المقاومات المتغيرة الجهد (Varistors)

Varistors (MOV)

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.

	Different varistors. 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.

However, 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.
Even 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..

Function

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 application.

Characteristic curve of a MOV. Is the voltage low also the current is low (the resistance is high). When the voltage reaches the voltage of the varistor the current gets high very fast (the resistor is extrem low. The connectors are short.

Specifications

Varistors are a kind of resistors but their specifications are not resistance ῼ and wattage W. For varistors the most important specifications is the clamping voltage.

Clamping voltage
This 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.

Energy absorption / dissipation
This 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 parallel.

Response time
Varistors 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. 

Application

	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).

Problems

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.

Alternatives

A gas discharge tube or gas tube is a kind of spark gap which contains air or a gas mixture.
When 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.
Compare to 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 longer.
Gas arrestors are commonly used in telecommunication equipment to protect against lightning strikes.

Sources and additional information

http://en.wikipedia.org/wiki/Varistor
http://en.wikipedia.org/wiki/Surge_Protector
http://www.nteinc.com/Web_pgs/MOV.html