Comment by dragontamer | Hacker News Reader

Comment by dragontamer | Hacker News Reader

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> We are talking about a significant amount of current.

Are we?

I'm not sure how they deliver more power in this specification. But traditionally, the additional power (from 5W to 20W to 100W) was through additional *voltage*, not current.

> And it's not something trivial that power supplies and phones can detect, there is not a way to determinate the voltage drop in the cable built into the standard (basically all they needed to do was to add a voltage sensing pin on the connector, to be shorted with VCC at the load side, so the power supply could sense the voltage at the other side and determinate if there too much drop, but as far as I know it doesn't have it).

Surely we just set a current-limit, written into the specification. Then we choose AWG wires / connectors as appropriate to support that current.

Voltage can't go up arbitrarily, but voltage can safely be increased to ~48V or so in most applications. After 48V, humans start to get shocked / hurt, so that's probably the reasonable limit.

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And I'm pretty sure you can sense the voltage drop across a cable. USB-3 delivers 5V by default, and then you send protocol commands to increase your voltage to 12V or whatever. If you detect that the power-supply is only supplying 11V (after the 12V command), then its either a PSU-error or a cable-error. And I'm not sure if it even matters which is which (either way, you're not getting enough power, so your device needs to probably shut off)

You can then disconnect / reset your device and maybe stick to 5V default specs.

It's 5 amps at 48 volts, defined in https://usb.org/document-library/usb-power-delivery

That level of power delivery is negotiated after the devices are connected. The cable also has to advertise that it is capable of that power delivery.

> Voltage can't go up arbitrarily, but voltage can safely be increased to ~48V or so in most applications. After 48V, humans start to get shocked / hurt, so that's probably the reasonable limit.

Car batteries in ICE cars are nominally 12v. Grab both terminals and tell me it doesn't hurt.

Ok, done. Many many many times.

12v can't exceed the resistance of human skin so it doesn't matter how much current capacity it has. You can hold onto those terminals all day.

It's settled science. If you doubt it, tests of this are all over YouTube. It's just true.

Now your tongue, perhaps...

So my cat doesn't have to worry?

If t you short a car battery, you might not get electrocuted, but the results are unlikely to be pretty.

You can't short a car battery by touching the terminals with your dry unbroken skin, anymore than you can short a AA battery by touching both ends.

You aren't shorting the battery in this case. There's still a significant resistance (your body, particularly the skin contact resistance) between the terminals.

...it doesn't hurt. Dry skin isn't conductive enough to pass any significant current at 12V. But if you inserted electrodes into your fingers so the electricity conducted through your wet inner bits, you can be killed by a lot less than 12V across the heart.

I have never felt anything holding 12V DC in my hands. Putting your tongue across 9V "transistor" battery terminals is another thing though.

I've left a 9V battery in my pocket once and it touched a penny. I definitely felt that, lol.

But yeah, 9V and 12V don't hurt your skin at all. The worst you'll get typically is when you accidentally short it with some metal, and something becomes burning hot really quickly.

But this "burning hot" issue can happen even with 1.5V NiMH batteries or 3.3V Li-Ion batteries (even hot enough to start a fire in your pocket! Like all those vaping accidents).

That's not "shock" or "electricity", that's literally heat from some other thing messing up the battery pack. So its not really the same.

I've done this. It doesn't hurt. In fact I didn't feel anything besides the battery terminals.

When I was a kid, we had this massive high-voltage transformer that produced about 2,000 volts, at some sick amperage.

We used it to make jacob's ladders. That was fun.

Until I touched the two bars.

I woke up on the other side of the room, smelling burning...me.

That wasn't fun...

I learned this recently:

> If you receive a shock, if can affect the rhythm of your heart and cause problems later. We’re taught to go and get an ECG after receiving a shock. Like jump in the car and go to the doctor or hospital, and have someone else drive you. If you’ve received a shock - go get checked out. -jaidan

https://news.ycombinator.com/item?id=27259589

yup, always work around high voltages with your left hand (the one closes to your heart) in your pocket, to discourage unwanted currents from passing through the heart

As the old joke goes, experienced electricians touch even their wife's breasts with only one hand, just in case.

The main reason being that you don't want to be working with both hands, since then current can travel in one and out the other, directly across your chest.

I saw that too but I wouldn't spread it as a fact without a better source.

I found this, but it's more than a mild shock:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2658458/

Bing's answer-mode points here:

https://healthcare.utah.edu/the-scope/shows.php?shows=0_esno...

Always hard to know how cautious to be, with so many variables.

I recently had some industrial training that made the same point. The recommendation there was to immediately hook up an AED.

You can go into tachycardia or bradycardia (fast and slow heartbeats), so feeling sort of okay isn't the end of it.

Hadn't heard of this, and initial results had no clear link to electricity:

https://wonders.physics.wisc.edu/jacobs-ladder/

Good god, you were lucky to live!

You have no idea...

That was nowhere near the stupidest thing I've done...

It just tingles a bit if anything.

It doesn’t tingle.

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