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Re: Power a test TTL circuit from USB?

Cross-posted to 2 groups.

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In sci.electronics.components James Harris <james.harris.1@gmail.com> wrote:
> On 16/10/2018 20:21, Jasen Betts wrote:
>> On 2018-10-16, James Harris <james.harris.1@gmail.com> wrote:
>>> Having not touched any of this kind of stuff for years I'm looking to
>>> power a small TTL test circuit which I intend to built on a breadboard.
>>> I wondered if I could run it off USB power. In other words, is it
>>> feasible to power a TTL cct via a USB lead and USB socket?
>>>
>>> I found a "USB Mini B Breakout Board"
>>>
>>> http://www.hobbytronics.co.uk/cables-connectors/usb-connectors/usb-minib-breakout
>>>
>>> but I'm not sure that the PCB bit of it is necessary and, besides, it's
>>> only rated for 100mA (which may be enough but I can't be sure at this
>>> stage). If not USB, I guess I should go for a wall wart and a 7805.
>> 
>> They can only claim 100mA as that's all that USB guarantees without
>> some sort of complicated negotiation. in actual use It can probably do
>> 500mA or more (not that soldeless breadboard is suited to currents
>> above about 200mA)
>> 
>>> So the question is whether there's a good way to power a circuit via
>>> USB, or whether you think I should go down the wall-wart or some other
>>> route.
>> 
>> USB-shaped "phone chargers" that produce 5V at 500mA or more are
>> easily had.
> 
> I've been trying to work out USB power negotiation without much success. 
> I recognise that an un-negotiated port can draw 100mA and I know that a 
> device can negotiate for more. I am not clear, though, on whether 
> there's a cable or a particular outlet which will allow higher current 
> to an endpoint which doesn't do negotiation.

100mA (or 150mA for USB 3) is the default limit that USB is _supposed_
to restrict devices to before they request more power. In practice this
is often omitted from computer motherboard designs to save cost, but
obviously they don't advertise this.

Many USB charger plug-packs don't restrict current, so they might be
easier as all you have to do is look at the current rating on the
sticker (sometimes this is only enough for the device it was
originally intended to be used with, rather than to conform to any
particular USB current limit).

Looking more boardly, there are lots of switch mode plug packs
without USB connectors, but which provide regulated 5V up to a
specified current, so they are an option too. Make sure they're
switch-mode, not transformer types (which I think you're already
aware of).

See "Low-power and high-power devices" here:
https://en.wikipedia.org/wiki/USB

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Power a test TTL circuit from USB? James Harris <james.harris.1@gmail.com> - 2018-10-16 14:04 +0100
  Re: Power a test TTL circuit from USB? Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2018-10-16 14:37 -0400
    Re: Power a test TTL circuit from USB? James Harris <james.harris.1@gmail.com> - 2018-10-17 14:27 +0100
      Re: Power a test TTL circuit from USB? Ralph Mowery <rmowery28146@earthlink.net> - 2018-10-17 10:19 -0400
      Re: Power a test TTL circuit from USB? Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2018-10-17 13:02 -0400
        Re: Power a test TTL circuit from USB? not@telling.you.invalid (Computer Nerd Kev) - 2018-10-17 22:06 +0000
          Re: Power a test TTL circuit from USB? Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2018-10-18 20:58 -0400
  Re: Power a test TTL circuit from USB? Jasen Betts <jasen@xnet.co.nz> - 2018-10-16 19:21 +0000
    Re: Power a test TTL circuit from USB? James Harris <james.harris.1@gmail.com> - 2018-10-17 14:31 +0100
      Re: Power a test TTL circuit from USB? not@telling.you.invalid (Computer Nerd Kev) - 2018-10-17 22:19 +0000

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