3-8
IM 253710-01E
3.5
For Making Accurate Measurements
Effects of Power Loss
By using an appropriate wiring method that matches the load, the effects of power loss
on measurement accuracy can be minimized.
We will consider the current source (SOURCE) and load resistance (LOAD) below.
When the measured 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 1 M
Ω
. If the input is 1000 V,
i
V
is approximately 1 mA (1000 V/1 M
Ω
). If the load current, i
L
, is 10 A or more (load
resistance is 100
Ω
or less), then the effect of i
V
on the measurement is 0.01% or
less. If the input is 100 V and 10 A, i
V
= 0.1 mA (100 V/1 M
Ω
) then the effect of i
V
on
the measurement accuracy is 0.001% (0.1 mA/10 A).
SOURCE
LOAD
This instrument
U
±
I
±
i
V
i
L
SOURCE
LOAD
Input terminal
(ELEMENT)
I
U
±
±
As a reference, the relationship of the voltages and currents that produce effects of
0.1%, 0.01%, and 0.001% are shown in the figure below.
1000
800
600
400
200
0
Measured current (A)
Measured voltage (V)
0.1% Effect
0.01% Effect
0 2 4 6 8 10 12 14 16 18 20
1 A
Smaller effect
0.001% Effect
When the measured 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
100 m
Ω
and 11 m
Ω
for the 5-A and 20-A terminals, respectively. For example, if the
load resistance is 1 k
Ω
, then the effects on the measurement accuracy is 0.01% for
the 5-A terminal (100 m
Ω
/1 k
Ω
) and 0.0011% for the 20-A terminal (11 m
Ω
/1 k
Ω
).
SOURCE
LOAD
U
I
This instrument
±
e
L
e
I
±
