A varistor, short for variable resistor, is an electronic component designed to protect electronic circuits from voltage spikes and transient overvoltages. It is also known as a voltage-dependent resistor (VDR) or a metal oxide varistor (MOV). The primary purpose of a varistor is to limit or divert excessive voltage away from sensitive electronic components, thus safeguarding them from damage caused by voltage surges.
The varistor's behavior is nonlinear, meaning its resistance changes with the applied voltage. Under normal operating conditions, it exhibits a relatively high resistance and has little impact on the circuit's performance. However, when a voltage spike occurs, such as during a lightning strike or sudden power surge, the varistor's resistance decreases significantly, allowing it to conduct large amounts of current.
Here's how a varistor protects electronic circuits from voltage spikes:
Response to overvoltage: When the voltage across the varistor exceeds its rated voltage, it begins to conduct heavily. The varistor acts like a valve, providing a low-resistance path for the excessive current caused by the voltage spike.
Absorption of energy: As the varistor conducts, it absorbs and dissipates the excess energy generated by the voltage surge. By doing so, it prevents the energy from reaching sensitive components in the circuit.
Voltage clamping: The varistor has a clamping voltage, which is the maximum voltage it can withstand before conducting. When the voltage spike exceeds this clamping voltage, the varistor effectively "clamps" the voltage at this level, preventing it from going any higher and keeping the circuit protected.
Fast response time: Varistors have rapid response times, reacting to voltage spikes in nanoseconds. This quick response is crucial to effectively divert the surge before it can cause damage.
It's important to note that varistors have limitations, such as a finite energy-handling capability and a maximum clamping voltage. Therefore, in some cases, additional protection components like transient voltage suppressors (TVS diodes) or gas discharge tubes may be used in conjunction with varistors to provide comprehensive protection against voltage spikes and transient overvoltages. Proper circuit design and the use of appropriate protection devices are essential to ensure the reliability and longevity of electronic systems.