Heatsinks and Relays 379tromechanical relay contacts and destroy solid-state
relays.13.2.3.1 The Effects of Various LoadsIncandescent Lamps. The cold resistance of a tung-
sten-filament lamp is extremely low, resulting in in-rushcurrents as much as 15 times the steady-state current.
This is why lamp burnout almost always occurs during
turn on.Capacitive Loads. The initial charging current to a
capacitive circuit can be extremely high, since the
capacitor acts as a short circuit, and current is limited
only by the circuit resistance. Capacitive loads may be
long transmission lines, filters for electromagnetic inter-
ference (emi) elimination, and power supplies.Motor Loads. High in-rush current is drawn by most
motors, because at standstill their input impedance is
very low. This is particularly bad when aggravated by
contact bounce causing several high-current makes and
breaks before final closure. When the motor rotates, it
develops an internal back emf that reduces the current.
Depending on the mechanical load, the starting time
may be very long and produce a relay-damaging in-rush
current.Inductive Loads. In-rush current is limited by induc-
tance; however, when turned off, energy stored in
magnetic fields must be dissipated.Dc Loads. These are harder to turn off than ac loads
because the voltage never passes through zero. When
electromagnetic radiation (emr) contacts open, an arc is
struck that may be sustained by the applied voltage,
burning contacts.13.2.4 Electromechanical RelaysRegardless of whether the relay operates on ac or dc, it
will consist of an actuating coil, a core, an armature, and
a group of contact springs that are connected to the
circuit or circuits to be controlled. Associated with the
armature are mechanical adjustments and springs. TheFigure 13-10. Methods of suppressing transients across
contacts. Courtesy Magnecraft Electric Co.
A. Resistor capacitor network (Use C or C[']
as preferred), ac/dc.B. Resistor, ac/dc.I. Capacitor-diode-resistor
for ac suppression.D. Diode and Zener, dc. E. Diode and Resistor, dc.F. Varistor, ac. G. Resistor-capacitor-diode
network, dc.H. Back-to-back diode
(zener or avalanche), ac.C. Diodes, dc.Relay
contactLoad
R C¾ CRelay
contactRelay
contactRelay
contactRelay
contactRelay
contactRelay
contactRelay
contactRelay
contactLoad
RLoadLoad LoadLoadLoad
R
VARLoadLoadRFigure 13-11. High in-rush current on turn-on can damage
relays.CurrentPeak current can
reach 15 × normalIncandescent lampsCurrentMax in-rush
current = V/RTime
Capacitor loadIn-rush current
3 to 6 × running currentInduction motor loadStarting Running