Message 30382 in TECH.WW
From: G8MNY
Date: Mon, 21 Sep 26 06:49:00 Z
Newsgroups: TECH.WW
Subject: Measuring 1 of 3 Rs
Message-ID: <5693_GB7CIP>
Path: N2MH4|GB7BED|EI2GYB|GB7CIP
R:260921/0652Z 37480@N2MH4.#SENC.NC.USA.NOAM LinBPQ6.0.24
R:260921/0651Z 18002@GB7BED.#21.GBR.EU LinBPQ6.0.25
R:260921/0651Z 59297@EI2GYB.DGL.IRL.EURO LinBPQ6.0.25
R:260921/0649Z @:GB7CIP.#32.GBR.EURO #:5693 [Caterham Surrey GBR] $:5693_GB7CIP
From: G8MNY@GB7CIP.#32.GBR.EURO
To : TECH@WW
By G8MNY (new Mar 06)
(8 Bit ASCII graphics use code page 437 or 850, Terminal Font)
I recently bought an old LAB DMM (Datron 1061a) meter. It has an unusual
feature, the ability to measure one Resistor R1 in a triangle of 3 (R2 & R3)
while all connected.
As the DMM has uProcessor you might expect the Guard line to be connected to
the Low then High and then disconnected, to give 3 values of Rx, then with 3
equations & 3 unknowns all the Rs can be calculated....
1/ Guard R1 + R2
to Low R1 x R2
2/ Guard R1 + R3
to High R1 x R3
3/ Guard R1 + R2 + R3
connected R1 x (R2+R3)
But that is not the purpose of the Guard line in a sensitive measuring circuit.
To measure the resistance the DMM actually uses an accurate constant current
source across Rx and measures the voltage to get the R1 value. But with R2 + R3
in circuit this will give a wrong reading as some of the accurate current will
flow through R2 + R3. But on this meter the Guard line has another current
generator with its control arranged to make the Guard the same potential as the
Low...
So as long as the Guard stray current generator can put the error current
through R2 the Reading of Rx = R1 and be correct.
See my tech bul "Lab Measuring DMM".
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
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