Brochure

21
Technical Information – Relays
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Contacts
The contacts are the most important constituent of a relay. Their
characteristics are significantly affected by factors such as the
material of the contacts, voltage and current values applied to
them (especially, the voltage and current waveforms when
energizing and de-energizing the contacts), the type of load,
operating frequency, atmosphere, contact arrangement, and
bounce. If any of these factors fail to satisfy predetermined
values, problems such as metal deposition between contacts,
contact welding, wear, or rapid increase in the contact resistance
may occur.
Switching voltage (AC, DC)
When a relay breaks an inductive load, a fairly high
counterelectromotive force (counter emf) is generated in the
relay’s contact circuit. The higher the counter emf, the greater the
damage to the contacts. This may result in a significant decrease
in the switching power of DC-switching relays. This is because,
unlike the AC-switching relay, the DC-switching relay does not
have a zero-cross point. Once arc has been generated, it does not
easily diminish, prolonging the arc time. Moreover, the
unidirectional flow of the current in a DC circuit may cause metal
deposition to occur between contacts and the contacts to wear
rapidly (this is discussed later).
Despite the information a catalog or data sheet sets forth as the
approximate switching power of the relay, always confirm the
actual switching power by performing a test with the actual load.
Switching Current
The quantity of electrical current which flows through the contact
directly influences the contact’ characteristics. For example,
when the relay is used to control an inductive load such as a
motor or a lamp, the contacts will wear more quickly, and metal
deposition between the mating contacts will occur more often as
the inrush current to the contacts increases. Consequently, at
some point the contacts may not be able to open.
Contact Materials
Selection of an appropriate contact material according to the load
to be opened or closed is important. Several contact materials
and their properties are listed below.
Contact Materials and Feature
Contact Protection Circuit
A contact protection circuit, designed to prolong the life of the
relay, is recommended. This protection will have the additional
advantages of suppressing noise, as well as preventing the
generation of carbide and nitric acid, which otherwise would be
generated at the contact surface when the relay contact is
opened. However, unless designed correctly, the protection circuit
may produce adverse effects, such as prolonging the release time
of the relay.
P. G. S. This material has excellent corrosion resistance
Alloy and is suitable for very small current circuits.
(Au : Ag : Pt = 69 : 25 : 6)
AgPd This material exhibits good corrosion and sulphur
resistance. In a dry circuit, it attracts organic gas
to generate a polymer, therefore it is usually
plated with gold or other material.
Ag This material has the highest electric and heat
conductivities among all metals. It exhibits low
contact resistance, but easily forms sulphide film
in a sulphide gas environment. This may result in
defective contact performance at a low-voltage
small-current operation.
AgNi This material exhibits the same high electric
conductivity as silver and excellent arc
resistance.
AgSnO
2
This material exhibits excellent deposition
resistance. It easily forms sulphide film in a
sulphide gas environment, the same as Ag
contact material.
AgSnIn This material exhibits excellent deposition
resistance and exhaustion resistance.
AgW This material exhibits a high hardness and
melting point. It also exhibits excellent arc
resistance and superior resistance to deposition
and transfer. However, it shows high contact
resistance and inferior environmental resistance.
PCB Relays
Omron A5 Catalogue 2007 1-282 11/9/06 10:16 am Page 21