Apple Power Supplies - why watts?

[RickNel]

In a current Conquests thread on a PB165 there has been discussion of correct ratings for power supplies that should be connected to various PB models. The official Apple support document seems to suggests you should not connect a PSU that exceeds the recommended level of "watts".

I don't get this. Normally PSUs are rated in voltage and amps, and these determine how much current the PSU can push through a given resistance. Are Apple's machines and/or PSUs designed without any over-current protection, current-limiting or fuses? To my understanding, any PSU at the correct voltage will only deliver the watts that the circuit requires, if it has the amps to do so. You'll only draw excess watts if you have a short circuit of some kind.

I've never heard of a PC, for example, that would be damaged by a PSU capable of delivering more watts than the machine actually needs. In fact, most PSUs are at way below capacity almost all the time.

Please enlighten me if I have missed something.

Rick

 

[coius]

You should correct that. Any system that uses a certain VOLTAGE will only take as much AMPS as it needs. Keep in mind AMPs x Voltage = Watts. if you have 10.8 Watts @ 7.2v you have 1.5Amps. Watts really should not be a measurement. You should should use Amps. A laptop is going to require a certain voltage (over voltaging will kill a system) however putting more amps (say 7.2v @ 3500ma) will not kill a system as long you use the proper voltage (7.2 with 7.2 systems, 9 with 9, 12 with 12, etc..)

A system will only draw as much current (amps) as it requires. A power supply uses amps as the max the power supply will put out under the best conditions. This is why a 265 Watt PSU on a computer might put out a consistent stream of 180 Watts, but if you go over 180 on a regular basis, it will overheat and break down the components. That said, a PSU For a laptop will only under the worst conditions draw the max amps from a power supply. Worst = charging battery, HDD access, optical access, screen/GPU/CPU at max, etc...

Even then, apple builds in extra current at the worst for good measure. Cooling on the other hand with apple power supplies are another matter altogether (remember the G3 and the PB 190/5300 PSUs setting on fire?)

 

[RickNel]

Thanks Coius, that confirms my understanding.

What I don't understand is why Apple's support document specifies its proprietary PSUs by watts instead of volts/amps. You could connect something with the nominated "watts" rating that would destroy your machine in a puff of smoke if the voltage was significantly too high.

Rick