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·
    23 hours 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.