current equals the same power.

That much I remember from electrical engineering school

But I just wonder if the power requirements are the same - and I can't think why they shouldn't be, why would our refrigerators use more or less power than yours? Or if the efficiency of the power supply is similar.

Oh, I see. Well, sticking with our PC power supplies, the modern ones can be plugged into either 120v or 240v just by changing the cord. No transformer necessary, they auto-switch. And they provide the same 90w regardless.

[Lance]

But I guess that answers the exact opposite of what you asked.

So never mind.

[Enzo]

The power supplies take the mains voltage and current, and turns it into the lower voltages the system needs to operate. The system itself is the same in there, regardless of what mains supply is involved. So in general there is no reason for a 100 watt thing to use more or less power, depending on mains voltage. A 240 watt system (number chosen for simple arithmetic) will draw 2 amps from 120v service and 1 amp from 240v service. Something like a refrigerator works with a motor. it is possible the 120v motor and the 240v motor have some small efficiency differences, but that is a separate issue.