In the context of a PCB (Printed Circuit Board), VDR stands for Voltage-Dependent Resistor, which is more commonly known as a varistor.
Understanding VDRs (Varistors)
As the name suggests, a Voltage-Dependent Resistor is an electronic component whose electrical resistance changes significantly depending on the voltage applied across it. This unique characteristic makes it highly effective for protecting electronic circuits.
Function and Characteristics
Based on the provided reference, a varistor (a.k.a. voltage-dependent resistor (VDR)) is a surge protecting electronic component with an electrical resistance that varies with the applied voltage. It has a nonlinear, non-ohmic current–voltage characteristic that is similar to that of a diode.
Key characteristics include:
- Surge Protection: Its primary function is to protect downstream components from voltage spikes (transients or surges).
- Variable Resistance: At normal operating voltages, the VDR has a very high resistance, acting almost like an open circuit. When the voltage exceeds a specific threshold (called the clamping voltage), its resistance drops dramatically, allowing the surge current to be diverted away from sensitive components, typically to ground.
- Non-linear Behavior: Unlike standard resistors (which follow Ohm's Law), the relationship between current and voltage in a VDR is non-linear and non-ohmic.
VDRs on a PCB
On a PCB, VDRs are typically placed across power supply lines or data lines near connection points where voltage surges might enter the board. Their role is critical in enhancing the reliability and lifespan of electronic devices by absorbing or diverting excessive energy from sources like lightning strikes, power line fluctuations, or inductive load switching.
Common applications on PCBs include protection for:
- AC and DC power inputs
- Data and communication lines
- Sensitive integrated circuits (ICs)
By quickly changing resistance from high to low during a voltage surge, the VDR effectively clamps the voltage seen by the protected circuit to a safe level, sacrificing itself if the surge is too large or enduring repeated smaller surges.
