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.
If the size of the transient pulse (often measured in joules) is too high, the device may melt, burn, vaporize, or otherwise be damaged or destroyed. For example, a nearby lightning strike which generates a conducted surge 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 varistors are used to protect communications lines (such as phone lines used for modems), their capacitance is also important because high capacitance would absorb high-frequency signals, thereby reducing the available bandwidth of the line being protected.
A typical surge protector power strip is built with MOVs. The cheapest kind may use just one varistor, from hot to neutral. A quality protector should contain at least three varistors, one across each of the three pairs of conductors (hot-neutral, hot-ground, neutral-ground).
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)
- 360VDC max clamp @ 5 Amp
- 400Amp max transient surge
- 180VDC max continuous
- 140VAC RMS max continuous
- 0.2W avg power dissipation
14-page datasheet for ZA Varistor Series
Note: 120VAC power line has a nominal peak voltage of 170VDC.
[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 | >500 pF | 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 |
Another method for suppressing voltage spikes is the transient voltage suppression diode (TVS). Although these diodes do not have as much capacity to handle large surges as MOVs, they have the advantage that they are not degraded by smaller surges. MOVs do suffer degradation from repeated exposure to surges. Both types are available over a wide range of voltages. MOVs tend to be more suitable for higher voltages, because they can handle the higher associated energies.
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
