usb fast charger question

Hi All,

I asked about the voltage of a USB fast charger.  I got back:

5V-3A, 9V-3A, 12V-3A, 15V-3A, 20V-3A

My Android tablet wants 5.2V, 2.0 A


Uhhhh.  Is there something about the USB cable that
tell the charger not to fry what I plug it into?

https://www.amazon.com/dp/B08786SHXV

Many thanks,
-T

T <T@invalid.invalid> wrote:

> I asked about the voltage of a USB fast charger.  I got back:
> 
> 5V-3A, 9V-3A, 12V-3A, 15V-3A, 20V-3A
> 
> My Android tablet wants 5.2V, 2.0 A

What is your Android tablet? Be specific...

> Uhhhh.  Is there something about the USB cable that
> tell the charger not to fry what I plug it into?

Source current refers to ABILITY to source current.

It doesn't FORCE current through a device.

Given a specific voltage, a device determines how much current flows through 
it.

You need a standard 5V USB charger that sources at least 2 A. Sourcing more 
than 2 A just means it is potentially more useful (for other devices).

On 11/17/21 00:12, John Doe wrote:

> What is your Android tablet? Be specific...

Lenovo:  A10-70F

On 17/11/2021 08.58, T wrote:
> Hi All,
> 
> I asked about the voltage of a USB fast charger.  I got back:
> 
> 5V-3A, 9V-3A, 12V-3A, 15V-3A, 20V-3A
> 
> My Android tablet wants 5.2V, 2.0 A
> 
> 
> Uhhhh.  Is there something about the USB cable that
> tell the charger not to fry what I plug it into?

Not the cable. The voltage is negotiated between the device and the
charger. Before that, you only have "1 Load unit" available.

<https://en.wikipedia.org/wiki/Battery_charger#USB-based_charger>
<https://en.wikipedia.org/wiki/USB#Power>
<https://en.wikipedia.org/wiki/USB_hardware#Power>
USB power delivery section, too.

<https://en.wikipedia.org/wiki/Common_external_power_supply>


-- 
Cheers,
       Carlos E.R.

On Wednesday, November 17, 2021 at 2:15:43 AM UTC-8, Carlos E. R. wrote:
> On 17/11/2021 08.58, T wrote: 
> > Hi All, 
> > 
> > I asked about the voltage of a USB fast charger.  I got back: 
> > 
> > 5V-3A, 9V-3A, 12V-3A, 15V-3A, 20V-3A 
> > 
> > My Android tablet wants 5.2V, 2.0 A 
> > 
> > 
> > Uhhhh.  Is there something about the USB cable that 
> > tell the charger not to fry what I plug it into?
> Not the cable. The voltage is negotiated between the device and the 
> charger. Before that, you only have "1 Load unit" available. 

There is very limited voltage range to negotiate anyway.

The 3A rating is mainly to protect the cable and connector, so that you don't drive 3A through a 2A device.

As for the battery, nothing wrong with 1.5C charging, although 1C is the optimized rate.

On 11/17/21 07:30, Ed Lee wrote:
> On Wednesday, November 17, 2021 at 2:15:43 AM UTC-8, Carlos E. R. wrote:
>> On 17/11/2021 08.58, T wrote:
>>> Hi All,
>>>
>>> I asked about the voltage of a USB fast charger.  I got back:
>>>
>>> 5V-3A, 9V-3A, 12V-3A, 15V-3A, 20V-3A
>>>
>>> My Android tablet wants 5.2V, 2.0 A
>>>
>>>
>>> Uhhhh.  Is there something about the USB cable that
>>> tell the charger not to fry what I plug it into?
>> Not the cable. The voltage is negotiated between the device and the
>> charger. Before that, you only have "1 Load unit" available.
> 
> There is very limited voltage range to negotiate anyway.
> 
> The 3A rating is mainly to protect the cable and connector, so that you don't drive 3A through a 2A device.
> 
> As for the battery, nothing wrong with 1.5C charging, although 1C is the optimized rate.
> 
> 

I am not following.  How does the charging block
know what voltage and current is allowed?  Is
the charging block a constant current device
that clips at 5V?

