Appendix 7 Unstable Measured Values
A18
(12) Measurement of current sensing resistors (shunt resistors)
When mounting a two-terminal type current sensing resistor
on a printed circuit board, separate the current and voltage
detection wires as shown in Fig. 10 in order to avoid the
effects of wiring resistance. To ensure that the current will
flow evenly to the sensing resistor, it is necessary to use the
same width for the current wire as the electrode and to avoid
bending the wire near the electrode (see Fig. 11). When
testing the current sensing resistor, wire probes are gener-
ally used (see Fig. 12). In this case, the measurement current will gradually expand inside
the current sensing resistor from the point of application (SOURCE B) and flow back again
to the probe point (SOURCE A) (see Fig. 13). Current density is high at the current applica-
tion points (SOURCE A, SOURCE B), and placing the voltage terminals (SENSE A,
SENSE B) near them will yield resistance values that tend to be higher than the actual
mounted value (see Fig. 14).
Figure. 10A Current Sensing
Resistor mounted on a
Printed Circuit Board
Figure. 11 Current Flow in the Mounted State
Figure. 12 Probing in the Test State
Voltage detection Voltage detection
Conductor
pattern
Electrode Resistor Electrode
Current
Conductor
pattern
Figure. 13 Flow of Current in the Test State
SOURCE A
SENSE A
SOURCE B
SENSE B
Current
Current probes
Figure. 14 Difference between
Mounted State and Test State
Resistance value in mounted state
Resistance value in test state
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