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HP laptop rejects Dell PSU on the basis of branding.. apparently. Crappy design, asshole design, or good design? (infosec.pub)

submitted 6 hours ago* (last edited 5 hours ago) by daveyOsborn@infosec.pub to c/askelectronics@discuss.tchncs.de

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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.

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[–] Onomatopoeia@lemmy.cafe 11 points 5 hours ago* (last edited 5 hours ago) (3 children)

Meh, those designs were to ensure you got the right power supply and didn't fry the mobo.

It's a little of asshole design, but it also prevented warranty issues from people using the wrong PSU.

Having been on teams depliying thousands of laptops and desktops, knowing everyone had the right power supply was important - this helped us too. End users can be oblivious and will just plug in shit willy-nilly. We only had a few spare machines, and no spare time. Imagine if using a different PSU could cause problems - we could be chasing a few rogue power supplies for days.

I'm ok with this choice, even with the bit of asshole design.

Fortunately USB C adoption is making this problem go away.

[–] wiccan2@thelemmy.club 4 points 4 hours ago (1 children)

USB-C has been game changing for us.

I can now confidently go to any desk or meeting room in the company and be fairly certain that my laptop will just work with whatever's been installed regardless of brands.

[–] daveyOsborn@infosec.pub 2 points 4 hours ago* (last edited 4 hours ago)

USB PD is also a shit show. You might want to read this:

https://goughlui.com/2025/12/01/tested-usb-c-barrel-connector-adapters-5-5mm-od-2-1-2-5mm-id/

Which shows those USB PD negotiations can get dicey. It chooses voltages thought to be close enough to what’s requested.

I’ve run into a nasty problem where a USB-C appliance needed 9v. Says in the manual something like “only use Kenwood power supplies on this device”. I thought, fuck that, USB PD is a standard for a reason. I’m not going to blow money on a proprietary OEM Kenwood USB-C PSU. Then found that many power supplies actually skip 9v. The USB PD standard makes some voltage steps optional, and some mandatory. IIRC, 9v was one of the required ones, yet it was easy to find USB-C power supplies that skipped 9v but offered 12v (or 15v, I forget). And yet 12v or 15v was one of the voltage steps that’s optional. And IIRC, the Kenwood OEM PSU /only/ did 9v, which is also not USB PD compliant, so the Kenwood PSU could not be used on other things.

So the USB PD strangely and arbitrarily makes some voltage steps optional, and manufacturers ignore the standard anyway.

I have an adapter that goes from barrel to USB-C. Recipe for disaster. Even if a 9v PSU uses that adapter and a USB-C appliance wants 9v, it will fry shit because any non-5v appliance expects to negotiate the voltage. I don’t recall how it leads to frying (I think b/c it tries to start at the 5v default before negotiating for 9v but instead it just gets hit with 9v), but the fact that there are USB-C-barrel adapters that just hardwire without the needed logic is scary.

[–] ChrysanthemumIndica@discuss.tchncs.de 2 points 4 hours ago

I just responded to the other post, and I definitely agree with you that when you're working with so many systems, knowing you have a baseline for the power supplies is important! (Especially given how sketchy those third-party adapters can be...)

But I will say I think it's pretty anti -consumer, and not accidental at all, that there is vendor lock in there. And as I said in that reply, I think a setting in the BIOS with a warning (and maybe a warranty invalidation?) that allowed it would have been the more friendly approach, but I can only imagine how much the lawyers would yell about why that's a bad idea, haha

USB-PD made me so happy, I was using my Apple power adapter with my Surface and Dell laptops and it was great!

[–] daveyOsborn@infosec.pub 1 points 4 hours ago* (last edited 4 hours ago)

So in the context of copious dumb users plugging in anything that fits the socket, the data PIN may¹ have prevented a lot of damage while at the same time enabled HP to rack in lots of money on replacement OEM proprietary PSUs. Indeed I have seen on many occasians clueless plebs at the street market selling 2nd-hand appliances and quickly trying barrels from a pile of tangled PSUs until one fits, then trying to include that with the device they are selling without looking at voltage or anything. It’s common and I’m sure lots of gear gets fried because the general population is just not smart enough.

I know how to match voltage, polarity, and current demands. And I got stung. Spent a lot of time disassembling a laptop, trying different RAM sticks, removing wifi cards and other components as I was baffled about what this fucking blicking LED means when it is not blinking in any way to convey an error code. Removed the CR2032 battery to reset the CMOS. Wondered if my slower than spec DDR3 RAM was causing this, so I went to the trouble of tracking down a RAM stick the precisely matched the specs. Still just got a steady non-stop blink, which the manual falsely states means it’s in sleep mode. So I was ready to conclude that the laptop was trapped in sleep mode and irreparably hosed. Perhaps I would have tossed a working laptop.

Whether it is a good design to protect from incorrect PSUs (despite that the obscure barrel connector is probably only 19.5v systems anyway), most certainly it’s a crappy design to not inform users. To fail to assign an error code. Sure, it vaguely says in the manual something like “use only approved HP power adapters” -- something /smart/ consumers do not take seriously because they know how to match PSUs to appliance and know that shit is always a branding hussle. The data pin should not be a secret that is concealed from both the user guide and the maintenance and service guide (which HP says is not for end users.. yet they still withhold the info from service people).

Perhaps good design as far as the PSU goes. But shitty to not document the situation and to not implement an error code.

¹ I stress /may have/ prevented damage because I have seen a lot of street market goods and only seen this obscure barrel tip on HP and Dell laptops, both of which are 19.5v.