torsdag den 18. november 2021 kl. 02.18.47 UTC+1 skrev T:
> On 11/17/21 07:30, Ed Lee wrote: 
> > On Wednesday, November 17, 2021 at 2:15:43 AM UTC-8, Carlos E. R. wrote: 
> >> On 17/11/2021 08.58, T wrote: 
> >>> Hi All, 
> >>> 
> >>> I asked about the voltage of a USB fast charger. I got back: 
> >>> 
> >>> 5V-3A, 9V-3A, 12V-3A, 15V-3A, 20V-3A 
> >>> 
> >>> My Android tablet wants 5.2V, 2.0 A 
> >>> 
> >>> 
> >>> Uhhhh. Is there something about the USB cable that 
> >>> tell the charger not to fry what I plug it into? 
> >> Not the cable. The voltage is negotiated between the device and the 
> >> charger. Before that, you only have "1 Load unit" available. 
> > 
> > There is very limited voltage range to negotiate anyway. 
> > 
> > The 3A rating is mainly to protect the cable and connector, so that you don't drive 3A through a 2A device. 
> > 
> > As for the battery, nothing wrong with 1.5C charging, although 1C is the optimized rate. 
> > 
> >
> I am not following. How does the charging block 
> know what voltage and current is allowed? Is 
> the charging block a constant current device 
> that clips at 5V?

if it is regular USB it is just 5V an the device will draw as much current as it needs up to the 2A USB charging limit

if it is USB-C the device can ask for higher voltages and will be told how much current it is allowed to draw

On Wednesday, November 17, 2021 at 5:18:47 PM UTC-8, T wrote:
> On 11/17/21 07:30, Ed Lee wrote: 
> > On Wednesday, November 17, 2021 at 2:15:43 AM UTC-8, Carlos E. R. wrote: 
> >> On 17/11/2021 08.58, T wrote: 
> >>> Hi All, 
> >>> 
> >>> I asked about the voltage of a USB fast charger. I got back: 
> >>> 
> >>> 5V-3A, 9V-3A, 12V-3A, 15V-3A, 20V-3A 
> >>> 
> >>> My Android tablet wants 5.2V, 2.0 A 
> >>> 
> >>> 
> >>> Uhhhh. Is there something about the USB cable that 
> >>> tell the charger not to fry what I plug it into? 
> >> Not the cable. The voltage is negotiated between the device and the 
> >> charger. Before that, you only have "1 Load unit" available. 
> > 
> > There is very limited voltage range to negotiate anyway. 
> > 
> > The 3A rating is mainly to protect the cable and connector, so that you don't drive 3A through a 2A device. 
> > 
> > As for the battery, nothing wrong with 1.5C charging, although 1C is the optimized rate. 
> > 
> >
> I am not following. How does the charging block 
> know what voltage and current is allowed? Is 
> the charging block a constant current device 
> that clips at 5V?

The charging current is limited by the internal resistance of the battery.  So, it's OK to hook up a 2A device to a 3A charger.

On Wednesday, 17 November 2021 at 17:18:47 UTC-8, T wrote:
...
> > As for the battery, nothing wrong with 1.5C charging, although 1C is the optimized rate. 
> > 
> >
> I am not following. How does the charging block 
> know what voltage and current is allowed? Is 
> the charging block a constant current device 
> that clips at 5V?

With USB-C there there is a wire in the cable that is used to communicate between the power supply and the device.
They negotiate what is the optimum voltage and current to use that is within both of their capabilities.

The power supply then provides that voltage and the device manages the charing current into its battery to not overload the power supply or overcharge the battery.

With the older USB there was a protocol for a device to negotiate up from 100mA to the maximum 500mA but that didn't suit charging so there were various other approaches either using resistors at the power supply end or a short between the two data lines that would give the device the information about how much power was available.

Again, the device would then manage the battery charging.

kw

On Wednesday, 17 November 2021 at 18:11:48 UTC-8, Ed Lee wrote:
...
> The charging current is limited by the internal resistance of the battery. So, it's OK to hook up a 2A device to a 3A charger.

That is wrong.

the charger within the device will manage the charging current to not exceed the capability of the power supply.

kw