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Installation - 3
37
Because the measurement circuits of the DVM are internally referenced to the minus terminal of the
main output, you must observe the following restrictions in order to guarantee accurate DVM
measurements (refer to figure 3-7).
66319D
66321D
OUTPUT
+
−
−−
−
36V
DC
6V
DC
DVM INPUT
DVM
DVM
DVM
DVM
R1
12V
lead resistance
LOAD
lead resistance
R2
12V
R5
2V
R6
2V
Test Fixture
(for illustration only)
+
−
−−
−
load
current
R3
12V
R4
2V
V
1
2
3
4
5
6
7
Node # V Common Mode
1
2
3
4
5
6
7
36 V + V
24 V + V
12 V + V
V
- 2 V + V
- 4 V + V
- 6 V + V
Minus
terminal
NOTE: The DVM common
mode voltage range is from
-4.5Vdc to +25Vdc.
voltages outside this range will
result in erroneous readings.
Figure 3-7. Measuring Circuits Not Powered by the Main Output
♦
You cannot measure voltages greater than +25 Vdc with respect to the negative terminal of the main
output. A situation where this could occur is illustrated by R1 in figure 3-7, which has only a 12 Vdc
drop across it but is 36 Vdc + Vlead with respect to the negative terminal of the main output.
♦ You cannot measure voltages less than −4.5 Vdc with respect to the negative terminal of the main
output. A situation where this could occur is illustrated by R6 in figure 3-7, which has only a −2 Vdc
drop across it but is −6 Vdc + Vlead with respect to the negative terminal of the main output.
♦ When calculating the common mode voltage between the point that you wish to measure and the
negative terminal of the main output, you must also include any voltage drop in the negative load
lead. For example, in figure 3-7, if the voltage drop in the negative load lead is 2 V, you would not be
able to correctly measure the 12 Vdc drop across R2. This is because when the voltage drop in the
load lead is added to the voltage drops across R2 and R3, the resultant voltage is 26 Vdc, which
exceeds the +25 Vdc common mode rating of the DVM.