Originally a Silicon Chip article but this full article and circuit is by
the original designer.

An extremely useful tool.
Reduces resistance of current measurement ranges by about 100 times.
At core it is "just" a 100x amplifier connected across a current sense
resistor - in practise it uses and extremely nice opamp to allow
measurements into the nA range.

http://alternatezone.com/electronics/ucurrent/uCurrentArticle.pdf

The very capable opamp is under $US5  at Digikey in ones.

Multimeters can have substantial resistance on current ranges, which can
cause substantial voltage drop when measuring. This reduces this effect ver=
y
substantially.

A meter resistance of 1 ohm drops 1 mV per mA or 1 V per amp.
At 200 mA a 1 ohm meter drops 200 mV. That's most if the range f a NimH
cell.
Good meters may have 1.8 mV/mA "burden voltage".
I have (had) a meter which I discovered the hard way had 17 ohms resistance
on the 200 mA range =3D 1.7V drop at 100 mA !!!- ridiculous and absolutely
unnecessary. BUT do you know the resistance of your current ranges?

I address this problem by placing a current sense resistor upstream of a
purpose built variable voltage supply. The system is fed by a bench psu
which provides overall current limiting. Rout ~=3D zero at any (sensible)
current.



 Russell McMahon
--=20
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