App-24
IM 760301-01E
Appendix 5 For Making Accurate Measurements
Effects of Power Loss
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. The input resistance of the voltage
measurement circuit of the instrument is approximately 10 M
Ω
. If the input is 1000 V,
i
V
is approximately 0.1 mA (1000 V/10 M
Ω
). If the load current, i
L
, is 1 A or more (load
resistance is 1 k
Ω
or less), then the effect of i
V
on the measurement is 0.01% or less.
If the input is 100 V and 1 A, i
V
= 0.01 mA (100 V/10 M
Ω
) then the effect of i
V
on the
measurement accuracy is 0.001% (0.01 mA/1 A).
SOURCE
LOAD
WT3000
U
±
I
±
i
V
i
L
LOAD
SOURCE
U
I
I
U
±
±
As a reference, the relationship of the voltages and currents that produce effects of
0.01%, 0.001%, and 0.0001% are shown in the figure below.
1000
800
600
400
200
0
Measured current (A)
Measured voltage ( V )
0.01% Effect
0 1 2 3 4 5 6 7 8 9 10
0.001% Effect
0.0001% Effect
Smaller effect
• 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 e
L
and
voltage drop e
I
across the current measurement circuit. e
I
is the amount of error in this
case. The input resistance of the current measurement circuit of the instrument is 500
m
Ω
and 5.5 m
Ω
for the 2-A and 30-A input terminals, respectively. For example, if the
load resistance is 1 k
Ω
, then the effects on the measurement accuracy is 0.05% for
the 2-A input terminal (500 m
Ω
/1 k
Ω
) and 0.00055% for the 30-A input terminal (5.5
m
Ω
/1 k
Ω
).
SOURCE
LOAD
U
I
WT3000
±
e
L
e
I
±
SOURCE
LOAD
U
I I
U
±
±