1

I’m looking to understand how to test and evaluate chargers so this is a bit technical a problem. Since MagSafe is proprietary, I can’t consult a USB power distribution specification to know how things are designed to work.

What is the voltage output of an Apple 85W MagSafe 2 model A1424 without a load?

migrated from electronics.stackexchange.com Aug 23 at 3:28

This question came from our site for electronics and electrical engineering professionals, students, and enthusiasts.

  • Feel free to edit in some of the specifics - how you are going to measure this might help others greatly. Knowledge is power and if you can caution people not to use power to start a fire or damage gear, even better. – bmike Aug 24 at 19:22
2

I believe these are smart chargers, and initially only supply about 3 volts in order to power a power management chip inside the device to be charged. Once powered by the 3 volts, the power management chip inside the device then communicates with the charger verifying that the device is compatible to be charged, and if that fact is verified, only then will the charger output full charge voltage.

Additional info provided by @MarcWilson.

  • Hi @Hitek. Thats what I read on internet too about sending a small volt at beginning and then the full. But whats strange is that even the small voltage sometimes is 0.2V and only one time it was 3V . I measured between Ground and Power according to this picture static.righto.com/images/magsafe/1-magsafe_connector_labels.png . Was wondering if someone knew the correct "no load voltage" for the model A1424. Thank you – AlberAls Aug 23 at 9:55
  • @AlberAls - From an engineering perspective, the charger has to provide power to the target device in order for communication to happen, in the case of the target device's battery being completely dead. In such a situation, if the charger didn't supply power, then the dead device could never be charged again. If you are not getting any significant voltage(BTW 0.2V is NOT enough) from the charger at times, then the charger may very well be faulty. It's rather hard to say specifically, though, as I don't have a Magsafe 2 charger, and Apple's standards are highly guarded secrets. – Hitek Aug 23 at 10:21
  • @AlberAls - Also, if you are getting 3 Volts sometimes and not others, you should hold a meter on the connector while having a second party flex the cable around, to make sure it isn't just an intermittent connection due to a bad cable that could easily be repaired. Another possibility is the fact that some chargers for other brand products actually send the standby voltage over the data(center in your case) pin rather than the power pins, so you might check that also. – Hitek Aug 23 at 10:25
  • @nekomatic - Sorry I don't have the reputation to comment directly to your answer. You are correct that the charger only produces it's full output voltage when it senses the resistance across the power lines, but from what I understand, the device will not introduce this resistance until after it has been verified that the charger is valid during the previous negotiation.... – Hitek Aug 23 at 21:31
  • 1
1

Based on the answers to this question the supply doesn't communicate with a power management chip - it simply looks for a resistance of approximately 40 kΩ between its output and ground, and if this is detected then the full output voltage is enabled.

Also based on the above answers, if you often measure 0.2 V rather than 3 V then an intermittent fault in the cable seems most likely.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .