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Appendix 11 Measuring Contact Resistance
A26
(1) Types of contacts
Switches, relays, and connector contacts can be broadly classified as either of two types:
Power contacts
Signal contacts
•
Power contacts
Lines carrying currents of several dozens of amperes consume power measured in watts,
even if they have a resistance of 1 m
Ω
. Consequently, switch contacts on high-current lines
such as circuit breakers have resistance values that are far below 1 m
Ω
. Power relays, cir-
cuit breakers, and other components are designed based on the assumption that they will
be used with high-current lines. Consequently, use of low currents (on the order of micro-
amperes) requires caution since gradual corrosion of the contacts will eventually compro-
mise their conductivity.
•
Signal contacts
Since switches and connectors used in standard electronic circuits typically carry currents
of 1 A or less, their contact resistance is on the order of several dozens of milliohms. These
contacts are usually gold-plated so that stable contact can be achieved even with microam-
pere-level currents. Switches that use conductive rubber exhibit resistance values that vary
dramatically with the pressure placed on them. They have a high contact resistance of
around 1 k
Ω
, but they are characterized by an extremely high level of durability.
(2) Measuring contact resistance
•
Power contacts
Unless otherwise defined, measurement can be accomplished at an adequate level of res-
olution by using a current of about 1 A. However, if there are local areas of high contact
resistance, it is necessary to observe heat generation at the contact while using a current
that approaches the conditions under which the contact will be used. Power contacts are
typically used at a relatively high voltage of at least 5 V. When measured with an ohmmeter
with a low open voltage, the current may be unable to pass through contaminants (oxide
film or dirt) on the contact that do not pose an issue during normal use, triggering a decision
of poor contact. For this reason, it is not desirable to measure power contacts with low-
power ohmmeters.
•
Signal contacts
Most signal contacts are connected to IC input terminals, and it is not unusual for them to
carry currents of less than 1
μ
A. As the contact is repeatedly opened and closed, vibrations
cause the plating on the contact surface to flake off, triggering rapid corrosion of the contact
(oxidation and sulfuration).
When contacts become corroded so that their contact resistance increases, measurement
at high currents such as 1 A may trigger a process by which the contact resistance gradu-
ally recovers. Measuring contacts with more advanced corrosion with an ohmmeter with a
high open voltage may allow the current to pass through the corrosion, leading to a decision
of good contact.
For this reason, when measuring signal contacts, the open voltage should be limited to the
extent possible, and measurement should be carried out using an extremely low current
(dry-circuit testing). The instrument can be used to perform dry-circuit testing by enabling
the low-power setting.
Appendix 11 Measuring Contact Resistance