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Chapter 8 Tutorial
Actuators and General-Purpose Switching
The maximum value for R
Therefore, the limits on R
V
< R
< R
p
L
I
max
Note that the actual value of the current (I
by the equation:
V
I
=
o
R
L
Where V is the peak value of the source voltage and R
of the load. The value for I
protection capacitor (C
p
In determining the value of the protection network capacitor (C
are several things that you will need to consider. First, the total circuit
capacitance (C
) must be such that the peak voltage across the open
tot
relay contacts does not exceed 300 Vrms. The equation for determining
the minimum allowable circuit capacitance is:
2
C
(I
/ 300)
x L
tot
o
where L is the inductance of the load and I
calculated earlier.
The total circuit capacitance (C) is actually made up of the wiring
capacitance plus the value of the protection network capacitor C
Therefore, the minimum value for C
the total circuit capacitance (C). Note that the actual value used for C
should be substantially greater than the value calculated for C.
Using Varistors
Use a varistor to add an absolute voltage limit across the relay contacts.
Varistors are available for a wide range of voltage and clamp energy
ratings. Once the circuit reaches the voltage rating of the varistor, the
varistor's resistance declines rapidly. A varistor can supplement an RC
network and is especially useful when the required capacitance (C
too large.
386
is usually made equal to the load resistance R
p
can be stated as:
p
) in a circuit is determined
o
will be used to determine the value of the
o
).
is the current value
o
should be the value obtained for
p
.
L
is the resistance
L
), there
p
.
p
p
) is
p
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