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Comment by peterwwillis | Hacker News Reader

Two things:

1. The data lines can be very important in regulating power output for different devices, and there are different maximums for different versions of USB. Some devices require data communication to charge. Some require proprietary protocols. Implementing Apple product charging is somewhat convoluted, for example, and has changed over time.

2. A friend of mine is a computer engineer, and tells me that correctly implementing USB in hardware is incredibly difficult. It's possible that devices like these might be skimping on parts of the spec to more easily get a working product out the door.

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I used to design these things. Yes they are difficult to design in the sense of RF engineering to pump that much data on a data line, but the power end of it is a very simplest design. It has a current cap to prevent damage to the regulator circuit and USB regular spec is 0.5A. Which isn't very much if you've ever tried to charge a phone off of that. Without the inner pins it'll default to the low spec (on a well design USB Host) but no damage or risk to having those inner pins float since they have internal pull up resistors. If the device decides to provide more all the better for you as your device is what regulates how much current it wants when an infinite amount is available.

>If the device decides to provide more all the better for you as your device is what regulates how much current it wants when an infinite amount is available.

If the host is capable of providing more power like 2.1A for example, how is that supposed to be signaled to the phone when the data lines are floating high? The way a USB charger that provides more current than the USB spec works is by pulling the data lines either to some voltage or shorting them together or to ground. Either way you need the data lines to negotiate that.

Yeah there's normally a negotiation via pins being grounded or through the level of resistance on that pull up which tells the device what spec is being used. That goes on to tell the device how much power it can pull without causing a power droop. If the pins are missing most devices won't try to pull more, like my phone won't... but I have a cheap external USB HDD enclosure that will pull what ever it needs from the line regardless of the pin setup. Most Mother Boards actually put out 1-2A on those lines so I can run the thing as normal, but the drive comes with this add on extension to hook it up to two USB lines to get around the 0.5A limit.

So it'll depend on how nice of a device you have.

The "condom" can be designed to be transparent with regard to one or more kind of power negotiation, though this would require a microcontroller, and possibly some analog circuicity for proprietary resistance-based negotiations.

Then the condom would be a target for hacking! You've just added another layer to the problem...

That's kind of like arguing that seat belts can cause strangulation.

There are always risks, so we just have to choose the less risky option available to us.

So between these two...

A. The black box charging station I know nothing about.

B. The simple device that I brought from home whose single purpose is to limit my exposure to external risk.

I'll take the latter.

Odds are I'm no worse off than had I plugged directly into the charging station.

A microcontroller actively doing the current negotiation would be nice (for things like working at the maximum power of both android and apple chargers) but a much simpler and dirt cheap solution would be to hook the data lines up so that they go through a simple filter that filters out everything other than the DC bias so power negotiation would work normally for chargers but no data could cross the cable.

There are ICs that do this negotiation already.

Like the MAX14636: http://www.maximintegrated.com/datasheet/index.mvp/id/7968

Or if you have a "nice" device like an Apple product, the voltages/resistances on the data lines will determine your charging speed. And then there's USB dedicated charging spec ports, like the ones that can put out 2100mA, but that's luckily out of scope of this device...

http://www.extremetech.com/computing/115251-how-usb-charging... http://learn.adafruit.com/minty-boost/icharging

> Implementing Apple product charging is somewhat convoluted, for example, and has changed over time.

If that and other things you claim are real issues then, every dumb usb power brick, every old computer/old charger, every non-apple charger, and others would all have problems.

> Implementing Apple product charging is somewhat convoluted, for example, and has changed over time.

It is? I've never had any problems with cheap AC to USB power adapters.

I think they are referring to the higher power ipad charging that pushes more amps down the line than the spec calls for. There is some kind of trigger used to tell the charger to send more juice. This device will probably cause that to not work. So, an ipad on this would only charger at the standard/lower rate.

The trigger is just grounding the two inner data pins together. This is ok since the data pins are loose pull up so even if the device tries to send data on something with those shorted no damage will occur.

Right, but my point is that this device won't work with those 'high current' charging devices (at the high current, it will only will only send the standard amps). Unless they included the logic to test this, which, I suppose, is possible.

Probably because those cheap AC to USB power adapters are specifically designed to charge iDevices http://learn.adafruit.com/minty-boost/icharging

You're correct about the need of the data lines for power negotiation. The LockedUSB kickstarter is creating a USB condom but with power management: http://www.kickstarter.com/projects/1137339450/lockedusb-ada...

Note: I am in no way affiliated with the above kickstarter

I have created my own battery usb charger (it takes d batteries and gives a usb port). I simply placed a resistor to a data line and that was it. I understand the idea of power negotiation, but felt like it was way more work than it was worth. I didnt need any complicated circuitry; just a battery, resistors, a couple of wires, and a small dc-boost circuit (to drain the batteries as much as possible).

You are right. I made a review of the USB Condom in my blog, talking about the shortcomings of their implementation: http://multigrad.blogspot.ca/2013/09/review-of-usb-condoms.h...

just like with a real one... it will be a boring experience and you will be limited to half Ampere of fun. But no harm.

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