Adaptive devices: USB chargers

[Dave2002]

Recently, out of curiosity, and since it was/is very cheap I bought one of these - http://www.amazon.co.uk/gp/product/B...ilpage_o00_s00 It's an LED display which shows the voltage and charging current being passed down a USB cable from a charger.

It's neat, and seems to work OK - at least it's reasonably self consistent, though I've not checked it independently.

Looking at the specs of different USB chargers (e.g the iPad ones), it seems that some do have adaptive behaviour, and to some extent this can be verified with the new gadget. For example, I think the iPad chargers do have different regimes for different current flows.

This has become particularly apparent over the last few days, as I used an iPad charger to charge up my Sony mobile phone. For a long while the voltage reading was over 5 V - typically around 5.13V, and finally the current was 0.05A with the phone green light showing a full charge. Then I switched to the charger which came with the phone, and the voltage dropped to under 5V - about 4.96V, and the current went up to around 0.5 A (10 times the iPad flow rate). The phone then charged for a further 30-60 minutes, at which point the charging current dropped to 0.00A - that with the supplied charger.

For most people this will be simply a curiosity, though I have discovered that some devices don't charge at all with some chargers, which may well be due to non-linear adaptive circuitry, and the gadget provides a means of detecting this. It might also stimulate curiosity in some as to what the relationship is between a full charge and current - and even if oldies like me think of this as of mild interest, it might provide a means of explaining to some young people how things work - and might, for some (probably very few) elicit a different response than "do I care?" or "am I bovvered".

[Frances_iom]

For most people this will be simply a curiosity, though I have discovered that some devices don't charge at all with some chargers, which may well be due to non-linear adaptive circuitry, .

no it is due, at least in Apple's case, to circuitry to detect that an Apple supplied charger is in use - this is merely a resistor across two of the pins - specific values for specific models - as usual with Apple the fanbois will willingly pay an inflated price.

[Dave2002]

no it is due, at least in Apple's case, to circuitry to detect that an Apple supplied charger is in use - this is merely a resistor across two of the pins - specific values for specific models - as usual with Apple the fanbois will willingly pay an inflated price.

For whatever reason, I would say it's non-linear. I can accept that it's not adaptive though, as its behaviour probably doesn't change in a state dependent way, though when I wrote that sentence I was thinking of a system which adapts to whatever is connected, which is just what you have described.

In fairness, also, the Apple charger may be detecting the measuring device, and that might also be why it modifies its behaviour. If you look at the specs for some of the Apple devices they do some possibly rather simple tests using perhaps rather simple circuitry to decide how much current to pass. I have no idea how the chargers behave without the measuring gadget in the chain. I do not know if the in-line gadget "looks" almost transparent to a source device, though surely it should appear almost transparent if it's reasonably well designed.

I think some chargers do detect the load as a protection mechanism in case too much current is taken, though in the particular case of the mobile phone as a load the charger seems to cut off when the current flow drops as the phone charges up. Some devices will not charge at all from Apple chargers I think, as they appear to not take enough current to turn them on - keep them on - there must of course be an initial flow - however small, but that's insufficient to do any serious charging.

Hiowever this is done, is it actually such a bad thing?

[Word]

When Apple introduced the iPhone the USB 2 spec still limited USB ports to 500mA so it was necessary or, at least, desirable for Apple to devise a way in which they could deliver higher current from their chargers and computer ports when connected to recognised Apple devices.
Had they not done so they would have run the risk of frying 3rd party peripherals plugged into their seemingly innocent USB ports.
(The original iPods used Firewire/IEEE1394 ports (as well as having lower capacity batteries) so didn't have that restriction.)

I too remember reading early descriptions of Apple's approach and love the idea that a resistor, something that could be identified and replicated by a school boy or girl, would be used as a form of hardware copy protection .

Not sure what was happening with your Sony mobile phone; when connected to iPads the older chargers deliver 2.1A and, I think, the newer ones 2.4A, but they’re happy to supply less. I have an old iPad charger that has been doing sterling work powering a Raspberry Pi.

p.s. Probably safest to to stick with official chargers: another-death-linked-to-fake-iphone-charger