VDR Silicon Carbide Varistors

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General Product Information

Silicon Carbide Varistors are voltage dependent resistors with symmetric voltage/current characteristics and are therefore suitable for use with direct and alternating current.

In case of voltage increase, the resistance value of these voltage dependent resistors gets inertia-free and exponentially lower, i.e. a slight increase in voltage effects a much higher increase in current.

This voltage dependence is represented by the nonlinear exponent n. The ideal varistor has the following current I/voltage U ratio.

U = B ^. I ^1/n

with B: Voltage for 1 A (so called B-Wert)

n: Exponent

If the varistor characteristic is plotted in a log-log U / I characteristic diagram, 1/n is the slope of the curve. n = 1 applies to a voltage independent ohmic resistor.

Voltage dependences rises as n increases. The exponent can be determined with two pairs of values (U / I) :

n = [l_g (I₁ / I₂)]  /  [Ig (U₁ /U₂)]

Silicon carbide varistors have good voltage dependencies and high energy absorption. They are manufactured mainly as discs with varying sizes, as the required electric values are most easily obtained with these geometric dimensions.

Silicon carbide varistors may be connected in series or parallel for various fields of application so that there is sufficient overvoltage protection for high power.

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Composition

Silicon carbide varistors are composed of approx. 90% silicon carbide of various grain and approx. 10% ceramic binder and additives.

After shaping, the varistors are sintered at high temperatures in specific ambient conditions. The electrical contact is manufactured by spraying on a layer of brass. The electrical connection for large discs is by means of clamps, while small discs have soft soldered tinned copper wires.

Discs with wire connections are laquered, but large discs can be supplied with an epoxy resin coating.

Silicon carbide varistors are extremely stable owing to the ceramic bonding.

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Application

Electrical systems and particularly electrical switchgear are seriously endangered by overvoltage pulses. This overvoltage may destroy or impair the function of parts of the system and construction components. This could cause a breakdown of the electrical system or, in cases of electronic circuits, a false instruction cycle with grave consequences.

Silicon carbide varistors offer reliable and economical protection against internal overvoltage (i.e. arising within the system or appliance itself) and external overvoltage (i.e. entering the system or appliance from outside).

Silicon carbide varistors are used as protection against:

• Overvoltage in the mains, caused by lightning, inductive or capacitive coupling
• Consumption in switch contacts with inductive load
• Excessive switch-off voltages in transformers, motor and generator windings and magnet coils

Fields of Application

The properties of Silicon carbide varistors, including high energy absorption, short reaction time, a symmetric characteristic curve and high power dissipation makes them suitable for use in a wide variety of applications, including:

• Communication Engineering
• Power Engineering
• Automation Technology
• Consumer Electronics
• Remote Control Technology