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Re: What sets the charging rate?

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On 27/11/2018 17.42, J. P. Gilliver (John) wrote:
> In message <35pucf-i3d.ln1@Telcontar.valinor>, Carlos E.R.
> <robin_listas@es.invalid> writes:
>> On 26/11/2018 17.52, J. P. Gilliver (John) wrote:
>>> In message <pth9uf.bag.1@ID-201911.user.individual.net>, Frank Slootweg
>>> <this@ddress.is.invalid> writes:
>>>> Tim <onelydad58@gmail.com> wrote:
>>>>> nospam <nospam@nospam.invalid> wrote in
>>>>> news:261120180852212967%nospam@nospam.invalid:
>>>>>
>>>>> > In article <XnsA9A6336EF683timothybilyahoocom@69.16.179.28>, Tim
>>>>> > <onelydad58@gmail.com> wrote:
>>> []
>>>>> >> But the USB 3 ports on my computer provide 1.5A, so why doesn't it
>>>>> >> charge at that rate rather than .5A?
>>>
>>> (Same reason your 40 watt light bulb doesn't draw the kilowatt or so
>>> your household lighting circuit can supply.)
>>
>> Well, not exactly: the light bulb doesn't have a controller, and it
>> doesn't negotiate. It is purely ohm law, with a relatively fixed plain
>> resistor (except that the resistor gets so hot, that resistance
>> increases notably with the temperature).
>>
>> So the house supplies a fixed voltage and you connect a fixed resistor:
>> the current is determined from both. There is no limitation in the
>> circuit on current or power: but if the resistor is too low, the fuse
>> blows.
>>
>> Intelligent devices can have batteries with regulation. The apparent
>> resistance the "battery" has varies so that it draws more or less
>> current as needed.
>>
> I know, it was a poor example: I was just a bit shocked that someone
> could say "my charger delivers 1.5A"; he didn't realise that the truth
> was that his charger _can_ deliver 1.5A, but doesn't necessarily do so
> if the device doesn't suck that much. I suppose I could have said "your
> charger doesn't deliver _anything_ if nothing is connected to it", but
> that wasn't as good, I thought.

Indeed :-)

> 
> Interesting off-topic sidebar: nearly forty years ago I briefly worked
> in the electricity supply industry, where they had (presumably do still)
> things called "current transformers", usually abbreviated to CT, which
> worked counter to how we're used to thinking: they delivered a _current_
> proportional to the current in the primary (often a "bar primary"!), and
> did so no matter what. They were intended for instrumentation (and thus
> control of course). Unlike conventional power sources, they were quite
> happy to be shorted: they'd deliver their rated current through the
> short. In fact you normally _did_ leave them shorted when not connected
> to anything - because they'd try to deliver that current regardless, and
> if you left them _open_ circuit, would overvoltage and damage
> themselves. I never did figure out how they worked (I don't remember
> trying very hard, I admit. I'm pretty sure they were passive devices
> though - just coils and cores, no electronics). Their advantage was
> that, no matter how long the cable run between them and the (say) meter
> you connected them to (could be many tens of yards - possibly miles I
> think), the current would indicate.

Interesting. I heard of them, but never saw one.

We did use "current sources" in circuit theory - and they were initially
a bit of a conflict to understand :-)

-- 
Cheers, Carlos.

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What sets the charging rate? Tim <onelydad58@gmail.com> - 2018-11-23 22:52 +0000
  Re: What sets the charging rate? Tim <onelydad58@gmail.com> - 2018-11-24 04:24 +0000
    Re: What sets the charging rate? micky <NONONOmisc07@bigfoot.com> - 2018-11-25 00:10 -0500
      Re: What sets the charging rate? "Carlos E.R." <robin_listas@es.invalid> - 2018-11-25 14:12 +0100
        Re: What sets the charging rate? micky <NONONOmisc07@bigfoot.com> - 2018-11-25 21:22 -0500
          Re: What sets the charging rate? "Carlos E.R." <robin_listas@es.invalid> - 2018-11-26 15:21 +0100
            Re: What sets the charging rate? micky <NONONOmisc07@bigfoot.com> - 2018-11-27 03:52 -0500
    Re: What sets the charging rate? nospam <nospam@nospam.invalid> - 2018-11-25 00:42 -0500
      Re: What sets the charging rate? Tim <onelydad58@gmail.com> - 2018-11-26 06:18 +0000
        Re: What sets the charging rate? nospam <nospam@nospam.invalid> - 2018-11-26 08:52 -0500
          Re: What sets the charging rate? Tim <onelydad58@gmail.com> - 2018-11-26 14:03 +0000
            Re: What sets the charging rate? nospam <nospam@nospam.invalid> - 2018-11-26 09:11 -0500
              Re: What sets the charging rate? Tim <onelydad58@gmail.com> - 2018-11-26 15:45 +0000
                Re: What sets the charging rate? nospam <nospam@nospam.invalid> - 2018-11-26 10:49 -0500
            Re: What sets the charging rate? Frank Slootweg <this@ddress.is.invalid> - 2018-11-26 16:19 +0000
              Re: What sets the charging rate? 123456789 <12345@12345.com> - 2018-11-26 09:33 -0700
                Re: What sets the charging rate? nospam <nospam@nospam.invalid> - 2018-11-26 11:57 -0500
              Re: What sets the charging rate? "J. P. Gilliver (John)" <G6JPG-255@255soft.uk> - 2018-11-26 16:52 +0000
                Re: What sets the charging rate? nospam <nospam@nospam.invalid> - 2018-11-26 11:57 -0500
                Re: What sets the charging rate? Frank Slootweg <this@ddress.is.invalid> - 2018-11-26 18:49 +0000
                Re: What sets the charging rate? "Carlos E.R." <robin_listas@es.invalid> - 2018-11-27 04:09 +0100
                Re: What sets the charging rate? "J. P. Gilliver (John)" <G6JPG-255@255soft.uk> - 2018-11-27 16:42 +0000
                Re: What sets the charging rate? Tim <onelydad58@gmail.com> - 2018-11-27 22:43 +0000
                Re: What sets the charging rate? "Carlos E.R." <robin_listas@es.invalid> - 2018-11-28 14:08 +0100
                Re: What sets the charging rate? Andy Burns <usenet@andyburns.uk> - 2018-11-28 13:27 +0000
                Re: What sets the charging rate? "Carlos E.R." <robin_listas@es.invalid> - 2018-11-28 14:06 +0100

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