[ltp] X61 power consumption

[James Knott]

Richard Neill wrote:
>
>
> Asheesh Laroia wrote:
>> On Fri, 28 Nov 2008, Leszek Koltunski wrote:
>>
>>>> That would be rather difficult, as in order to measure the power,
>>>> you have to measure current and that means cutting the power
>>>> connection to the chip, to insert the meter in series.  Now if you
>>>> wish to volunteer your ThinkPad to be butchered...
>>>
>>> Oh, come on, I am not an electrical engineer, but I am sure it's
>>> possible to do that in a non-invasive way, for example doing precise
>>> thermal imaging of the mainboard.
>>>
>>> It seems to me such hardware-level measurements would be very
>>> enlightening...
>>
>> And indeed, if someone wants to do this and has experience and
>> know-how but lacks some hardware (possibly just lacking a ThinkPad to
>> test), I could possibly be inclined to put money toward buying it....
>
>
> Having thought about this a bit more, I reckon the equipment could be
> obtained for about $25. 3 cheap digital voltmeters, an op-amp, some
> resistors and batteries should be enough. [If someone wants to do the
> experiment, I'll write a longer post.]
>
> That would let one measure the power consumption of a given chip to
> within < 10% accuracy, in a calibrated manner (provided that the PCB
> traces carrying the current can be acccessed - which isn't necessarily
> true for BGA chips).
>
> Richard
Electrical power is the product of voltage and current, plus power
factor in AC circuits.  Voltage is very easy to measure, with any
multimeter.  On the other hand, current is not.  The traditional method
is to open the circuit and insert the meter, but this really isn't
practical on circuit boards.  It is possible to measure the magnetic
field produced by the current, but then you'd have to do a bit of math
to apply it.  Another method is to measure the voltage drop along the
lead supplying power to the chip, but that method requires a sensitive
voltmeter and you also need to know the resistance of the lead.  Again,
it's not always practical with circuit boards.  If you try to use
temperature, there are a lot of different factors you need to know, to
calculate power dissipation, including cooling etc.  In short, there is
no simple way to do what he wants.  Measuring power required by the
entire system is easy.  Incidentally, many years ago, I used to repair
core memory boards.  On those boards you have to be able to measure the
current in the wires that go through the cores.  On those boards, there
were wire loops which could be used with an appropriate oscilloscope
probe that could measure AC, but not DC currents.  The loops were
necessary, as the probe had to close around the wire.


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