A varistor is an electronics component with an electrical resistance that varies with applied voltage. Also known as a voltage-dependent resistor (VDR), it has a non-linear, non-ohmic current characteristic that is similar to a diode. Unlike a diode, however, it has the same characteristic for both directions of current flow. At low voltage, it has a high electrical resistance, which decreases with increasing voltage.
Varistors are used as control or circuit compensators to ensure optimal operating conditions or to protect against excessive transient stresses. When used as protection devices, they isolate the current caused by the excessive voltage of the sensitive components when they are triggered.
The voltage drop describes how the power of a voltage source is reduced which is reduced because the electric current moves through the passive elements (non-supplying elements) to an electrical circuit.
The dropping voltage of the internal resistances of the source, between conductors, between contacts and between connectors is undesirable because the supplied power is lost (dissipated). Voltage drops on loads and other active circuit elements are desirable as the supplied energy is useful.
For example, an electric heater can have a ten ohm resistance, and the wires that supply it can have a 0.2 ohm resistor, about 2% of the total resistance of the circuit. This means that approximately 2% of the supplied voltage is lost in the wires itself. An excessive voltage drop can result in unsatisfactory operation and damage to electrical and electronic equipment.
National and local electrical codes may set guidelines for the maximum allowable voltage drop in electrical wiring to ensure distribution efficiency and proper operation of electrical equipment. The maximum allowable voltage drops from one country to another. In electronic design and power transmission, different techniques are employed to compensate for the effect of voltage drops on long circuits or where voltage levels need to be preserved.
The simplest way to reduce the voltage drop is to increase the diameter of the conductor between the source and the load, which leads to a decrease in total resistance. In power distribution systems, a certain amount of energy can be transmitted with a lower voltage drop if a higher voltage is used. More sophisticated techniques use active elements to compensate for the drop in unwanted voltage.