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.
It’s possible both 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? 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.
As a person who has worked at Dell as a firmware engineer, with the very same 1-wire connection over that third data pin.
Yes, it’s true that at least HP and Dell will only allow use of an “authentic” adapter. The internal reasoning for this is that third party adapters may SAY they are compatible with system voltage/current, but in reality it might be insufficient or damage the board. (I personally think this is not a good enough reason, and that a BIOS setting with a safety disclaimer would be enough, but I digress)
The second and much more legitimate reason is that yes the adapter reports to the system what voltage/current it supports, so the system can set it’s charging and power targets accordingly (e.g. if wattage is too low, the system will know to pull from the battery a well) This data connection is proprietary for these three pin connectors (there’s just not a standard, and no one wanted to negotiate one), so they just don’t work between different companies. And this “accidentally” works as a vendor lock (I say accidentally because internally we always talk about it as if we are providing a safe service for customers, but I know the reality of capital so…)
What I said is true of Dell, and to my understanding it’s also true of HP (as they both constantly try to copy each other, good and bad). So yes, both things are true at once… it is in fact a design that is useful for these pre-usbpd systems, and very “conveniently” it has allowed vendors to lock in their adapters
That said, my understanding is that it’s pretty easy to get a third party adapter that intentionally mimics that data pin, and they’re much cheaper than the oem versions but of course buyer beware (personally, I’ve been horrified, completely horrified, at what I’ve seen on the insides of various cheap power adapters 😅)