The maximum value for Rp is usually made equal to the load resistance R
L
.
Therefore, the limits on R
p can be stated as:
V
I
max
< R
p
< R
L
Note that the actual value of the current (Io) in a circuit is determined
by the equation:
I
o
=
V
R
L
Where V is the peak value of the source voltage and R
L
is the resistance
of the load. The value for I
o will be used to determine the value of the
protection capacitor (C
p).
In determining the value of the protection network capacitor (C
p), there
are several things that you will need to consider. First, the total circuit
capacitance (C
tot
) must be such that the peak voltage across the open
relay contacts does not exceed 300 Vrms. The equation for determining
the minimum allowable circuit capacitance is:
C
tot
≥ (I
o
/ 300)
2
x L
where L is the inductance of the load and Io is the current value
calculated earlier.
The total circuit capacitance (C) is actually made up of the wiring
capacitance plus the value of the protection network capacitor C
p.
Therefore, the minimum value for C
p should be the value obtained for
the total circuit capacitance (C). Note that the actual value used for C
p
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 (Cp) is
too large.
Chapter 8 Tutorial
Actuators and General-Purpose Switching
386
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