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A varistor is an electronic component with a significant non-ohmic current-voltage characteristic. The name is a portmanteau of variable resistor

SMD resistors are manufactured by a number of different companies. Accordingly the specifications vary from one manufacturer to the next. It is therefore necessary to look at the manufacturers rating for a specific SMD resistor before deciding upon exactly what is required. However it is possible to make some generalisations about the ratings that might be anticipated.

Power rating: The power rating needs careful consideration in any design. For designs using SMD’s the levels of power that can be dissipated are smaller than those for circuits using wire ended components. As a guide typical power ratings for some of the more popular SMD resistor sizes are given below. These can only be taken as a guide because they may vary.

• 1206: 1/8 watt
• 0805: 1/10 watt
• 0603: 1/16 watt

Some manufacturers will quote higher power levels than these. The figures given are conservative.

Tolerance: In view of the fact that SMD resistors are manufactured using metal oxide film they available in relative close tolerance values. Normally 5%, 2%, and 1% are widely available. For specialist applications 0.5% and 0.1% values may be obtained.

Temperature coefficient: Again the use of metal oxide film enables these SMD resistors to provide a good temperature coefficient. Values of 25, 50 and 100 ppm / C are available.

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A MOV inside a TVSS device does not provide equipment with complete power protection. In particular, MOV device provide no protection for the connected equipment from sustained over-voltages that may result in damage to that equipment as well as to the protector device.

A varistor provides no equipment protection from inrush current surges (during equipment startup), from over current (created by a short circuit), or from voltage sags (also known as a brownout). A varistor neither senses nor controls such events. Susceptibility of electronic equipment to these other power disturbances is defined by equipment design. Protection from these power disturbances is installed inside that equipment or is provided by other external devices such as an UPS, some voltage regulators and Surge Protectors with built in over voltage protection that make use of a voltage sensing circuit and a relay for disconnecting the AC input when voltage reaches a danger threshold.

Suntan Technology Company Limited
---All Kinds of Capacitors

Varistor is one kinds of transient voltage suppression devices which can be used instead of transient suppression diodes, zener diode and capacitor combination. Varistor IC and other devices can protect the circuit, to prevent electrostatic discharge, surges and other transient current (such as lightning, etc.) the damage caused by them. Simply use varistors and connected to the protected circuit IC or device, when the voltage value above a certain moment, the varistor resistance decreased rapidly, conducting high current, thereby protecting the IC or electrical equipment; when the voltage is lower than the varistor voltage value, the varistor resistance high, almost open, and thus will not affect the device or the normal operation of electrical equipment.

The most common type of varistor is the Metal Oxide Varistor (MOV). This contains a ceramic mass of zinc oxide grains, in a matrix of other metal oxides (such as small amounts of bismuth, cobalt, manganese) sandwiched between two metal plates (the electrodes). The boundary between each grain and its neighbour forms a diode junction, which allows current to flow in only one direction. The mass of randomly oriented grains is electrically equivalent to a network of back-to-back diode pairs, each pair in parallel with many other pairs. When a small or moderate voltage is applied across the electrodes, only a tiny current flows, caused by reverse leakage through the diode junctions. When a large voltage is applied, the diode junction breaks down due to a combination of thermionic emission and electron tunneling, and a large current flows. The result of this behaviour is a highly nonlinear current-voltage characteristic, in which the MOV has a high resistance at low voltages and a low resistance at high voltages.

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A varistor is an electronic component with a "diode-like" nonlinear current–voltage characteristic. The name is a portmanteau of variable resistor. Varistors 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. A varistor is also known as Voltage Dependent Resistor or VDR. A varistor’s function is to conduct significantly increased current when voltage is excessive

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Suntan Technology Company Limited
---All kinds of Capacitors

A varistor is a type of resistor with a significantly non-ohmic current-voltage characteristic. The name is a portmanteau of variable resistor*, which is misleading since it is not continuously user-variable like a potentiometer or rheostat, and is not a resistor but in fact a capacitor. Varistors are often used to protect circuits against excessive voltage by acting as a spark gap.

The most common type of varistor is the metal oxide varistor, or MOV. This contains a mass of zinc oxide grains, in a matrix of other metal oxides, sandwiched between two metal plates (the electrodes). The boundary between each grain and its neighbour forms a diode junction, which allows current to flow in only one direction. The mass of randomly oriented grains is electrically equivalent to a network of back-to-back diode pairs, each pair in parallel with many other pairs. When a small or moderate voltage is applied across the electrodes, only a tiny current flows, causes by reverse leakage through the diode junctions. When a large voltage is applied, the diode junctions break down because of the avalanche effect, and a large current flows. The result of this behaviour is a highly nonlinear current-voltage characteristic, in which the MOV has a high resistance at low voltages and a low resistance at high voltages.

If the size of the transient pulse (often measured in joules) is too high, the device may melt, or otherwise be damaged. For example, a nearby lightning strike may permanently damage a varistor.

Important parameters for varistors are response time (how long it takes the varistor to break down), maximum current and a well-defined breakdown voltage. When used in communications lines (such as phone lines used for modems), high capacitance is undesirable since it absorbs high frequency signals, thereby reducing the available bandwidth of the line being protected.