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Varistor

Zinc Oxide Varistor can offer different diameters like 05D, 07D, 10D, 14D and 20D. You can make easy choice upon what you need to protect the circuit.

Common packing is Bulk, Ammo and Tape & Reel Packing are also available for some items upon special request.

Varistor List

Series SCV
Dimension D05, D07, D10, D14, D20
Varistor voltage 18V ~ 1800V
Tolerance ±10%
Packing Bulk, Ammo, T & R
Photo Zinc Oxide Varistor

About Varistors

Varistors are nonlinear two-electrode semiconductor voltage-dependant resistors. The current in a varistor is proportional to applied voltage raised to a power. These devices are normally made of zinc oxide. Upon application of a high voltage pulse (such as lighting) they conduct a large current, thereby absorbing the pulse energy in the bulk of the material with only a relatively small increase in voltage, thereby protecting the circuit.

Important physical specifications to consider when searching for varistors include mounting options, lead types, and diameter. Mounting options include through hole and surface mount (SMT/SMD). Through hole varistors connect to a printed circuit board by inserting a terminal or lead through a hole in the board and soldering it to the opposite side. Surface mount components are a direct response to cost reduction efforts that center around improved circuit board production. Automatic or robotic pick and place equipment can pick up and place surface mount style components on a printed circuit board faster and more accurately than previous technology would allow. Instead of a pin or terminal passing through a printed circuit board and being soldered on the opposite side, surface mount components utilize a flat solderable surface that is soldered to a flat solderable pad on the face of the printed circuit board. The pad on the circuit board is usually coated with a paste like formulation of solder and flux. With careful placement, surface mount style components on solder paste will stay in position until elevated temperatures, usually from an infrared oven, melt the solder paste and solder the mount's flat terminals to the circuit board's pad. Lead types include axial leads, radial leads, and no leads (SMT). The diameter of the varistor is an important dimension to consider.

Performance specifications that should be considered for varistors include maximum AC RMS voltage, maximum clamping voltage, and operating temperature. The maximum RMS voltage is the maximum continuous sinusoidal RMS voltage that may be applied. The maximum clamping voltage is the peak voltage across the varistor measured under conditions of a specified peak pulse current and specified waveform.

Semec Varistor application

Our Varistor voltage is from 18V to 1800V, wide voltage make it with wide area of application:

  1. Power Supply
  2. Cable TV system
  3. Motor Control System
  4. Communication Equipment
  5. Smoke Detector
  6. Microprocessor protection
  7. Consumer Electronics
  8. Measuring equipment
  9. Electronic home appliance etc.

And we have different dimenion sizes for choice, range from 05D to 20D, including 05D, 07D, 10D, 14D and 20D.

Varistor Information

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.

A varistor is an electronic component with a significant non-ohmic 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.

Metal oxide varistor

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.

For example, follow-through current as a result of a lightning strike may generate excessive current that permanently damages a varistor. In general, the primary case of varistor breakdown is localized heating caused as an effect of thermal runaway. This is due to a lack of conformality in individual grain-boundary junctions, which leads to the failure of dominant current paths under thermal stress.

Varistors can absorb part of a surge. How much effect this has on risk to connected equipment depends on the equipment and details of the selected varistor. Varistors do not absorb a significant percentage of a lightning strike, as energy that must be conducted elsewhere is many orders of magnitude greater than what is absorbed by the small device.

A varistor remains non-conductive as a shunt mode device during normal operation when voltage remains well below its "clamping voltage". If a transient pulse (often measured in joules) is too high, the device may melt, burn, vaporize, or otherwise be damaged or destroyed. This (catastrophic) failure occurs when "Absolute Maximum Ratings" in manufacturer's datasheet are significantly exceeded. Varistor degradation is defined by manufacturer's life expectancy charts using curves that relate current, time, and number of transient pulses. A varistor fully degrades typically when its "clamping voltage" has changed by 10%. A fully degraded varistor remains functional (no catastrophic failure) and is not visibly damaged.

Ballpark number for varistor life expectancy is its energy rating. As MOV joules increase, the number of transient pulses increases and the "clamping voltage" during each transient decreases. The purpose of this shunt mode device is to divert a transient so that pulse energy will be dissipated elsewhere. Some energy is also absorbed by the varistor because a varistor is not a perfect conductor. Less energy is absorbed by a varistor, the varistor is more conductive, and its life expectancy increases exponentially as varistor energy rating is increased. Catastrophic failure can be avoided by significantly increasing varistor energy ratings either by using a varistor of higher joules or by connecting more of these shunt mode devices in parallel.

Important parameters are a varistor's energy rating (in joules), response time (how long it takes the varistor to break down), maximum current and a well-defined breakdown (clamping) voltage. Energy rating is often defined using 'industry standard' transients such as 8/20 microseconds or 10/1000 microseconds. MOVs are intended for shunting short duration pulses. For example, 8 microseconds is a transient's rise time; 20 microseconds is the fall time.

To protect communications lines (such as telephone lines) transient suppression devices such as 3 mil carbon blocks (IEEE C62.32), ultra-low capacitance varistors or avalanche diodes are used. For higher frequencies such as radio communication equipment, a gas discharge tube (GDT) may be utilized.

A typical surge protector power strip is built using MOVs. A cheapest kind may use just one varistor, from hot (live, active) to neutral. A better protector would contain at least three varistors; one across each of the three pairs of conductors (hot-neutral, hot-ground, neutral-ground). A power strip protector in the United States should have a UL1449 2nd edition approval so that catastrophic MOV failure would not create a fire hazard.

What varistors don't do

Some consumers assume that a MOV inside a TVSS device provides equipment with complete power protection. Unfortunately, an MOV device and other types of surge suppressors 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 potential fire hazard also exists.

A varistor provides no equipment protection from inrush current surges (during equipment startup), from overcurrent (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.