ITT - Index of

SELENIUM CONTACT PROTECTORS
Figure 5 shows the change in reo
verse resistance with application
of reverse voltage to a typical
sem iconductor contact - protector
cell. At the working voltage of 24
volts, the reverse resistance is approximately
1 megohm, When S,
is opened, the voltage across the
coil b.uilds up to approximately
-200 volts, At this voltage the reverse
resistance of cell B will be
less than 500 ohms, Much of the
stored energy is dissipated in this
low-resistance region of the cell
c ha racteri stic.
As the energy is dissipated and
the current decreases, the induced
voltage also decreases, The remainder
of the stored energy is
d i ssi pated ina h igher-resista nce
region than initially. For example,
as the Induced voltage decays to
-25 volts, the cell resistance rises
to approximately one megohm.
Thus the effect of the nonlinear
resistance of cell B is to provide,
at the instant of switching, a path
of relatively low resistance to prevent
the induced voltage from rising
to a value which will cause
arcing. As the stored energy is
dissipated the low resistance in-
creases, due to the nature of the
cell characteristics, and provides
a damping effect on the discharge
current. The result is to reduce
the coil release time as compared
to the action of the half-wave-type
contact protector,
Figure 6. Oscillograph trace voltage
across a set of contacts breaking
the circuit of a 48-volt telephone
relay, without contact protection.
Sparking tlegins at 300
volts (see oscillations), continues
as contacts are separated and
rises to a voltage peak at 500 volts.
Voltage decay is exponential.
Figure 7. Oscillograph trace of
voltage across same set of contacts,
protected by ITT half-wave
semiconductor contact protector.
Figure 8. Oscillograph trace of
voltage across same set of contacts,
protected by ITT back-toback
semiconductor contact protector.
10·
J
10' L
I
I
10·
10'
L V
L
10'
-150 -100 - ,0
vOLTAGE
FIG.5
/
~
FFRr=++=1
[E~
o 10 20 a 10 20
TIME MILLISECONDS
FIG.6 FIG.7 FIG.8
Back-to-back configuration for
AC applications
The back-to-back configuration is
readily adapted to use in AC circuits
by providing the same number
of cells in each arm of the
device. The number of cells in
series per arm will depend on the
applied AC voltage. The requirement
for the same number of cells
in each arm arises from the fact
that both arms alternately are required
to block the input voltage.
This arrangement differs from the
DC back-to-back configuration in
which one arm has sufficient cells
to block the battery voltage and
the other has only one or two cells
to provide the surge suppression.
Factors relating to the opening
of inductive circuits
It can be shown mathematically
that the time required for the coil
current to drop to zero, after the
switch contacts have opened, is
inversely proportional to the sum
of the coil resistance and protector
resistance. Also, the magnitude
of the voltage induced in the
coil at the instant of switching is
directly proportional to the sum
of these resistances.
When the circuit energizing an inductance
is opened the polarity
of the voltage across the coil is
reversed and is indicated by the
equation E = - L9i
dt
After the circuit opens, and assuming
no arcing occurs, the time
for the cu rrent to fa II to a percentage
of its initial value is given by
the following equation:
t=~log~
RL+r EI
When R = Coil resistance
r = Forward resistance of
rectifier
L = Indudance of coil
t = Time for current to
decrease to i
i = Current at time t
I = Steady-state current
through the coil
E = Supply voltage
I
12-5