USB Charger Amperage

[DerbyBorn]

I have just bought a 13amp socket with built in USB Charger and realise now that it is 3.1 amps.

Will it fry my Samsung which specifies 2 amps?

[newshound]

It shouldn't.

[Andy Burns]

No, devices pull whatever current as they require (subject to agreeing the maximum with the charger) the charger doesn't "ram" all it's capable of into the device ...

[Chris Hogg]

I would presume that the USB charger can supply a current of up to 3.1 amps, and the Samsung draws 2 amps and needs a charger that can supply accordingly.

[Graham.]

No. For the same reason that a car battery can crank the starter motor that requires 100A or more yet also operate the interior light and radio that doesn't.

[Martin Brown]

The only one that can sometimes cause trouble is a charger only good for the basic 1.2A current and an impolite device that tries to take 2.4A from it without negotiating the higher charge rate.

I don't have any device that bad, but I do have some that will slowly discharge if put onto a basic 1.2A capable charger whist being used.

[whisky-dave]

That's one of the thimngs our studetn have trouble understanding at first, maybe they have those galaxy note 7 phones ;-)

[DerbyBorn]

whisky-dave wrote in news: [e-mail]:

I am okay on ordinary electrics - but thought their might be a "gotcha" with phones/ tablets. There were scare storys some time back about Apple devices catching fire on incorrect chargers.

[whisky-dave]

Any device attached to the wrong charger could cause problems. I have a few PSU for raspberry Pi whose output is 5.2V at 2amps.

I'm not sure what the maxium voltage an iPhone or any other phone can take. But over Xmas I was charging a small cylidrical 2200ma battery left it on 24 hours and a little later the charge went pop and I saw a few sparks there was a black mark on the USB lead and the charger now rattles a 5V 1 amp Gear4.

Incedently the batter was cold and there was no sign that it was being charging when it went pop.

[Dave Plowman (News)]

Does a 13 amp socket fry a 60 watt table light?

[Andy Burns]

The USB spec is 5V +/- 5% so 5.2V is within tolerance.

[<frde]

Nope. That specifies the maximum current that it can supply. It?s the voltage that can fry things if its wrong.

[Brian Gaff]

I would hope not. It t would be a very dodgy design that could draw more current than its supposed to. Or am I too trusting? Brian

[Brian Gaff]

If you are really worried an in car fuseholder with a slo blow 2A fuse should set your mind at rest. Brian

[therustyone]

The opposite problem can arise, like my iPad mini will not charge at all on a 2A charger, but is OK on a 2.4A Belkin one, even with a third party non-Apple cable. It charges when switched off completely, but then overcharges as its regulator is also off.

[whisky-dave]

Denps which student fits it ;-)

[whisky-dave]

Ask samsung.

[whisky-dave]

I have an old iphone chareg iphone4 I think, that when conected to an ipad air 2 charges for about half a second then the ipad goes off then back on again then off like a ticking clock.

[Johnny B Good]

No, it won't (even if there was any need)!

This question ("Can I use this 2A USB rated socket to charge my half amp rated gadget or charger?") comes up with monotonous regularity in the news groups. It's akin to asking if a 240v 40W table lamp (0.167A incandescent lamp load) can be safely plugged into a 240v 13A socket. The answer in both cases is "Yes!" and for essentially the same reason.

When supplying electrical power to an electrical appliance or gadget, the primary consideration is the matching of the supply voltage to the voltage required by the appliance or gadget. With mains powered appliances and gadgets fitted with 13A plugs, the 230/240v is implied in the case of UK domestic supply and appliances and gadgets (and always marked in some way on the plug in appliance or gadget itself for the benefit of any "Doubting Thomases").

The same rule applies in the case of low powered gadgets intended to be powered from low level DC voltages (typically 3 to 30 but more commonly 5 or 12 volts) using 'wallwarts' which may take the form of a "USB Charging 'wallwart'" (plug in or built into something else like a PC or a 13A double outlet wall socket).

Naturally, due to the variance in DC voltage levels used by such low voltage gadgets (commonly 5 or 12 volts but possibly as low as 3 to as high as 30 volts) end users tend (and most definitely ought) to verify that the voltages match between any chosen 'wallwart' and its associated gadget to guard against the possible mix up of an identical looking but different voltage wallwart[1]

It's understandable that such concerns should arise out of this scenario (mistakenly using the wrong voltage wallwart). However, with USB gadgets and chargers, the voltage level has long been standardised on the classic five volt standard originating with the TTL chips used by the original desktop computers (Apple and IBM and all the toy computers of the late

70s onwards to the present day).

As with wallwarts designed not for charging but simply to act as a low voltage power supply, the only critical specification for 'safe' operation is the voltage level. With USB this is defined as 5 volts[2]. The charging amperage specified by such chargers are a maximum they can supply before they either current limit or else foldback into an overload induced tripped off state.

If you plug a gadget, designed to charge its own built in battery from a standard USB port specified to supply no more than half an amp per socket [3], into a 2 or 3 amp rated USB charger, said gadget will still only draw the maximum of half an amp unless it has the means to identify whether the supply is capable of supplying more current than the nominal half amp limit of a standard USB port *and*, importantly, the ability to make use of the higher charging current.

IOW, a 2A rated Samsung gadget will work just fine with a 3.1A rated USB charging port. Indeed, a 2A (maximum charging rate) Samsung gadget will work just fine (at least for charging purposes) off a half amp limited USB2 port. The Samsung will use whatever it can get to charge its battery, even if it's an unnegotiated 100mA from a laptop, netbook or tablet USB port (it'll just take a lot longer to charge its battery compared to how fast it can charge from a 2 or 3 amp rated port).

