Voltage dependent resistors (VDRs) are electronic components that have a nonlinear resistance that depends on the applied voltage. They are commonly used to protect electronic circuits from voltage surges and spikes. VDRs are also known as Varistors.

How VDRs Work:
When a low voltage is applied across a VDR, it behaves like a high resistance. As the voltage across the VDR increases, its resistance decreases. This nonlinear resistance characteristic makes VDRs useful in suppressing high voltage surges and spikes.

Applications of VDRs:
VDRs are used in a wide range of electronic applications, including:

1. Surge suppression: VDRs are commonly used to protect electronic equipment from voltage surges and spikes. They are connected in parallel with the equipment and act as a shunt to divert the surge current away from the equipment.

2. Overvoltage protection: VDRs are used to protect sensitive electronic components from overvoltage conditions. They are connected in series with the component and act as a voltage divider to limit the voltage across the component.

3. Power supply filtering: VDRs are used in power supplies to filter out high frequency noise and spikes. They are connected in parallel with the power supply and act as a shunt to divert the noise away from the load.

4. Transient suppression: VDRs are used to suppress transients in high-speed digital circuits. They are connected in parallel with the circuit and act as a shunt to absorb the transient energy.

Conclusion:
Voltage dependent resistors (VDRs) are important components in electronic circuits that protect equipment from voltage surges and spikes. They are widely used in a range of applications, including surge suppression, overvoltage protection, power supply filtering, and transient suppression.