USB chargers

[ARWadsworth]

How do they work?

I trapped the USB connector for my Sat Nav charger in the van door. The charger will now turn on the SatNav but will not charge it (in fact the SatNav battery disharges even when the charger is plugged in).

The cable is a two core cable so I assume that the 5V power supply is split between certain pins on the connector to allow charging to occur and that one of these pins no longer work/makes contact.

It is not a problem as I went and bought a new charger, I was just interested in how they worked.

[Andrew Gabriel]

5V across the outer two contacts. The max current draw is rather low (can't remember but something like 100mA). The device can negotiate over the USB bus to be supplied with up to 500mA, if the source is capable. Rather few sources actually police the current draw against what's been negotiated, but a few do. Quite a lot of devices draw what the hell they like without negotiating.

A newer standard allows for a simple passive resistor connection on the data bus connections which signals to the device that the lead is only supplying power (no USB bus), and larger current may be drawn without negotiating. This was developed for the cross-manufacturer USB charging connections for mobile phones. Prior to that, some mobile phone power supplies used 5.5V to signal to the phone that it was a dumb power supply connected which could deliver high current (in contrast to the max permitted voltage for a real USB connection which is 5.25V, and would trigger the phone to do only a slow charge).

[Tim Watts]

Are the resistor values actually standardised now?

I saw a slashdot link to a bit of "research" someone did to determine the relative resistor values to emulate an iPhone charger. Be nice if it did not vary from manufacturer to manufacturer.

Here's a link that explains the mysteries of Apple charging:

[Andy Burns]

[Theo Markettos]

The standard is that a short between the data lines says that the device can take a higher charge current (1A? I forget exactly). My phone says it takes 74mA when plugged into a PC USB socket and 581mA on such a fast charger.

That's Apple being 'special'. Samsung also have a series of resistor values for putting Galaxy phones in different modes (car cradle, no-sleep, etc). I don't think such have been standardised.

Theo

[Andy Burns]

Not seen any standards for docking cradle modes etc, the USB Battery Charging specs do cover resistor values to allow "dumb" dedicated charger ports

don't know (or care) Whether Apple devices conform to that or do their own thing.

[Dave Plowman (News)]

IIRC, for charging from an external charger rather than a PC USB port, the satnav is merely using the USB terminal as a connector for DC. Not 'clever' in any way.

[Andy Burns]

Depends if you count sensing some resistors as clever, but for dedicated charging ports it certainly doesn't need to be clever enough to send and receive USB packets