Some gadgets may draw so much current when turned on that they leave so little current available from a half amp supply as to take a day or two to fully recharge their battery. In this case, the benefit would be to extend the operating time without exhausting the battery.

Using a 3.1A rated USB charging port would allow the device to be powered up and *still* recharge its battery at the full 2 amp rate with

600mA to spare for another USB device. However, a gadget with a 2A battery charging capability may be forced to reduce the battery charging rate when also switched on and active at the same time due to limitations in its power management circuitry. [1] Due to the mass market nature of these 'handy gadgets', it's not unusual to land up with a collection of identical looking wallwarts with different DC voltage outputs (commonly, though not restricted to, a mix of 5 and 12 volts). Luckily, other than for kit that depends on the supply being a specific voltage to power a motor or solenoid, the innards often need one or more supply voltages such as 3.3v and 5v demanding some form of voltage regulator which, these days will be a low loss, high efficiency switching converter.

A surprising amount of electronic kit (SoHo ethernet switches and routers for example) will run just fine off almost any wallwart regardless of the voltage over the range of 6 to 16 volts DC despite what is specified by the gadget itself provided the wallwart's VA rating equals or exceeds that specified by the gadget. However, it's a given that anything designed to be powered via a USB2 socket cannot be safely subjected to a voltage in excess of 5.5v (nominal 5v +/- 10% tolerance).

[2] The USB3 standard not only permits a higher current demand to be met, it also allows for a higher negotiated supply voltage, presumably starting off with a 5 volt level for safe interoperability with USB1 and 2 and 'dumb' USB3 devices. [3] IME thus far, the USB2 ports on desktop PC motherboards supply their USB sockets and headers via a common 5v bus, protected by a 3 or 4 amp rated polyfuse[4] which not only powers the USB sockets but also, where fitted, the 5v pins on the PS/2 keyboard and mouse sockets.

It seems the 'Negotiated 500mA' protocol mechanism is restricted to laptop/tablet usage where power consumption is a more critical consideration. In theory (which only happens in practice with portable devices), a usb gadget can only draw a maximum of 100mA without negotiating for the additional 400mA it may desire.

[4] In some cases, notably those infamous "PC Chips" branded motherboards, the cost of a polyfuse was deemed an unnecessary expense by the bean counters who felt their safety obligations could be met adequately by reliance on either a very thin circuit trace or a small smd inductor to act as a 'one shot' safety fuse to obviate the risk of conflagration in the event of a short circuit fault on the 5v USB power bus.

[Johnny B Good]

The loads (eg TTL chips and their cmos equivilents) have a +/-10% voltage tolerance rating whilst it's common practice to specify PSU voltage tolerances at half of such loads' tolerance rating to absolutely guarantee that the chips will operate to their specified limits with a margin to spare, hence the +/-5% voltage tolerance commonly specified for PSUs.

However with USB powered kit, relying on a 'nominal five volt supply' with a +/-5% tolerance (4.75 to 5.25 volts) to power 'five volt kit' with its own 'nominal five volt' +/-10% requirement (4.5 to 5.5 volts) over the vagaries of ill defined cable and contact resistances can lead to out of tolerance conditions on the supply pins of the chips used in the USB device or gadget since the 250mV margin between the lower limits of 4.75v supply and 4.5v load can be entirely consumed (and then some) by volt drops in the cable resistance and socket contact resistances depending on the amount of current demanded. Even just a half amp loading could result in more than 250mV being lost in the connecting cabling alone.

Awareness of this cabling volt drop issue leads to USB wallwarts being specified to a higher and more tightly controlled voltage level with 5.2v

+/-2.5% being typical (5.07v min and 5.33v max) allowing a worst case cable volt drop allowance of 507mV.

An obvious way to eliminate the cable volt drop issue is to use remote voltage sensing wires in the cable between the wallwart PSU and the pins of the usb plug on the end of a permanently attached cable (dedicated high current USB chargers). This means the 5.2v supply can be maintained at the USB plug contacts independently of volt drops arising out of the normal current variations supplied through the cable, leaving only contact resistance as the final 'unknown' parameter.

Whilst a connector will eventually start producing excessive volt drops due to wear and tear, using remote voltage sensing will mitigate this by allowing for longer and more flexible (but higher resistance) leads reducing the stresses transferred into the plug and socket assembly. Also, the higher resistance of the lead will reduce the peak arcing currents due to contact movement, reducing electrical erosion of said contacts, leading to an improved reliability all round.

There can be a world of difference between a "Cheap 'n' Cheerful" "No Frills" "2A USB charger" and that of a well designed dedicated "Fast" 2 or 3 amp USB charger using remote sensing at the USB plug on the end of a

2 metre flexible cord.

Since those 13A wall sockets with built in USB charging ports can't provide a remote voltage sensing facility, they seem a rather pointless add-on feature imho. A decent plug in wallwart is a much better option since it can include the refinement of remote sensing and a more optimised tightly controlled voltage of 5.2 or 5.3 volts at its permanently connected USB plug.

With a little more expense, even the dedicated USB charging lead could be made a user servicable part of such a fast charging USB wallwart. However, I can see a case for not adding such expense on the basis that by the time such a well designed plug and fly-lead has worn itself out, it's very likely time to replace the smoothing capacitors in the wallwart itself and therefore the whole damn thing anyway.