The power extension has two USB slots. I noticed that when a Xiaomi phone is plugged first, and then iPhone next, the iPhone doesn't charge. However, if it's done in reverse, iPhone first, Xiaomi second, the iPhone charges and so is the Xiaomi phone.

Why plugging in a particular sequence makes a difference whether the other phone will charge or not?

I also tried to swap the plugging of phone on the USB slots, same thing happens.

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closed as off-topic by pipe, tomnexus, Leon Heller, brhans, Harry Svensson Jan 5 '18 at 11:59

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design." – pipe, tomnexus, Leon Heller, brhans, Harry Svensson
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    \$\begingroup\$ @jsotola Changed the "My power extension" to "The power extension" so you'll not get triggered. I hazard a guess you downvoted the question just for this reason? \$\endgroup\$ – Michael Buen Jan 5 '18 at 5:35
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    \$\begingroup\$ @jsotola Didn't understand your comment at first read. At first I thought this is literal: “everybody has that power extension” so I inferred the second phrase “so we all know what you are talking about” that you already know that all power extensions have USB slots now. Might as well be explicit with your question’s requirement than be in a non-helpful tone of “oh sure … everybody has that power extension” For a non-native English speaker, that could be inferred as literal. How hard it is to ask me: “What brand and model is the power extension, what’s the ampere?” Something like that. \$\endgroup\$ – Michael Buen Jan 5 '18 at 6:06
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    \$\begingroup\$ Anyway, I edited the question and attached a picture of the power extension \$\endgroup\$ – Michael Buen Jan 5 '18 at 6:06
  • \$\begingroup\$ apologies. your English is flawless, so i thought that a sarcastic hint would work. What brand and model is the power extension? .... there may be a user manual that makes a mention of any precautions. \$\endgroup\$ – jsotola Jan 5 '18 at 6:15
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    \$\begingroup\$ I guess that the Xiaomi draws enough current to cause a voltage drop. the iPhone detects the low voltage and refuses to charge. The Xiaomi does not care about charging voltage, so it happily charges when plugged in second. \$\endgroup\$ – jsotola Jan 5 '18 at 6:19

I can offer the following explanation of your phenomenon. If you would search the EE site, you would find out that chargers do have various different "signatures", and phones might understand one signature but not other signature.

Since your iphone did take charge when plugged first, I would assume that your "power extension" ports have the Apple signature, likely in parallel. The Apple standard signature is 45k/75k voltage divider (or of this order of values) on D+ and D- lines; different combinations signify different charger capabilities. This is a high-impedance signature, so it could be easy to screw/skew it up with some other device plugged in. If this signature is compromised and exhibits wrong levels, iphone won't take charge.

Apparently your other Xiaomi phone distort this signature when plugged first (it might behave as a host and have 15 k pull-downs), so if the iphone is second, it gets confused. If you plug the iphone first, it recognizes the Apple signature, and charges. Apparently your other phone doesn't care about signatures.


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