YOKOGAWA WT210, User Manual, Page: 56 / 316

3-7
IM 760401-01E
Before Starting Measurements
3
3.4 For Making Accurate Measurements
By wiring the circuit to match the load, you can minimize the effect of the power loss on
the measurement accuracy. We will consider the current source (SOURCE) and load
resistance (LOAD) below.
When the Measurement Current Is Relatively Large
Wire so that the voltage measurement circuit is connected to the load side. The
current measurement circuit measures the sum of current i
L
flowing through the load
of the circuit under measurement and the current i
V
flowing through the voltage
measurement circuit. Because the current flowing through the circuit under
measurement is i
L
, i
V
is the amount of error. For example, WT230’s input resistance
of the voltage measurement circuit of the instrument is approximately 2 M
Ω
. If the
input is 600 V, i
V
is approximately 0.3 mA (600 V/2 M
Ω
). If the load current, i
L
, is 300
mA or more (load resistance is 200
Ω
or less), then the effect of i
V
on the
measurement is 0.1% (0.3 mA/300 mA) or less. If the input is 600 V and 3 A, the
effect on the measurement accuracy is 0.01% (0.3 mA/3 A).
Source
Load
WT210/WT230
V
±
C
±
i
V
i
L
Source Load
C
±
±
Input terminal
(Element)
V
V: VOLTAGE terminal
C: CURRENT terminal
As a reference, the following figure shows the relationship between the voltage and
current that cause 0.1% or 0.01% effect on the measurement accuracy.
600
500
400
300
200
100
0
Measured current (A)
0.1% effect 0.01% effect
0 1 2 3 4 5
0.3A
Effect decreases
Measured
voltage (V)
When the Measurement Current Is Relatively Small
Wire so that the current measurement circuit is connected to the load side. In this
case, the voltage measurement circuit measures the sum of the load voltage eL and
voltage drop e
C
across the current measurement circuit. e
C
is the amount of error in
this case. For example, WT230’s input resistance of the current measurement circuit
is approximately 6 m
Ω
. If the load resistance is 600
Ω
, the effect on the measurement
accuracy is approximately 0.001% (6 m
Ω
/600
Ω
).
Source
Load
V
C
WT210/WT230
±
e
L
e
C
±