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G8MNY  > TECH     27.04.26 15:45l 66 Lines 2037 Bytes #4 (0) @ WW
BID : 58227_GB7CIP
Read: GUEST
Subj: Measuring 1 of 3 Rs
Path: JH4XSY<N3HYM<W9GM<KC9UHI<VK2RZ<GB7CIP
Sent: 260427/0640Z @:GB7CIP.#32.GBR.EURO #:58227 [Caterham Surrey GBR]
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.

ﾚﾄﾄﾄﾄﾄﾄﾄﾄｿ
ｳ    Highﾃﾄﾄﾄﾄﾂﾄﾄﾄｿ
ｳ        ｳ    ｳ  R2
ｳ     Rx>ｳ   R1   ﾃﾄﾄｿ
ｳ DMM    ｳ    ｳ  R3  ｳ
ｳ     Lowﾃﾄﾄﾄﾄﾁﾄﾄﾄﾙ  ｳ
ｳ        ｳ           ｳ
ｳ   Guardﾃﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾙ
ﾀﾄﾄﾄﾄﾄﾄﾄﾄﾙ

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
   connected         Rx = ﾄﾄﾄﾄﾄﾄﾄ
   to Low                 R1 x R2

2/ Guard                  R1 + R3
   connected         Rx = ﾄﾄﾄﾄﾄﾄﾄ
   to High                R1 x R3

3/ Guard                  R1 + R2 + R3
   not               Rx = ﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄ
   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...

ﾚﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄｿ
ｳ        Highﾃﾄﾄﾄﾄﾂﾄﾄﾄｿ
ｳ          / ｳ    ｳ  R2
ｳ        DVM ｳ   R1   ﾃﾄﾄｿ
ｳ          \ ｳ    ｳ  R3  ｳ
ｳ  Ref I> Lowﾃﾄﾄﾄﾄﾁﾄﾄﾄﾙ  ｳ
ｳ       /ｳ_ /ｳ   No /ｳ\  ｳ
ｳ  ﾚﾄﾄ<' ｳ_  ｳ  Voltage  ｳ
ｳ \ｳ/  `\ｳ  \ｳ      \ｳ/  ｳ
ｳ Stray I> Guﾃﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾙ
ﾀﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾄﾙ

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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