A VDR is also known as Voltage Dependent Resistor or VDR. VDRs are often used to protect circuits against excessive transient voltages by incorporating them into the circuit in such a way that, when triggered, they will shunt the current created by the high voltage away from the sensitive components. The name is a portmanteau of variable resistor. (**A VDR (Voltage Dependent Resistor) is an electronic component with a “diode-like” nonlinear current–voltage characteristic. Two neon pilot lamps are also provided with the circuit diagram to show the status of input and load supply. The circuit is designed to protect sensitive electronic devices against overvoltage transients in normal mains voltage and overload/ short circuit. After the current is normal, the fuses resets and continue its functioning (protection against short circuit and overload). If the flowing current is more than the specified value of main self-resettable fuse, the fuse will break and the circuit will be protected. When the voltage again goes back to normal values G1 and VDR1, the conducting stops and the circuit remains normal. Thus the current cannot be raised much over that predetermined value. If more the voltage rises then more current starts to flow through GDT and VDR1.When the current is normal, the circuit is reset and resumes it’s functioning. When the voltage between the terminals is higher than the sum of voltage ratings of GDT and VDR1 (here both GDT UZ470B and VDR S20K250 has 250v 16A rating), current starts to flow through those components. Normally no current flows through GDT and VDR1. The protection circuit is connected between live and mains lead. The basic surge suppression circuit shown below consists of a VDR** (Voltage Dependent Resistor) and gas surge suppressor (GDT) connected in series.

Gas discharge tube lightning protection