"Anthony Fremont" wrote in
:

MassiveProng wrote:
On Sun, 04 Mar 2007 10:11:09 GMT, Robert Baer
Gave us:

Here is a good "trick" to measure low currents with your DVM.
Make a switchable shunt box with (at least) the following full
scale ranges: 200nA (shunt resistor 1.11 megs), 2uA (shunt resistor
101K), 20uA (shunt resistor 10.0K), 200uA (shunt resistor 1.00K).
Put a twisted pair of leads (red, black) with banana plugs (red,
black) running out of the box via a small grommet, to plug into your
DVM set to the 200mV scale; a pair of (red, black) banana jacks with
0.75 "spacing is mounted on the box for your test leads.
Hint: add to the legend the parallel resistance of the system
(200nA/1M, 2uA/100K, etc) as a reminder of the resistance of this
current meter scheme.
Added hint: the 200MV scale is good for 20nA full scale, just
remember the meter resistance is 10 megs.


Tell us, oh master... what does placing a 1,1 meg resistor in
series with a circuit's power source do to the voltage presented to
the circuit?

http://www.google.com/search?hl=en&q=ohms+law

It drops it like any other shunt would.

all current measuring devices add resistance to the circuit,affecting it.
Just like any DMM parallels its input resistance with what it's put
across,loading the circuit under test.

If you were designing
precision equipment, would you attempt to measure nA by using a .01Ohm
resistor? Read Ed's post, he is right about being able to measure low
currents that way, just as Robert is here. They just don't work if
the current is varying over a wide range. But for static current
measurements, THEY WILL WORK FINE.

Shunt resistors are typically less than an ohm. Show me where
ANYONE uses a 1.1 meg resistor os a current shunt.

(you use the 1.1 shunt value externally because the DMMs 10 Meg parallel
resistance brings the effective R down to 1.0.)




--
Jim Yanik
jyanik
at
kua.net