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Metal Oxide Varistor (MOV)
--------------------------
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
Contents:
1) Metal oxide varistor
2) Varistors compared to other transient-suppressors
1) 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 isn't 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
isn't 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 UL 1449 2nd edition approval so that
catastrophic MOV failure would not create a fire hazard.
2) Potential hazards
While a MOV is designed to absorb significant amounts of power for very
short durations (~8/20 microseconds), such as caused by lightning strikes,
it typically doesn't have the capacity to handle sustained energy dissipation.
Under normal utility voltage conditions, this isn't a problem. However,
certain types of faults on the utility power grid can result in sustained
over-voltage conditions. Examples include a loss of a neutral conductor or
shorted lines on the high voltage system. Application of sustained
over-voltage to a MOV can cause high dissipation, potentially resulting in
the MOV device catching fire. The National Fire Protection Association (NFPA)
has documented many cases of catastrophic fires that have been caused by MOV
devices in surge suppressors, and have issued bulletins on the issue.
There are several issues to be noted regarding behavior of transient
voltage surge suppressors (TVSS) incorporating MOVs under over-voltage
conditions. Depending on the level of the applied voltage, the heat generated
may not be sufficient to cause failure, but may degrade the MOV device,
shortening its life. If the over-voltage is abruptly applied to the MOV, it
may explode inside the case, keeping the load connected but now without any
surge protection. Typically, the user has no indication when the surge
suppressor has failed. Under the right conditions of over-voltage and line
impedance, it may be possible to cause the MOV to burst into flames, the root
cause of many fires and the main reason for NFPA's concern. Properly designed
TVSS devices should contain the flames, eventually resulting in the operation
of a safety fuse.
It should be noted that many consumers assume that the TVSS devices,
especially the more expensive devices in the market, provide the connected
load equipment with complete power protection. Unfortunately, MOV devices or
other types of surge suppressors, provide no protection for the connected
equipment against sustained over-voltages, often resulting in damage to the
equipment as well as to the protection device itself, and causing a potential
fire hazard.
MOV devices also provide no protection for equipment against
current-inrush surges that can flow from the utility grid into the equipment,
following power disturbances such as voltage sags. Susceptibility of
electronics equipment to current-inrush at power-on is well known. However,
recent publications have shown that voltage sag events occur very frequently
on the grid, and can cause extremely high levels of current-inrush surge,
significantly in excess of safe values. This is another mechanism that can
degrade components inside the equipment, shortening its life and reliability,
and potentially resulting in equipment failure. Given the confusion in the
market in the understanding of voltage surges, current surges and power
surges, and how these are caused and how they can damage equipment, it is
important to understand the limitations of transient voltage surge
suppressors, and to remember that they don't protect against sustained
over-voltage or against current-inrush surges, all of which represent power
disturbances that can damage expensive electronic equipment.
2) 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 diameter 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
Note: 120VAC power line has a nominal peak voltage of 170VDC and can be
as high as 185V peak.
2) Varistors compared to other transient-suppressors
Type Metal-oxide Avalanche Gas tube
varistor (MOV) diode
Lifetime Up to 70,000 Amps @ 50 Amps @ 50 > 20,000 Amps
(number of surges) 100 Amps, 8x20 uS Amps, 8x20 uS @ 500 Amps, 8x20 uS
Response time Sub-nanosecond++ Sub-nanosecond < 5 microseconds
Shunt capacitance Typically 50 pF < 1 pF
100 - 1000 pF
Leakage current 10 microamps+++ 10 microamps picoamps
(approximate)
Capability pulse shape: pulse width: pulse shape:
(typical) 1000 surges 200 surges infinite
++ 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.
+++ Typical capacitances for consumer-sized (7-20mm diameter) varistors are
in the range of 100 - 1000 pF. Smaller, lower-capacitance varistors are
available with capacitances of ~1 pF for microelectronic protection,
such as in cellular phones. These low-capacitance varistors are, however,
unable to withstand large surge currents simply due to their compact
PCB size.
Another method for suppressing voltage spikes is the transient voltage
suppression diode (TVS). Although diodes don't have as much capacity to
conduct large surges as MOVs, diodes aren't 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.
Extracted from Wikipedia.com.
Compiled and translated to ASCCI by LW1DSE Osvaldo
Almirante Brown
Buenos Aires
Argentina
08/09/2007
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