26
HP laptop rejects Dell PSU on the basis of branding.. apparently. Crappy design, asshole design, or good design? (infosec.pub)
submitted 2 months ago by to c/askelectronics@discuss.tchncs.de
21 comments fedilink hide all child comments
 

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.

you are viewing a single comment's thread
view the rest of the comments
[–] [S] 2 points 2 months ago* (last edited 2 months ago) (7 children)

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.

  • permalink
  • fedilink
  • source
  • parent
  • hideshow 7 child comments
    •
  • [–] 3 points 2 months ago* (1 child)

    So I have an adapter that goes from barrel to USB-C

    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.

  • permalink
  • fedilink
  • source
  • parent
  • hideshow 1 child comment
    •
  • [–] [S] 1 point 2 months ago* (last edited 2 months ago)

    That’s no part of USB spec

    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.

    as there’s no way to communcate requirements through the barrel.

    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).

    Hard to blame USB for when we side-step it’s design.

    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.

  • permalink
  • fedilink
  • source
  • parent
  • [–] 2 points 2 months ago (3 children)

    I'm getting the sense from your comments that you think that USB-PD will deliver a higher voltage than requested. This is not accurate. A device can request any of the voltages specified by PD, and the power supply will supply that voltage if it is capable of doing so. If not, it will provide 5V. It's very possible to get a sippy that won't power your device, but a working, PD-compliant power supply will not damage a working, PD-compliant device.

    Obviously, if you get a barrel jack adapter that requests 20V and then plug it into a 5V device, you're going to let the magic smoke out, but that's because barrel jacks don't do negotiation. The power supply correctly supplies 20V as requested, and then you plug that 20V into the wrong thing.

  • permalink
  • fedilink
  • source
  • parent
  • hideshow 3 child comments
    •
  • [–] [S] 1 point 2 months ago* (last edited 2 months ago) (2 children)

    I’m getting the sense from your comments that you think that USB-PD will deliver a higher voltage than requested. This is not accurate. A device can request any of the voltages specified by PD, and the power supply will supply that voltage if it is capable of doing so. If not, it will provide 5V. It’s very possible to get a sippy that won’t power your device,

    See the experiment I linked in the post that you replied to. If 19.5v is requested, 20v is supplied. If 12v is requested, 9v is supplied (not 5v). But also since 9v is optional, some PSUs would push 5v. So the voltage you get depends on what the PSU makers feel like sending.

    but a working, PD-compliant power supply will not damage a working, PD-compliant device.

    Yet they will fail because compliance is subject to interpretation and the ambiguities are disputed. Try plugging a USB-C bicycle light into a Lenovo USB-C PSU. Even though they both handle 5v, it fails. The Lenovo device makers believe all voltages, even 5v, must be negotiated. The bicycle light makers believe a 5v “default” /means/ they should get 5v without negotiation. The Lenovo PSU makers believe “default” means 5v will be the result of a failed negotiation (but not absence of negotiation).

    And other device makers believe USB PD requires supplying the closest voltage when negotiation fails, not 5v. Those people probably also believe 5v should be sent in the absence of negotiation.

    Obviously, if you get a barrel jack adapter that requests 20V and then plug it into a 5V device, you’re going to let the magic smoke out, but that’s because barrel jacks don’t do negotiation. The power supply correctly supplies 20V as requested, and then you plug that 20V into the wrong thing.

    You’re mixing up the different adapters. You can have a USB→barrel and you can have a barrel→USB. The interesting case is what I described, when the appliance and PSU are compatible in terms of voltage but still fail because of the protocol. Also consider that a 20v barrel PSU could feed a dumb usb-c adapter which then fries a 5v device. In principle, the adapter /could/ do a negotiation and refuse to supply 20v to a 5v device, or even just cut-off anything above 5v, but the shitshow that we have is a marketplace with dumb adapters. And in both situations (usb→barrel and barrel→usb), it’s not foolproof. Countless consumers cannot handle the voltage matching task so if the plugs fit, they will try it.

  • permalink
  • fedilink
  • source
  • parent
  • hideshow 2 child comments
    •
  • [–] 2 points 2 months ago (1 child)

    If 19.5v is requested, 20v is supplied.

    You're misreading the blog post. That adapter has 19.5V written on it, but it requests 20V. 19.5V is not a PD voltage.

    You can have a USB→barrel and you can have a barrel→USB.

    The latter is pretty much impossible to make to spec, because it can't negotiate voltage. A barrel jack is single-voltage by necessity.

    The interesting case is what I described, when the appliance and PSU are compatible in terms of voltage but still fail because of the protocol.

    Respectfully, I'm going to have to disagree. It's much more interesting when it catches fire, and I'd rather have a protocol that sometimes doesn't operate in order to avoid the excitement of one that sometimes explodes

    Also consider that a 20v barrel PSU could feed a dumb usb-c adapter which then fries a 5v device.

    No matter what charging standard you use, some idiot could cut off the end, wire it to a stun gun, and send 100,000V into your phone. That doesn't make the standard bad, it makes the adapter bad.

