A varistor or metal
oxide varistor (MOV) is a special resistor that is used to protect
circuits against high transient (short term) voltage. These surges and
spikes attacks the equipment by the power line and will destroy the
power supply of the equipment. A varistor is able to short these surges
and spikes and keep them away from the following application.
A varistor is also known as Voltage Dependent Resistor or VDR.
The short circuit voltage is printed on the housing.
Schematic of varistor.
Surges and Spikes
A power surge or a spike is an increase in voltage significantly above the standard
voltage of 230 volts. The precise definition is:
When the increase lasts 3 ns or more, it's called a surge.
When it only lasts for 1-2 ns it's called a spike.
if the surge or spike is high enough, it will damage a device or
machine. And in fact power-line surges can easily reach 6,000 volts.
if the increased voltage doesn't immediately break your machine, it may
put extra strain on the components and wearing them down over time.
|Spikes on a AC voltage.
Surge on a AC voltage.
A cause of surges and spikes on the power line is the
operation of high-power electrical devices, such as, air conditioners,
refrigerators and elevators. These high-powered equipments require a
lot of energy to switch on and off motors and compressors. This
switching creates sudden, brief demands for power, which upset the
steady voltage flow in the electrical system.
These surges and
spikes can damage electronic components, immediately or gradually and
are a common problem in most building's electrical systems.
Beside power lines also telephone lines and antenna cables are affected
by high voltage pulses caused by strokes of lightning.
It's a good idea to use surge protectors for all sophisticated electronic devices electronic equipment, such as
computers, entertainment center components and of cause biomedical equipment. A surge protector will generally extend the life of these devices..
Under normal conditions the resistance of the varistor is very high. When the
connected voltage gets higher than the specification of the varistor
the resistance immediately gets extrem low. This circumstance is used
to protect electronic applications from over-voltage. The varistors is
simply added to the power supply input. When high voltage surges and
spikes appear the varistor will short them and protect the following
Characteristic curve of a MOV.
Is the voltage low also the current is low (the resistance is high).
the voltage reaches the voltage of the varistor the current gets high
very fast (the resistor is extrem low. The connectors are short.
Varistors are a
kind of resistors but their specifications are not resistance ῼ and
wattage W. For varistors the most important specifications is the
is the voltage which short circuit the varistor. A lower clamping
voltage indicates better protection. But on the other hand the voltage
must not be that low, that smaller power changes destroy the varistor.
For 230 V mains a varistor of 275 V clamping voltage is a good choice.
absorption / dissipation
rating is given in joules and shows how much energy the varistor can
absorb. A higher number indicates greater protection. Varistors with
200 to 400 joules offer good protection, better protection is given
with devices of 600 joules or more.
For extending the energy absorption two or three varistors can put
switch fast but not immediately. There is always a very slight delay as
they respond to the power surge. The longer the response time the
longer the connected application is exposed to the surges. A response
time of 1 ns or faster is fine.
Varistor at the input of a power supply.
The varistor is simply connected between line and
neutral but after the fuse. If the varistors gets a short circuit the
fuse will blow and disconnect the main from the following application.
Simple solution for effective protection.
The original high current fuse should be replaced with one matching
with the equipment.
A better protector contains three varistors: One across each of the
three pairs of conductors (line, neutral and ground).
Varistors can be
destroyed by too many surges. They wear out a little with each surge
above the threshold and some day they are completely destroyed.
Over-voltage is also a common problem. The varistors burned but also
let the fuse blow and so save the connected equipment.
Defective varistor. Too many surges over long time destroy varistors.
The normal failure of MOV is overheating. This can cause fires.
A gas discharge tube or gas tube is a kind of spark gap which contains
air or a gas mixture.
the voltage surges reaches a certain level, the gas will ionize the
gas, making it a very effective conductor. It passes the current to the
ground line until the voltage reaches normal levels.
varistors gas tubes have higher breakdown voltages. They can handle
significantly higher fault currents and withstand multiple high-voltage
hits without self destruction. On the other hand the response times is
Gas arrestors are commonly used in telecommunication equipment to
protect against lightning strikes.
Sources and additional information