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G8MNY  > TECH     13.05.22 07:36z 66 Lines 2031 Bytes #27 (0) @ WW
BID : 21578_GB7CIP
Subj: Measuring 1 of 3 Rs
Path: SP7YDD<SR1BSZ<IW0QNL<ON0AR<DB0RES<DB0ERF<DK0WUE<GB7CIP
Sent: 220513/0709Z @:GB7CIP.#32.GBR.EURO #:21578 [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.

┌────────┐
│    High├────┬───┐
│        │    │  R2
│     Rx>│   R1   ├──┐
│ DMM    │    │  R3  │
│     Low├────┴───┘  │
│        │           │
│   Guard├───────────┘
└────────┘

As the DMM has uProcessor you might expect the Guard line to be connected to
the Low then High & 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 = ───────
   to Low                 R1 x R2

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

3/ Guard                  R1 + R2 + R3
   not               Rx = ────────────
   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 & 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...

┌────────────┐
│        High├────┬───┐
│          / │    │  R2
│        DVM │   R1   ├──┐
│          \ │    │  R3  │
│  Ref I> Low├────┴───┘  │
│       /│_ /│   No /│\  │
│  ┌──<' │_  │  Voltage  │
│ \│/  `\│  \│      \│/  │
│ Stray I> Gu├───────────┘
└────────────┘

So as long as the Guard stray current generator can put the error current
through R2 the Reading of Rx = R1 & 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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