Analyzing circuits with varistors for overvoltage protection involves understanding the behavior of varistors and their role in protecting electronic components from excessive voltage. Varistors are voltage-dependent resistors with a highly nonlinear current-voltage characteristic. They are designed to have a high resistance at normal operating voltages but become low resistance when exposed to overvoltage conditions.
Here are the general steps to analyze circuits with varistors for overvoltage protection:
Identify the varistor in the circuit: Locate the varistor component in the circuit schematic. It is usually represented as a two-terminal device, and its symbol resembles a diode or a resistor with a non-linear characteristic curve.
Obtain varistor specifications: Look up the datasheet or specifications for the varistor being used. Pay attention to the following parameters:
a. Maximum continuous voltage rating (Vc): This is the voltage up to which the varistor can operate continuously without damage.
b. Maximum clamping voltage (Vc): The voltage at which the varistor begins to conduct significantly and starts protecting the circuit.
c. Maximum peak current (Imax): The maximum current the varistor can handle without damage during an overvoltage event.
d. Varistor voltage (V1mA): The voltage at which the varistor conducts a specified current, typically 1 mA. This parameter helps understand the varistor's resistance at lower voltages.
Circuit analysis at normal operating voltage: At normal operating voltages, the varistor should exhibit a very high resistance, effectively acting as an open circuit. Analyze the circuit with the assumption that the varistor is non-conductive.
Circuit analysis during overvoltage conditions: When the voltage across the varistor exceeds its clamping voltage (Vc), it starts conducting significantly, effectively becoming a low-resistance path. Analyze the circuit with the assumption that the varistor is now conductive and clamping the voltage to a safe level.
Calculate power dissipation: During an overvoltage event, the varistor dissipates power due to the current flowing through it. To ensure that the varistor can handle this power dissipation safely, calculate the power using P = Ipeak^2 * Rv, where Ipeak is the peak current during the overvoltage event and Rv is the resistance of the varistor at that voltage (which can be derived from the datasheet's I-V curve).
Select appropriate varistor: Ensure that the selected varistor can handle the maximum peak current and power dissipation without exceeding its rated limits.
Verify protection effectiveness: Simulate the circuit or conduct tests to ensure that the varistor clamps the voltage to a safe level during overvoltage events, thereby protecting the sensitive components in the circuit.
It's important to remember that varistors have a finite lifespan, and after absorbing several surges, their performance may degrade. Regular monitoring and, if necessary, replacement of varistors in critical circuits are essential for maintaining effective overvoltage protection. Always refer to the varistor's datasheet and follow the manufacturer's guidelines for proper usage and application-specific considerations.