    And in both situations (usb→barrel and barrel→usb), it’s not foolproof.

    If you do it correctly, a USB power supply being adapted to a barrel jack won't destroy any device that can tolerate 5V. It may not operate the device, but as long as nothing breaks, I'd consider it as foolproof as anything reasonably can be.

    Countless consumers cannot handle the voltage matching task so if the plugs fit, they will try it.

    That's just how electricity works. You can get 5V devices that use 5.5x2.5 barrel jacks and you can get 20V 5.5x2.5 power supplies. You can get C13 leads that connect to 240V wall outlets and you can get C14 devices that only accept 100V. If someone plugged things in without reading before USB, they broke things and started fires. PD simply doing nothing when incompatible devices are connected is a massive upgrade over that, even if it can be frustrating sometimes.

  • permalink
  • fedilink
  • source
  • parent
  • hideshow 1 child comment
    •
  • [–] [S] 1 point 2 months ago* (last edited 2 months ago)

    You’re misreading the blog post. That adapter has 19.5V written on it, but it requests 20V. 19.5V is not a PD voltage.

    We know the /result/ is 20v, not what was requested. It’s a lie and abuse to print 19.5v on an adaptor that is actually just requesting 20v. It’s certainly /possible/ that is what is happening, which obviously proves my point nonetheless. It’s yet another demonstration of the USB PD shitshow. Either way in fact. Whether the negotiation is a lie, or whether the source is saying “close enough, will send 20v”, it’s a shitshow either way.

    But now it’s worse because consumers who know how to pay attention to voltage are getting a product that’s not delivering what they signed up for.

    Worth noting that the PPS spec of the PD3 standard allows for 20mV step adjustments to the fixed voltages. If the 19.5v adapter is not lying, it’s trying to down-step the 20v (which will only work if the source PSU is PD3 or higher). And indeed the author’s PSU supports PPS. Yet despite that, consumers are still not getting what they believe they signed up for.

    The latter is pretty much impossible to make to spec, because it can’t negotiate voltage. A barrel jack is single-voltage by necessity.

    First of all, the adapters exist. Not impossible. If you meant to say a smart negotiating one is impossible, I don’t believe that either. It could detect whatever the barrel passes and use that result to negotiate only for that available voltage. It could even have a voltage converter that supplies voltages that differ from the source. In fact I have a DC-DC adaptor. You probably have one and don’t know it. A USB charger that plugs into a car’s cigarette lighter converts 12v to 5v. I have one that has a variety of tips and a range of voltages. Detecting the source voltage may not be cheap, but not impossible. It would also be possible to have a cheap manual switch to specify input voltage.

    Respectfully, I’m going to have to disagree. It’s much more interesting when it catches fire, and I’d rather have a protocol that sometimes doesn’t operate in order to avoid the excitement of one that sometimes explodes

    If you find explosions interesting, you can go play with fireworks or watch war movies. It’s not academically interesting that tech illiterate consumers find countless ways to shoot themselves in the foot. When engineers

    who /should/ know what they are doing, push a shitty standard that burns even consumers who have the basic knowledge to match devices, that’s academically interesting because it’s people we expect to be smart fucking up. When a whole industry fucks up, that’s academically interesting. I cannot charge a 5v bicycle light using a USB PD spec’d PSU because the industry fucked up on the meaning of “default”.

    No matter what charging standard you use, some idiot could cut off the end, wire it to a stun gun, and send 100,000V into your phone. That doesn’t make the standard bad, it makes the adapter bad.

    Dumb consumers are not in short supply. It used to be that only tech-illiterate consumers were getting burnt. Which created an incentive for consumers to gain knowledge. The nannying attempt at a remedy has enabled informed consumers to get burnt. At the same time, it dumbs down society. When expert consumers can get burnt, of course it reflects a poor standard.

    And poor practices. A good standard anticipates what the market will do. They did not anticipate different product makers having a different interpretation of “default”.

    but as long as nothing breaks, I’d consider it as foolproof as anything reasonably can be.

    It’s now clear why you have no problems with the standard -- you have a low expectation. The Lenovo PSU that supports 5v /should/ be able to charge a 5v appliance. But they botched the negotiation and absence of it. When both sides of the negotiation are compatible in terms of physics but they cannot work together because of a miscommunication, it’s a shitty standard.

    The whole point of a standard is to establish an expectation that different components /function/ together. Doing that without fires is a given but not the end game.

    The blog shows situations where voltage is substantially less. It’s a general rule of thumb that supplying too little voltage usually does not damage things, but I do not think that is absolute.

    That’s just how electricity works.

    I was describing how /people/ work.

  • permalink
  • fedilink
  • source
  • parent
  • [–] 2 points 2 months ago

    12V is optional now. It was part of PD revision 1.0 and dropped in later revisions, but many chargers still support it. 9V has been required since PD rev 2.0.

    With the device that uses the 9V only USB adapter, I would either glue the plug in or replace it with something else. Its too easy to accidentally fry something with that.

  • permalink
  • fedilink
  • source
  • parent