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308
Chapter 7 Tutorial
Actuators and General-Purpose Switching
The maximum value for R
p
is usually made equal to the load resistance
R
L
. Therefore, the limits on R
p
can be stated as:
Note that the actual value of the current (I
o
) in a circuit is determined by
the equation:
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:
where L is the inductance of the load and I
o
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 (C
p
) is
too large.
V
I
max
-----------
R
p
R
L
 
Io
V
R
L
------- =
C
tot
I
o
300
  
2
L
 
34970A Refresh UG.book Page 308 Wednesday, February 17, 2010 12:34 PM