Being a form of a resistor, varistors are two-terminal semiconductor components that protect the electrical and electronic devices from overvoltage transients. In fact, the word is derived from the term variable and resistor, which is why it is also known as Voltage-dependent resistor VDR. Varistors are nonlinear varying resistance, dependent on the voltage applied. Their primary functions are to protect the transient voltage in a circuit.
Today you’ll get to know the definition, applications, functions, diagram, symbol, specifications, characteristics, types, and working, of varistors.
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What is a varistor?
Varistors are considered as a form of a resistor in which the resistance changes significantly as a result of the applied voltage. They are voltage-dependent resistor VDR and their resistance is variable and depends on the voltage applied, thus their name is “Variable Resistor”. When the voltage increases, their resistance reduces, and in case of excessive voltage increases, their resistance drops dramatically. Therefore, varistors are protective electrical devices as they are suitable to protect circuits during voltage surges.
So, varistor can be defined as nonlinear two-element semiconductors that drop in resistance as voltage increases. They are often used as surge suppressors for sensitive circuits. Surges are often caused by lightning strikes and electrostatic discharges.
Applications of varistors
Just as earlier stated, varistors are used as surge protectors due to their nonlinear characteristics. They are also used in surge protector power strips to protect high voltage transients such as lightning strikes, electrostatic discharge (ESD), or inductive discharge from motor or transformers. Some types of VDR are designed to protect low capacitance communication lines. Below are some common applications of varistor:
- Radio communication equipment transient suppression.
- Cable TV system surge protectors.
- Surge protector power strips.
- Telephone and other communication lines protection.
- Microprocessor protection.
- Electronics equipment protection.
- Industrial high-energy AC protection.
- Car electronics protection.
- Low voltage board-level protection.
Furthermore, in the applications of varistors, they are able to provide protection for electronic circuits that can be exposed to impulses and voltage spikes. Also, they can divert the energy to the ground and in such a manner, equipment is protected. VDR is used in many items like surge-protected sockets and related items. Finally, they are used in some circumstances as microwave mixers for modulation, detection, and also frequency conversion.
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The symbol of a varistor circuit is very similar to that of a thermistor. It consists of a basic resistor symbol of a rectangle with a diagonal line through it that has a small added section parallel to the body of the resistor symbol which indicates the nonlinear nature of the varistor. Although some other symbols may be used, the one shown below is the generally accepted one. It is depicted as a variable resistor that is dependent on voltage, U.
See the symbol of varistor below:
Below are the major features of a varistor:
- Nonlinear varying resistance
- High impedance under nominal load conditions
- Low impedance when a voltage threshold or breakdown voltage is exceeded.
- Circuit protection against excessive transient voltages.
- Varistors conduct and clamp the transient voltage to a safe level when high voltage transient occurs.
- Incoming surge energy is partially conducted and absorbed.
- The sintered matrix of zinc oxide ZnO grain in the design of metal oxide varistor provide P-N junction semiconductor characteristic.
- Little current flow when a low voltage is applied
- Varistors perform short circuit duration protection.
- They cannot deal with sustained surges.
- If transient energy is measured in joules J the absolute maximum ratings are exceeded, so the device can melt, burn, or explode.
- Some selection parameters include clamping, voltage, peak current, maximum pulse energy, rated AC/DC voltage, and standby current.
- Stray capacitance should be considered when used on communications lines.
- High capacitance acts as a filter for high-frequency signals or induces crosstalk. This limits the available bandwidth of the communication line.
- Varistors degrade when exposed to repeated surges and their clamping voltage decrease after each surge.
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There are a few things to consider while choosing a varistor for applications. below are some specifications of varistors and their functions:
Clamping voltage – the voltage at which the varistor starts to show significant conduction.
Rated voltage – is either stated as AC or DC and is the maximum voltage at which the device can be used. Normally, it is important to have a good margin between the rated voltage and the operating voltage.
Peak current – this is the maximum current that a varistor can handle. It may be expressed as a current for a given time.
Maximum pulse energy – is the maximum energy of a pulse. It is expressed in Joules that the device can dissipate.
Response time – this is the time for the varistor to start conduction after the pulse is applied. Although this is not an issue in many situations.
Capacitance – metal oxide varistor has a relatively high capacitance across the device. This is not an issue for low-frequency applications, but problems may occur when used with lines carrying data, etc.
Standby current – this is the level of current that is drawn by the varistor when it is operating below the clamping voltage. although the current will be specified at a given operating voltage across the device.
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Types of varistors
The various types of varistors can be determined by the material used for their body. The two most common types of varistors are silicon carbide varistor and metal oxide varistor (MOV)
Silicon carbide varistor:
As the name indicates, silicon carbide, the body of the varistor is made from silicon carbide (SIC). It is one of the most used in the time period before the MOV took the market. However, they are used intensively in high power, high voltage applications. one drawback of these types of varistors is the significant standby current they draw which is why a series gap is required to limit the standby power consumption.
Metal oxide varistors (MOV):
The metal oxide varistors have advantages over the silicon carbide types as they provide very good voltage transient protection. They are quite popular and their body is made of metal oxide, Zinc oxide grains often. The material is pressed as a ceramic mass, with 90 % of zinc oxide grains and 10% of other metal oxides like cobalt bismuth and manganese.
It is then sandwiched between the two metal plates. The 10% of metal oxides of cobalt bismuth and manganese act as a binding agent to zinc oxide grains so that it is kept intact in between the two metal plates. The connecting terminals or the leads are connected to the two metal plates.
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The working of a varistor is less complex and can be easily understood. Just as earlier mentioned, they are used for surge protection in many areas where they are placed across the lines to be protected, or down to the ground from the line. Normally, the device draws little current, but when a surge occurs, its voltage rises to above the knee or clamping voltage and they draw current, thus dissipating the surge and protecting the equipment. The actual surge is part absorbed by the varistor and partly conducted away.
The metal oxide and silicon carbide varistors operate on the grain boundaries between the grains of the material act as PN junctions. The components act as a large mass of small diodes in series and parallel. When a low voltage is applied, very little current flows because the junctions are reverse biased and the only current is the leakage current. When a surge takes place across the device that exceeds the clamping voltage, the diodes experience avalanche breakdown and a large amount of current will be able to flow through the device.
Furthermore, varistors are suitable for short-duration pulses, and cannot be used to handle sustained surges. The device size determines the amount of power they can dissipate. Exceeding the rated period or voltage can cause the devices to burn out or can even explode. This is why they should be operated within their ratings.
Watch the video below to learn more on the working of varistors:
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Varistors are considered as a form of a resistor in which the resistance changes significantly as a result of the applied voltage. They are voltage-dependent resistor VDR and their resistance is variable and depends on the voltage applied, thus their name is “Variable Resistor”. Being a form of a resistor, they are two-terminal semiconductor components that protect the electrical and electronic devices from overvoltage transients. That is all for this article, where the definition, applications, functions, diagram, symbol, specifications, characteristics, types, and working, of varistors being discussed.
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