Agilent Technologies 34970A manual Using Varistors

Chapter 8 Tutorial

Actuators and General-Purpose Switching

The maximum value for Rp is usually made equal to the load resistance RL. Therefore, the limits on Rp can be stated as:

V

Imax < Rp < RL

Note that the actual value of the current (Io) in a circuit is determined by the equation:

V

Io = R L

Where V is the peak value of the source voltage and RL is the resistance of the load. The value for Io will be used to determine the value of the protection capacitor (Cp).

In determining the value of the protection network capacitor (Cp), there are several things that you will need to consider. First, the total circuit capacitance (Ctot) 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:

Ctot ≥ (Io / 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 Cp. Therefore, the minimum value for Cp should be the value obtained for the total circuit capacitance (C). Note that the actual value used for Cp 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.

386