Re: Calibration Of Electronic Equipment In The Home Workshop
- From: Jim Yanik <jyanik@xxxxxxxxx>
- Date: 5 Mar 2007 02:02:23 GMT
Robert Baer <robertbaer@xxxxxxxxxxxxx> wrote in
news:WjJGh.6609$PL.2202@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx:
MassiveProng wrote:
On Sun, 04 Mar 2007 10:11:09 GMT, Robert BaerI think i said nothing about a series resistor.
<robertbaer@xxxxxxxxxxxxx> 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?
Shunt resistors are typically less than an ohm. Show me where
ANYONE uses a 1.1 meg resistor os a current shunt.
Take that handheld DVM and note (rare exceptions) that its input
resistance is 10 megs onany of the voltage scales.
Take further note that the most sensitive scale is (almost always)
200mVFS.
So, by the simple application of ohms law, driving the meter for
full
scale reading, the current thru the meter is 20nAFS.
Now, if one places a 1.11Meg resistor in parallel with the DVM,
then
the equivalent input rtesistance would then be 1.00Megs and that would
mean, by the simple application of ohms law, driving the meter for
full scale reading, the current thru the meter is 200nAFS.
Actually,the current *thru the meter* is still 20naFS(on the voltage
rage),the other 180na goes thru the shunt 1.1111MegR.
but I know Rob't knows that.
Und so wieder.
Now if you happen to have a *different* meter that has current
scales
moer sensitive than 2mAFS, then this "trick" would not be needed.
Or....if you have absolutely no need for 3.5 or4.5 digit readings
of
low currents via your handheld DVM, then this is moot.
--
Jim Yanik
jyanik
at
kua.net
.
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