They are nonlinear resistors and can be used in circuit protection and many other applications such as power supplies, microprocessor protection, surge protection for consumer and industrial electronics, and many other applications.
The VDR is a device that becomes a conductor when the voltage applied to it crosses a determined value.
Symbols and types
The symbols and examples of common VDRs are shown in Figure 1.
The size of the VDR determines the amount of energy it can absorb when becoming conductor.
Specifications
The main specification of a VDR is the breakdown voltage.
It is the voltage at which the device becomes conductive, passing from a high-resistance state to a very low-resistance state.
This component can be found with voltage specifications between 18 and 1800 volts.
Where they are found and how they are used
The reader will find VDRs at the input of many electronic devices operating as a protective element. In fact, when voltage spikes appear in the AC power line, if their value is high enough to make the VDR conductive, the snap action of this component puts a short in the spike.
So, the high voltage that could damage the equipment is shunted to the ground, or the energy of the spike absorbed by the VDR.
Computer outlet sockets with surge protection use this device as a main element in the protection circuit.
Outlets for computers with surge protection such as the one shown in Figure 2 use VDRs as protection elements.
Testing
The effect of the absorption of the energy spikes coming from an AC power line or telephone line makes the VDR dissipate their energy as heat.
This effect is cumulative, and after a lag in time of use, the VDR becomes conductive independently from the input voltage acting as a short.
In this case, it must be replaced.
The replacement type must have the same voltage as the original and be the same size.
The multimeter cam be used for testing.
A good VDR presents a high resistace above a predetermined hundreds thousand megΩ.
