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

[Gary E. Miller]

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:

https://www.solo-labs.com/diy-usb-line-power-meter-stick/

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:

https://www.adafruit.com/products/1852

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

http://www.green-trust.org/arduino/DIY-Amp-Hour-Meter-Arduino.pdf

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.

RGDS
GARY
---------------------------------------------------------------------------
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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