Hioki RM3542-50, Instruction Manual

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Appendix 3 Unstable Measurement Values
A
5
Appendix
(4) Wider/Thicker DUTs
If the measuring object has a certain width or thickness like
boards or blocks, it will be difficult to measure it accurately
using Clip Type Probes or Pin Type Probes. By using such
measurement probes, there may be considerable fluctuation of
the measurement value due to contact pressure or contact
angle.
For example, when measuring a W300 x L370 x t0.4 mm metal
plate, the measurement values are fairly different, even if mea-
suring the same points, as shown below:
0.2 mm pitch Pin Type Probe: 1.1 m
Ω
0.5 mm pitch Pin Type Probe: 0.92 to 0.97 m
Ω
Model 9287-10 Clip Type Lead: 0.85 to 0.95 m Ω
This does not depend on the contact resistance between
probes and the measuring object, but on the current distribu-
tion of the measuring object.
Figure 7. is an example plot of equipotential line for a metal
plate. Similar to the relation between atmospheric pressure dis-
tribution and wind on a weather forecast diagram, current den-
sity is higher in locations where the equipotential lines are
narrowly spaced, and lower in locations where they are widely
spaced. The figure illustrates how the electric potential gradient
varies significantly near the point at which current is injected.
This variation is due to the fact that the current is in the process
of spreading out into the metal plate, increasing the current
density in that area. For this reason, when the voltage detec-
tion terminals are placed close to the point at which current is
injected, minute changes in the position of contact are associ-
ated with significant changes in the measured value.
It is known that such effects can be minimized by detecting the
voltage within the space between the current injection points.
Generally, if the distance between the voltage detection points
and their corresponding current injection points is greater than
the width (W) or thickness (t) of the DUT, current distribution
may be considered uniform.
As shown in Fig. 8, it is desirable for the POT terminals to be
positioned at least 3W or 3t inside the CUR terminals.
(5) Unstable Temperature of the DUT
Copper wire resistance has a temperature coefficient of about 0.4%/ ° C. Just holding a copper wire in the hand
raises its temperature, causing its resistance to be increased as well. When the hand is removed from the
wire, temperature and resistance decrease. Varnished windings are more susceptible to temperature
increase, so the resistance tends to be relatively high. If the temperature of the measured object is different
from that of the probe, thermal emf occurs and causes an error. Try to measure after the temperature of the
measured object becomes close to that of room temperature. Use an instrument with a temperature-compen-
sation function such as the Hioki RM3544, RM3545 and RM3548 to minimize temperature dependence of
copper etc.
Current
injection
Figure 7. Equipotential line of the metal
plate
(W300 mm x L370 mm x t0.4 mm)
∗ Apply a current of 1 A to the end point and plot
the equipotential line for 50 µV levels
0.1 m
Ω
0.2 m
Ω
0.3 m
Ω
0.4 m
Ω
3 W, 3 t or more
3 W, 3 t or more
Figure8. Probe Positions for Wider/
Thicker measuring objects
H
CUR
H
POT
L
POT
L
CUR