ITT - Index of

SELENIUM CONTACT PROTECTORS
CAUSE OF CONTACT ARCING
Consider the circuit shown in Figure
1. Close the circuit contacts SI
and the current flow as indicated
by the ammeter MI builds up to
400 mAo The flow of current has
set up a magnetic field through
the core. Electrical energy has
been stored in the magnetic field
by the current, analogous to the
mechanical energy stored in a
spring by compression.
Now open the contacts SI thus
breaking the steady flow of current.
Just as the compressed
spring will start to release its
stored mechanical energy, the
magnetic field starts to collapse,
releasing its stored energy. While
the magnetic field is collapsing it
induces a voltage in the coil that
acts to maintain the current
through the coil in the same
direction as the original current
which built up the field. The magnitude
of this induced voltage is
eq ua I to the prod uct of the co iI
current, and the coil and circuit
resistance; and it can attain very
high values since the circuit resistance
approaches infinity with
the switch contacts open.
Theoretically, when the flow of
current is suddenly stopped by the
opening of Slo the voltage that
appears across the coil could go
up to infinity. Approximately 300
volts is required to produce arcing
across contacts in free air. Since
air is not a perfect insulator, the
instant that SI is opened and the
high induced voltage appears
across the contacts, an arc occurs.
In typical applications, air breakdown
across the contacts will limit
the voltage rise to the regIon of
300 to 1,000 volts in an average
24-volt coil circuit. See Figure 2.
CONTACTS
FIG.1
FIG.2
IRON}
CORE
WIRE
COIL
R=60 OHMS
HOW ITT SELENIUM CONTACT PROTECTORS WORK
II: 400MA
FIG. J
Half-wave configuration for
DC applications
Consider the circuit of Figure 3,
showing contact protector cell A
connected across the coil terminals.
While contacts SI are held
closed a current of 400 milliamperes
flows through the coil, but
no part of the coil current flows
through the contact protector.
When SI is opened coil current
flows in the low-resistance direction
of the contact protector, thus
dissipating the energy stored in Half-wave configuration, continued
the coil through the resistance of
the coil and contact protector. As In the circuit shown in Figure 4,
the resistance of the contact pro-. cell B has been added in such a
tector is very low, usually a frac- direction as to oppose the distion
of the coil resistance, the charge current I". A semicond.uctor
voltage across the contacts rises cell has the unique characteristic
only. slightly, to a few volts above of exhibiting decreasing resistbattery
voltage.
12-4
ance with increasing voltage in
the blocking direction.
I At the instant switch SI is opened,
the induced voltage rises to approximately
150 volts, due to the
blocking action of cell B. This induced
voltage is indicated in Figure
4 and appears as a reverse
voltage across cell B, i.e., the h ighresistance
direction.
1
----. II =400MA
r4V.
~I
FIG.4
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