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I'm supplying power to a MacBook Pro via a magsafe cable from a broken 60 W power supply.

Is it safe (1) to omit the initial high impedance 6 V or 3 V and simply apply "14,5V to 18,5V on Magsafe 1 and up to 20V on Magsafe 2"? Do I have to match the voltage of the power supply reported by the cable connector (16.5 V)?

Powering your (Apple) laptop in the wild
Teardown and exploration of Apple's Magsafe connector
What voltage readings should i get from my 60 watt magsafe

(1) Assuming I'm not shorting the pins with a spoon. This risk seems to be why the PS only turns on full power after it senses 40 kOhm across the power pins for 1 s. Is there a risk of shorting the pins when the plug is not properly seated? In that case I'd apply power after nicely seating the plug.

  • If its broken, don't use it! Why take a chance and damage your expensive laptop! – IconDaemon Sep 12 '18 at 13:35
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Is it safe (1) to omit the initial high impedance 6 V or 3 V and simply apply "14,5V to 18,5V on Magsafe 1 and up to 20V on Magsafe 2"? Do I have to match the voltage of the power supply reported by the cable connector (16.5 V)?

The "initial high impedance 6V or 3V" is handled by the power supply that sends the 3/6V until it detects a "power line low" condition (the MagSafe is connected) at which point it sends the full 16-20V DC

Do you need this initial 3/6V? No. What you need to do is supply clean, stable 16-20V DC to the MagSafe cable (shielding to negative, white inner to positive) because that's what's required to power your Mac.

This risk seems to be why the PS only turns on full power after it senses 40 kOhm across the power pins for 1 s. Is there a risk of shorting the pins when the plug is not properly seated? In that case I'd apply power after nicely seating the plug.

Yes, there's a risk - a low one. It's extremely difficult to short the pins on a MagSafe connector. I'm not saying it's impossible, but the design makes it so it's extremely difficult to do so. However, if you do short the points, the damage will most likely be minimal (you'll blow the MagSafe DC-In board and not the whole logic board), but in my experience, it's not worth it.

What I have done is connect a MagSafe to a switched AC adapter "wall wart" that supplied 18V at 3A to my MagSafe and powered my "benchtop" MagSafe diagnostic power supply. I did it this way so I could insert a multi meter in line so I could measure amperage draw. I've never shorted pins and this worked great.

Standard disclaimer: Don't try this at home. I only play a professional on TV.

  • Can I go up to 20 V even with Magsafe (not magsafe 2)? – darsie Sep 12 '18 at 13:08
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    Yes. You can go up to 20V because the DC in board will "step it down" to the proper voltage. Also, I wouldn't do this with a 60W. I would do it with an 85W as it's compatible with "everything". You can use a higher wattage adapter on a lower wattage device, but not vice versa. – Allan Sep 12 '18 at 13:11
  • Well, the thing is, I have a 24 V laptop charger and a 20 V one. With the 24 V one I need a step down converter, the 20 V one I could use directly. – darsie Sep 12 '18 at 13:15
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    The 20 will be fine. I've never tried with a 24V – Allan Sep 12 '18 at 13:18

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