[gpsd-dev] GR-701W hangs on Pi-3

Gary E. Miller gem at rellim.com
Fri Jun 3 19:02:45 UTC 2016

Yo Hal!

On Fri, 03 Jun 2016 03:09:04 -0700
Hal Murray <hmurray at megapathdsl.net> wrote:

> > And what do you call a 'real' meter?  
> Something that is likely to be accurate.  It will cost more than $5.
> I have a Fluke 112.

I guess you didn't know I used to work at Fluke.  :-)

Yes, real quality stuff, espeially if you need 9 digits or acurate 
precision.  3 digits is all we need here.  

Plus I have a bunch and always compare them.  Just in case one gets
dropped a bit too hard.

> I got it because we had one at work and I liked
> it. It's old enough that they don't make them any more.  I'm not sure
> what I would get today.

Danaher bought them, and then also bought the Philips meter biz.  Still
good stuff.

The Fluke meters and calibrators always beat the HP/Agilent equivalents.

> > Any modern meter should have a really, really, small drop resistor.
> > But, a good question.  I'll open up a few of my meters to check.   
> The size of the drop resistor gets scaled by the current range.  The
> question is what is the lowest voltage the meter can read full
> scale.  Typical numbers are 200 mV.

The cheap one are fixed at 9.99V full scale.  So they do not need a
big drop resistor.

I guess they are too cheap to be Hall Effect, like the sensors in my
$30 chinese meters.  Not as good as a $300 Fluke, but good enough for
my use, and I don't cry when they are stolen...

I found the schematic for a DIY USB meter:


They use 0.01 Ohm for the drop resistor.

28AWG is 0.068 Ohm/foot
20AWG is 0.015 Ohm/foot

So the drop resistor is like:

2" extra 28AWG line length
8" extra 20AWG line length

Put another way, that is an extra voltage drop at 2A of 0.02 Volts

Good to know, and I can live with that.

I see the Adafruit sensor uses a 0.05 Ohm drop resistor, that seems a bit
large, adding 0.10 extra voltage drop at 2A:


If that bothers you, go with a DIY hall sensor:


Since I measure the Volts at the target, as long as I still see
5V I'm happy.  I care that the power supply has not folded and the
drop in the cable is not too high.

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