MD-5060
11
Then the resulting circuit will be:
V
I
R
R
R
G
R
FIG. 3
R
i
x
1
2
A
A
In the circuit of Fig. 3 it may be noted that R1 is in parallel with a low-value resistance (the
one from the built-in microammeter) therefore its influence is reduced during reading.
Through resistance R2 circulates a current which is not passing through the meter and
consequently does not affect the reading. In fact, current through R2 originates a certain
error, since it creates an additional voltage drop in R1 which was not regarded during
megohmmeter calibration.
As regards the practical use of megohmmeter, it shall be considered that if R1 and R2 are
higher than 100 M
Ω
, any value of Rx will be measured with an error lower than 10%. For
example: Let us consider Rx = 3.000 M
Ω
and R1 = R2 = 100 M
Ω
, the reading without
using the GUARD terminal would be 187.5 M
Ω
, which is quite wrong. On the other hand, if
the GUARD terminal is properly used, we would have 3.000 M
Ω
.
