Keysight 34972A - Using Varistors

Tutorial 7
Keysight 34970A/34972A User’s Guide 323
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
<<
I
o
V
R
L
----=
C
tot
I
o
300⁄()
2
L×≥