Raspberry PI Power

[William Weaver]

I totally agree that it's easier to measure the Voltage than the current,
the problem is for the situation the RPi is going to keep the voltage
constant on the 5V rail. It'll do that by upping the current draw, if he's
capping out the 1A power draw of the supply, he won't know that unless you
either get a current measurement or you can accurately determine the power
draw of the USB port. Then you have P = VI.

Putting two leads inline a USB connector to get a current measurement
shouldn't be to terrible.

Will


On Fri, Aug 2, 2013 at 1:24 PM, Zbigniew Koziol <softquake-Re5JQEeQqe8AvxtiuMwx3w at public.gmane.org> wrote:

> On 02/08/13 20:46, William Weaver wrote:
>
>> Colin,
>>
>> My bet is that the logitech unifying receiver and the wifi are both
>> requesting a high current usb connection, which is then saturating your 1A
>> power source when the RPis load gets high. Would it be possible for you to
>> measure your current draw from the externally powered power supply?
>>
>
> It is much easier to measure voltage than current. For this one may
> connect voltmeter to two points where the first capacitor is connected,
> just after USB power supplier. I have for instance the following results:
>
> When no any USB is connected, and nothing else, voltage is 5.18 V with my
> USB power supplier.
> When I connect USB keyboard only - voltage drops to 5.17
> When I connect SSD card only - voltage drops to 5.15 V
> With USB keyboard, SSD card and wireless UB mouse - voltage remains 5.15.
>
> These results do not prove much but could indicate on a problem.
>
> Actually, I do have a problem. I would expect that after connecting
> keyboard I should have some response from it (LED lights up/down on
> keyboard after CapsLock is pressed, etc). Unfortunately, I do not see that.
>
> [...]
>
> zb.
>
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