Yo Hal! On Fri, 03 Jun 2016 03:09:04 -0700 Hal Murray <[email protected]> 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 [email protected] Tel:+1 541 382 8588
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