rasPi Stretch Lite install notes

Gary E. Miller gem at rellim.com
Tue Jan 9 20:01:39 UTC 2018

Yo Hal!

On Mon, 08 Jan 2018 19:11:31 -0800
Hal Murray <hmurray at megapathdsl.net> wrote:

> >>> Who cares about the dynamic peak?  That is handled by the RasPi on
> >>> board capacitors.  When the capacitors can not hold up the DC any
> >>> longer then the meter sees the problem.  
> >> No, the meter is much too slow for that.  (Or at least mine is.)  
> > I'd love to test that.  Maybe the $100 USB meters are worth it?  I
> > can always put a scope on it.  Do you have a RasPi dynamic overload
> > scenario I can try to duplicate?  
> There are two issues in this discussion.  (Sorry I wasn't clear the
> first time.)
> The first problem is the capacitor.  Do the back of the envelope.
> Assume the power jumps from X to X+0.1 A and you want the voltage to
> drop less than 0.1 V in 0.1 second.  V=I*T/C  Right?  So you need one
> Farad.  There is nothing like that on a Pi.  The regulator has to
> catch that sort of change.

Yes, I oversimplified by just saying capacitor.  Of course there are
regulators, cables, capacitors, etc.

Which is why I test all my cables and regulators, and ony use a
few of them.  And yes, my meter does see the difference well enough
for me to separate better from worse.  So good enough for my common
cases.  I always have the scope if I need to dig more.

> The other issue is the meter response time.  What sort of glitch do
> you expect to see?  What do you think the response time of the
> display is?  Most low cost meters average over a long time.  That
> gives you the average voltage by filtering out the noise.  Long is a
> significant fraction of a second.

My meters seem to respond at about 0.1 second, as you assumed above.  I
can actually see the current go up and down as I do different
reads/writes to my USB devices.

> (Years ago, a friend told me the story of helping a student.  He put
> an old mechanical meter on the power supply and you could see the
> fuzz in the needle.)

Yup.  I'd rather have the analog needle, but digital is what is
cheap and easy now.  And the scope if need be.
> I don't have a handy test case.  You might try whatever you have
> handy.  Plug in a GPS and see what it says.

I do that all the time.  I test everythong.  What I see on my meter
correlates with when I see things fail, or act marginal.

So until you have a real test case, you'll just have to live with my
data. :-)

> The other problem with USB meters is that they drop the voltage 
> significantly.  They are actually measuring the voltage across a
> small resistor.

I just put two USB meters in series, then added a 1A load.  The voltage
difference was 0.07V.  I can live with that.  2A was 0.14V.  If my
RasPi dies on 0.14V drop I got bigger problems than a USB stick.

Then I swapped the meters.  1A drop 0.20V, 2A drop 0.34V.  At the same
time my regulator dropped 0.26V!  Next time I want precision I'll check
for my best meter first.

But all negligible since the test case we were talking about, USB
sticks and SC cards, was 0.10A and under.

>  If you put one in upstream of your Pi you are
> increasing your chances of power problems.  

Which is why I do not leave extra stuff hooked up.  This is test gear,
not continuous monitoring.

> If you have a normal
> meter with probes, you can probably measure the voltage going in and
> coming out.

I got meters, scopes, etc.  The USB meter is just another tool in the
toolbox, with its own strengths and weaknesses.  For checking SD
card current draw, the perfect tool.  Even OK for checking other meters.

Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
	gem at rellim.com  Tel:+1 541 382 8588

	    Veritas liberabit vos. -- Quid est veritas?
    "If you can’t measure it, you can’t improve it." - Lord Kelvin
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