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G8MNY > TECH 21.08.08 21:04l 87 Lines 2378 Bytes #999 (0) @ WW
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From: G8MNY@YB8EW..#MKR.IDN.OC
To : TECH@WW
Path: !VK6HGR!GB7YFS!GB7CIP!
From: G8MNY@GB7CIP.#32.GBR.EU
To : TECH@WW
By G8MNY (new Mar 06)
(8 Bit ASCII graphics use code page 437 or 850)
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.
̆̊̊̊̊̊̊̊³ HighÃÃäÃäÃäÃäÂÃäÃäÃä¿
³ ³ ³ R2
³ Rx>³ R1 ÃÃäÃä¿
³ DMM ³ ³ R3 ³
³ LowÃÃäÃäÃäÃäÃüÃäÃäÃäÃÖ Â³
³ ³ ³
³ GuardÃÃäÃäÃäÃäÃäÃäÃäÃäÃäÃäÃäÃÖ
ÀÃäÃäÃäÃäÃäÃäÃäÃÖ
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...
̆̊̊̊̊̊̊̊̊̊̊̊³ HighÃÃäÃäÃäÃäÂÃäÃäÃä¿
³ / ³ ³ R2
³ DVM ³ R1 ÃÃäÃä¿
³ \ ³ ³ R3 ³
³ Ref I> LowÃÃäÃäÃäÃäÃüÃäÃäÃäÃÖ Â³
³ /³_ /³ No /³\ ³
³ ÃÜÃäÃä< ³_ ³ Voltage ³
³ \³/ \³ \³ \³/ ³
³Stray I> GuÃÃäÃäÃäÃäÃäÃäÃäÃäÃäÃäÃäÃÖ
ÀÃäÃäÃäÃäÃäÃäÃäÃäÃäÃäÃäÃÖ
So as long as the Guard stray current generator can put the error current
through R2 the Reading of Rx = R1 & be correct.
Why Don't U send an interesting bul?
73 De John, G8MNY @ GB7CIP
[End of Message #643728 from G8MNY]
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