Varistor
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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. 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.
*Note: only non-ohmic variable resistors are usually called varistors. Other, ohmic types of variable resistor include the potentiometer and the rheostat.
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[edit] 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 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.
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.
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 unacceptable (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 visually damaged.
Ballpark number for varistor life expectancy is its energy rating. As MOV joules increase, then number of transient pulses increase and "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 is 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 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.
Specifications
The ZA Varistor Series has a Wide Operating Voltage Range VM(AC)RMS of 4V to 460V, DC Voltage Ratings of 5.5V to 615V.
Typical parameters for a V220ZA05 Metal oxide varistor:
- 5mm dia disc
- 220VDC nominal (198-253V @ 1mA)
- 6 joules for a 10/1000 microsecond pulse
- 360VDC max clamp @ 5 Amp
- 400Amp max transient surge
- 180VDC max continuous
- 140VAC RMS max continuous
- 0.2W avg power dissipation
- 90 pf capacitance
14-page datasheet for ZA Varistor Series
Note: 120VAC power line has a nominal peak voltage of 170VDC and can be as high as 185V peak.
[edit] Varistors compared to other transient-suppressors
| Type | Surge capability (typical) | Lifetime - number of surges | Response time | Shunt capacitance | Leakage current (approximate) |
|---|---|---|---|---|---|
| Metal-oxide varistor (MOV) | Up to 70,000 Amps | @ 100 Amps, 8x20 uS pulse shape: 1000 surges | Sub-nanosecond++ | >= 0.6 pF (low cap version) | 10 microamps |
| Avalanche diode | 50 Amps | @ 50 Amps, 8x20 uS pulse shape: infinite | Sub-nanosecond | 50 pF | 10 microamps |
| Gas tube | > 20,000 Amps | @ 500 Amps, 8x20 uS pulse width: 200 surges | < 5 microseconds | < 1 pF | picoamps |
++ The response time of the MOV is largely ambiguous, as no standard has been officially defined. The sub-nanosecond MOV response claim is based on a transient having an 8 microsecond rise-time, thereby allowing ample time for the device to slowly turn-on. When subjected to a very fast, <1 ns rise-time transient, response times for the MOV are in the 40-60 ns range.
Another method for suppressing voltage spikes is the transient voltage suppression diode (TVS). Although diodes do not have as much capacity to conduct large surges as MOVs, diodes are not degraded by smaller surges and can be implemented with a lower "clamping voltages". MOVs degrade from repeated exposure to surges and generally must have a higher "clamping voltage" so that leakage does not degrade the MOV. Both types are available over a wide range of voltages. MOVs tend to be more suitable for higher voltages, because they can conduct the higher associated energies at less cost.
Another type of transient suppressor is the gas tube suppressor. This is a type of spark gap that may use air or an inert gas mixture and, often, a small amount of radioactive material, such as Ni-63, to provide a more consistent breakdown voltage and reduce response time. Unfortunately, these devices may have higher breakdown voltages and longer response times than varistors. However, they can handle significantly higher fault currents and withstand multiple high-voltage hits (for example, from lightning) without significant degradation.
[edit] References
http://ieeexplore.ieee.org/search/wrapper.jsp?arnumber=1350537 Capacitance changes in degraded metal oxide varistors Jaroszewski, M.; Wieczorek, K.; Bretuj, W.; Kostyla, P.; Solid Dielectrics, 2004. ICSD 2004. Proceedings of the 2004 IEEE International Conference on Volume 2, 5-9 July 2004 Page(s):736 - 738 Vol.2
[edit] See also
- Electronic Components
- Transient voltage suppression diode
- Transil
- Polyswitch -- a current-sensitive (rather than voltage-sensitive) device
