Heatsinks and Relays 377Shield, Magnetic. An optional plate or shell constructed
of magnetically permeable material such as nickel-iron
or mu-metal, fitted external to the relay’s coil. Its func-
tion is to reduce the effects of magnetic interaction
between adjacent relays and to improve the efficiency of
the relay coil. A magnetic shell also reduces the influ-
ence of external magnetic fields, which is useful in secu-
rity applications. Magnetic shields can be fitted
externally or may be buried inside the relay housing.Soft Failure. Intermittent self-recovering failure of a
contact.
Sticking (Contacts). A switch failure mechanism,
whereby a closed contact fails to open by a specified
time after relay de-energization. Can be subclassified as
hard or soft failures.
Switch AT. The ampere turns required to close a reed
switch, pull-in AT, or just to maintain it closed, drop-out
AT, and is specified with a specific type and design of
coil. Switch AT depends on the length of the switch
leads and increases when the reed switch leads are
cropped. This must be taken into account when speci-
fying a switch for a particular application.
Switching Current. The maximum current that can be
hot-switched by a relay at a specified voltage without
exceeding its rating.
Switching Voltage. The maximum voltage that can be
hot-switched by a relay at a specified current without
exceeding its rating. Generally lower than breakdown
voltage, since it has to allow for any possible arcing at
the time of contact breaking.
Transmission Line. In relay terms an interruptible
waveguide consisting of two or more conductors,
designed to have a well-controlled characteristic RF
impedance and to efficiently transmit RF power from
source to load with minimum losses, or to block RF
energy with minimum leakage. Structures useful within
RF relays include microstrips, coplanar waveguides,
and coaxial transmission line elements.
VSWR (Voltage Standing Wave Ratio). The ratio of
the maximum RF voltage in a relay to the minimum
voltage at a specified frequency and calculated from
(13-13)
where,
Uis the the voltage reflected back from a closed relay
terminated at its output with a standard reference
impedance, normally 50:.13.2.2 Contact Characteristics6,7,8Contacts may switch either power or dry circuits. A
power circuit always has current flowing, while a dry
circuit has minimal or no current flowing, such as an
audio circuit. A dry or low-level circuit typically is less
than 100 mV or 1 mA.
The mechanical design of the contact springs is such
that when the contacts are closed, they slide for a short
distance over the surfaces of each other before coming
to rest. This is called a wiping contact, and it ensures
good electrical contact.
Contacts are made of silver, palladium, rhodium, or
gold and may be smooth or bifurcated. Bifurcated
contacts have better wiping and cleaning action than
smooth contacts and, therefore, are used on dry circuits.
There are various combinations of contact springs
making up the circuits that are operated by the action of
the relay. Typical spring piles are shown in Fig. 13-9.
As contacts close, the initial resistance is relatively
high, and any films, oxides, and so on further increase
the contact resistance. Upon closing, current begins to
flow across the rough surface of the contacts, heating
and softening them until the entire contact is mating,
which reduces the contact resistance to milliohms.
When the current through the circuit is too low to heat
and soften the contacts, gold contacts should be used
since the contacts do not oxidize and, therefore, have
low contact resistance. On the other hand, gold should
not be used in power circuits where current is flowing.
The contact current specified is the maximum
current, often the make-or-break current. For instance,
the make current of a motor or capacitor may be 10–15
times as high as its steady-state operation. Silver
cadmium oxide contacts are very common for this type
of load. The contact voltage specified is the maximum
voltage allowed during arcing during break. The break
voltage of an inductor can be 50 times the steady-state
voltage of the circuit.
To protect the relay contacts from high transient
voltages, arc suppression should be used. For dc loads,
this may be in the form of a reverse-biased diode (recti-
fier), variable resistor (varistor), or RC network, as
shown in Fig. 13-10.
The R and C in an RC circuit are calculated with the
following equations:(13-14)VSWR= 1 +U e 1 – UC I210----- - μF=