Device identification in electronics: What is a varistor commonly defined as in circuit protection and voltage-clamping applications?

Introduction / Context:
A varistor is widely used to protect circuits from voltage transients such as surges and spikes. Recognizing its basic definition helps technicians select appropriate surge protection (e.g., MOVs across mains lines, relay coils, or DC rails) and understand how clamping behavior differs from that of zener diodes and transient-voltage-suppression diodes.

Given Data / Assumptions:

• We are classifying a varistor as a component type.
• General engineering usage (e.g., metal-oxide varistors, MOVs).
• Focus is on basic definition rather than construction details.

Concept / Approach:
A varistor (variable resistor) exhibits a nonlinear, voltage-dependent resistance. At normal line or signal voltages, its resistance is very high and current is negligible. When a transient pushes the voltage above its knee, the device’s resistance drops dramatically, clamping the voltage by conducting surge current and thereby protecting sensitive electronics downstream.

Step-by-Step Solution (behavioral view):
At nominal voltage: R_varistor is very large → negligible current.At surge voltage: R_varistor collapses → current diverts through the varistor.Voltage at the protected node is held near the clamping level.After the surge: voltage returns to nominal → varistor returns to high resistance.

Verification / Alternative check:
Datasheets show V–I curves with a pronounced knee: current rises steeply past the clamping region. This is characteristic of voltage-dependent resistance, not a diode’s unidirectional conduction curve.

Why Other Options Are Wrong:
Voltage-dependent diode: describes zener/TVS diodes, which conduct in a defined direction and mechanism.Current-dependent resistor/diode: does not match the primary varistor control variable, which is voltage.

Common Pitfalls:
Confusing varistors with zeners; both clamp voltage, but varistors are bidirectional (symmetrical V–I), while zeners are diodes with polarity and specific breakdown behaviors.

Final Answer:
a voltage-dependent resistor