85W power supply on a 13" MBP?

You can safely use an 85W power supply on a system that needs a 60W supply; you can't safely use a 60W supply on a system designed for 85W power supplies.

"you can't safely use a 60W supply on a system designed for 85W power supplies."

There's no safety risk. There is a possibility that the 60W power adapter won't provide sufficient power to both charge the battery while you are using the computer. Accordingly, it is possible that your battery will drain, and your computer will shut down.

There is a possibility that the 60W power adapter won't provide sufficient power to both charge the battery while you are using the computer.

yeh, like you can put a 25 amp fuse in a circuit rated for 15 amps.... NOT


The brick is rated at 60 watts; there will always be some possibility of overheating.

"yeh, like you can put a 25 amp fuse in a circuit rated for 15 amps.... NOT"

You are confusing CURRENT with POWER.

You are confusing CURRENT with POWER.

LOL

For any given voltage, power is directly proportional to current. Since the output voltage of the charger is the same for the 60 and 85 watt models, it's the safe current rating that is different.

Sorry if I offended your nit picking sensibilities.

"Sorry if I offended your nit picking sensibilities."

Confusing CURRENT with POWER is a profound misunderstanding of electrical theory. Nit picking, it is not.

I understand electrical theory very well; guess YOU don't understand. By using a 60 watt power supply on a machine designed for the 85 watt power supply - since both computers need the same voltage - you run the risk of trying to draw too much CURRENT from the power supply. (A well designed power supply should include thermal overload protection that will shut it down in over current situations.)

Um, no.

Power = Voltage x Current

Assuming both power supplies operate the same voltage, current is directly proportional to the power provided by the adapter. Current and power and not separate concepts.

F Shippey wrote:
I understand electrical theory very well; guess YOU don't understand. By using a 60 watt power supply on a machine designed for the 85 watt power supply - since both computers need the same voltage - you run the risk of trying to draw too much CURRENT from the power supply. (A well designed power supply should include thermal overload protection that will shut it down in over current situations.)

I understand your concern, but I don't think there's any real problem since Apple has already designed their circuits to work even if the adapter can't supply enough current. It's supposed to just tap into the battery for supplemental power. I'd be surprised if Apple didn't already factor the possibility of a lower rated adapter being used on the bigger MBP. There would be a liability issue if they didn't since the adapter itself isn't locked out and they're known to work with each other rather than refusing to work.

Already there are times where the 60W power supply isn't sufficient for the 13" MBP. It's supposed to simply draw some power from the battery in those cases.

And the current/power discussion is just plain stupid. In my background in electronics, whenever a constant-voltage DC power supply is discussed, current and power are proportional and pretty much interchangeably discussed since P=VI. Most people here seem to get that.

"By using a 60 watt power supply on a machine designed for the 85 watt power supply - since both computers need the same voltage - you run the risk of trying to draw too much CURRENT from the power supply."

Nope. The Apple power adapter will only produce as much power as it is designed to.

"A well designed power supply should include thermal overload protection that will shut it down in over current situations."

The Apple power adapter has several protection circuits.

Despite your claim that you are knowledgeable of electrical theory, power, current, and voltage are different. I'm not nitpicking.

The original poster asked if they could use the 85W power supply on a 13" MBP designed for a 60W power supply, and I answered YES. This answered his question.

I would personally NEVER recommend someone use a power supply in a potentially over-current situation - even though the power supply probably contains thermal overload protection - as such protective circuits can fail. Thus, I will never recommend someone use a 60W supply on a machine designed for an 85W supply.

carl wolf wrote:
Despite your claim that you are knowledgeable of electrical theory, power, current, and voltage are different. I'm not nitpicking.

In a fixed voltage system, power and current are proportional and effectively interchangeable. Once you know the max current allowed, you'll know the max power. It's a simple matter of applying Ohm's law.

"This answered his question."

But then you added your opinion, and it was wrong.

Before I left Apple, I was the reliability engineer responsible for all portable computers, and their power adapters. I have forgot You might have everyone else convinced in this forum - along with your mom and dad - that you understand electrical theory, and the operation of Apple's equipment. Your posts indicate otherwise, and you'll never convince me.

OK, Carl, lets make sure we all understand what you are saying. You are saying, based on your experience as 'a former reliability engineer responsible for all portable computers, and their power adapters at Apple', that it is perfectly fine to use a 60W power adapter on a computer designed for a 85W adapter and that there is NO chance of any damage to the computer or power adapter.

I'm sure the tech folks at Apple will be very interested in your reply.