YOKOGAWA WT500 User Manual page 341

Appendix 4
Effects of Power Loss
SOURCE
IM 760201-01E
How to Make Accurate Measurements
By wiring a circuit to match the load, you can minimize the effects of power loss on
measurement accuracy. We will discuss the wiring of the current source and load
resistance shown in the figure below.
When the Measured Current Is Relatively Large
Connect the voltage measurement circuit between the current measurement circuit and
the load. The current measurement circuit measures the sum of i
flowing through the load of the circuit under measurement, and i
through the voltage measurement circuit. Because the current flowing through the circuit
under measurement is i , only i
L
the voltage measurement circuit of the WT500 is approximately 2 M
is 1000 V, i is approximately 0.5 mA (1000 V/2 M
V
(load resistance is 200 or less), the effect of i
Ω
or less. If the input voltage is 100 V and the current is 5 A, iV = 0.05 mA (100 V/2 MΩ), so
the effect of i on the measurement accuracy is 0.001% (0.05 mA/5 A).
V
U
I
±
I
As a reference, the relationships between the voltages and currents that produce effects
of 0.01%, 0.001%, and 0.0001% are shown in the figure below.
0.01% effect
1000
800
600
400
200
0
0 5 10 15
When the Measured Current Is Relatively Small
Connect the current measurement circuit between the voltage measurement circuit and
the load. In this case, the voltage measurement circuit measures the sum of e
e . e is the load voltage, and e
I L
circuit. Only e reduces measurement accuracy. The input resistance of the current
I
measurement circuit of the WT500 is approximately 5 m
the effect of e on the measurement accuracy is approximately 0.0005% (5 m
I
U
SOURCE
U
±
±
I
reduces measurement accuracy. The input resistance of
V
). If the load current i
Ω
on the measurement accuracy is 0.01%
V
SOURCE
U
LOAD
±
Smaller effect
20 25 30 35 40
Measured current (A)
is the voltage drop across the current measurement
I
Ω
SOURCE
LOAD
I
and i . i is the current
L V L
is the current flowing
V
. If the input voltage
Ω
, is 5 A or more
L
U
LOAD
i
V
±
i
I
±
WT500
0.001% effect
0.0001% effect
and
L
. If the load resistance is 1 k
Ω
/1 k ).
Ω Ω
LOAD
U
e
±
I
±
e
I
WT500
App-21
1
2
3
4
5
6
7
8
L
9
10
11
12
13
,
14
L
App
Index