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G8MNY > TECH 09.03.25 11:30z 74 Lines 2369 Bytes #999 (0) @ WW
BID : 26450_GB7CIP
Subj: Measuring 1 of 3 Rs
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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.
ÚÄÄÄÄÄÄÄÄ¿
³ HighÃÄÄÄÄÂÄÄÄ¿
³ ³ ³ R2
³ Rx>³ R1 ÃÄÄ¿
³ DMM ³ ³ R3 ³
³ LowÃÄÄÄÄÁÄÄÄÙ ³
³ ³ ³
³ GuardÃÄÄÄÄÄÄÄÄÄÄÄÙ
ÀÄÄÄÄÄÄÄÄÙ
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 = ÄÄÄÄÄÄÄ
to Low R1 x R2
2/ Guard R1 + R3
connected Rx = ÄÄÄÄÄÄÄ
to High R1 x R3
3/ Guard R1 + R2 + R3
not Rx = ÄÄÄÄÄÄÄÄÄÄÄÄ
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...
ÚÄÄÄÄÄÄÄÄÄÄÄÄ¿
³ HighÃÄÄÄÄÂÄÄÄ¿
³ / ³ ³ R2
³ DVM ³ R1 ÃÄÄ¿
³ \ ³ ³ R3 ³
³ Ref I> LowÃÄÄÄÄÁÄÄÄÙ ³
³ /³_ /³ No /³\ ³
³ ÚÄÄ<' ³_ ³ Voltage ³
³ \³/ `\³ \³ \³/ ³
³ Stray I> GuÃÄÄÄÄÄÄÄÄÄÄÄÙ
ÀÄÄÄÄÄÄÄÄÄÄÄÄÙ
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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