How does a macbook power adapter know the voltage to output to a Macbook?

Rodion15 · Jul 3, 2015

How does the power adapter know what voltage it's to output to the MacBook?. I mean: and 85w adapter, for example, will give 14 to 20 volts depending of which MacBook model it's connected to. I read "Teardown and exploration of Apple's Magsafe connector" and this article mentions that the 1pin wire communicates the adapter wattage to the MacBook, but how can it know what voltage to output to the MacBook?.

i7Baby · Jul 3, 2015

Its an AC adapter. The voltage regulation is done by the controller in the laptop.

Rodion15 · Jul 3, 2015

i7Baby :

Macbooks have an input voltage of roughly 14v, 16v, 18v or 20v that then goes to the regulator you say. My doubt is how the power adapter actually knows which power to output for each model.

Someone Somewhere · Jul 6, 2015

i7Baby :

No it's not. It's an AC adapter in the sense that it takes in AC power. It's output at a fixed DC voltage.

I don't know about macs specifically, but usually all the regulation to e.g. 18V is done in the adapter - it's just a simple switch-mode power supply.
The laptop then has a pile of DC-DC convertors (with their own regulation) to give it the voltages needed for its components from either the adapter or the battery, and to charge the battery.

How variable voltages are usually done is you have a resistor inside the device, between two pins (or one pin and ground). The controller chip in the adapter reads the value of this resistor by applying a known current through it and measuring the voltage drop across it, then regulates the output voltage based on this.

Guest · Jul 6, 2015

Someone Somewhere :

Hi, thanks again for your answer: You're probably right, but in order to understand my doubt you'd need to read that article "Teardown and exploration of Apple's Magsafe connector" (it's a well known article, the only out there about Mac magsafe adapters that I know, you'll find it on Google).
You're probably right, so it looks like the AC adapter gives one of its four available DC voltages based on the resistive load at the MacBook input. Notice that in MacBook AC adapters, the sense pin goes only to the Magsafe connector, not to the AC/DC adapter. This article seems to say that the resistive load is 39.41 KΩ for all Macbooks, that's why I'm confused.

Someone Somewhere · Jul 6, 2015

When the Magsafe connector is plugged into the Mac, the Mac applies a resistive load (e.g. 39.41KΩ), pulling the power input low to about 1.7 volts.

I think this is saying that the macbook could apply a resistive load of for example39.41KΩ, not that every macbook does. This is likely it.

Someone Somewhere · Jul 6, 2015

I found this, which seems to be by you.

Where were you measuring with the voltmeter? Sometimes you can get inaccurate readings.

Were they all at the same state of charge? Did you get the same voltage for the same model of macbook, or did it vary within models?

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How does a macbook power adapter know the voltage to output to a Macbook?

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