cross-posted from: https://infosec.pub/post/51092505

I have a 3-pin Dell PSU spec’d as 19.5v 3.34A and DMM measures at 20v unloaded. The power reqs on the HP laptop are 19.5v 3.33A. AFAICT, that’s a match. Even the 3-pin barrel connector geometry matches. So what’s the problem?

The center pin is a “smart pin” aka “data pin”, as I came to learn. According to @over_clox@lemmy.world, this PIN basically passes brand info so HP can reject non-HP PSUs. But according to this article, the data pin is used to “regulate” the voltage – which I take to mean it negotiates the voltage for transmission as USB-PD does. If it’s merely negotiating power reqs, then the Dell should work.

It’s possible both sources are correct… perhaps HP has its own proprietary way of negotiating voltage and Dell has theirs.

Someone even claims that the data channel is used to communicate temp info so a PSU does not overheat. If that’s true, then perhaps it was a good design (noting that the USB-PD standard did not emerge yet).

But is it true about the communication? Considering there are hacks with a resistor or something to satisfy the data channel, it’s hard to believe that the communication is that sophisticated.

This page advises HP owners to always buy HP-branded PSUs. But it’s unclear if that’s just overly safe nonsense.

    • daveyOsborn@infosec.pubOPEnglish
      1·
      11 小时前

      Oh, so to be clear, an asus laptop that expects a USB PD supply can still detect whether or not it’s an OEM charger? And when it’s non-asus, it distrusts the capability and only draws 65w even if the PSU can do 100w?

      IIUC, that’s a bit fucked up. I’ve read claims that either USB PD or the proprietary 3-pin predecessors are supposed to be smart enough so appliances can go easy on PSUs that are not up to the full load, but I was expecting USB PD to not have vendor favoritism problems like that. I appreciate the tip. Seems like another gotcha that can even burn above average consumers who know to a reasonable extent what they are doing.

      (update) found this

      Myth #2: “Original Chargers Are Always Best” Truth: Not anymore.

      Many manufacturers use cheap chargers to cut costs. A quality USB PD charger often beats what’s in the box.

      So I have to wonder if an asus laptop would actually nanny a non-asus PSU that is better than the asus oem psu.

      • Cort@lemmy.worldEnglish
        2·
        9 小时前

        Yes, I believe you understand correctly.

        And yes Asus laptops/tablets reject full power charging on any non Asus USB PD PSU, even when the alternative brand is better. I’ve tried a couple including the 180w framework