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.
That's no part of USB spec as there's no way to communcate requirements through the barrel. Hard to blame USB for when we side-step it's design.
I do stuff like this too, but only for devices that just need power, nothing like a laptop or with circuitry that could get fried by the wrong voltage.
Do you mean the USB PD spec? It does not violate the USB spec which sets a default voltage of 5v. The adapter /might/ violate the USB PD spec if it were to claim to be PD compliant. But the shit-show we have is that USB-C does not imply USB PD. And so the market is full of USB-C things that are not USB PD. Often they do not claim to be USB PD as consumers are not wise enough to demand it. AFAICT, USB-C devices that do not mention USB PD would stand up in court.
Note as well the adapter I mentioned is quite dumb -- just hardwires pins for the situation that the appliance expects 5v without negotiation. Naive consumers could get burnt for sure whenever exceeding 5v.
But it would be possible to implement a USB PD-compliant adaptor with the smarts to handshake, which would then refuse to complete the handshake in the event that the voltage supplied is not that requested. Of course it would be a bit strange to put that much sophistication into it which I suppose would drive the cost to that of a whole compliant PSU anyway (thus defeating the purpose). But notice the opposite has been done in a compliant manner, whereby the barrel adapter negotiates a USB voltage on behalf of a barrel device.
IIRC, it’s disputed whether the hanshake negotiation is needed for 5v. Some appliances expect to do a handshake /no matter what/, and some 5v appliances are designed to skip the negotiation entirely. If you are in the camp that says even 5v must have a handshake negotiation, then you would condemn the simple barrel to usb-c adapter (but perhaps not one that is only for a fixed input and the circuitry to do the negotiation).
The blame was not pin-pointed. I pointed out a shit show in the marketplace. There are bits of the standard you can blame (ambiguity and also having optional voltage steps), and you can also blame market actors. You can probably also blame regulators for not preventing the shit show that I described. And perhaps even blame consumers for not insisting that what they buy is tagged as USB PD complaint.