rasPi Stretch Lite install notes

Hal Murray hmurray at megapathdsl.net
Tue Jan 9 03:11:31 UTC 2018

>>> 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 

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.

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.

(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 

I don't have a handy test case.  You might try whatever you have handy.  Plug 
in a GPS and see what it says.

The other problem with USB meters is that they drop the voltage 
significantly.  They are actually measuring the voltage across a small 
resistor.  If you put one in upstream of your Pi you are increasing your 
chances of power problems.  If you have a normal meter with probes, you can 
probably measure the voltage going in and coming out.

These are my opinions.  I hate spam.

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