Varistor

Key takeaways

Varistor belongs to science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Varistor to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Varistor from memory before moving on to harder problems.

Reference excerpt

A varistor (a.k.a. voltage-dependent resistor (VDR)) is a surge protecting electronic component with an electrical resistance that varies with the applied voltage. It has a nonlinear, non-ohmic current–voltage characteristic that is similar to that of a diode. Unlike a diode however, it has the same characteristic for both directions of traversing current. Traditionally, varistors were constructed by connecting two rectifiers, such as the copper-oxide or germanium-oxide rectifier in antiparallel configuration. At low voltage the varistor has a high electrical resistance which decreases as the voltage is raised. Modern varistors are primarily based on sintered ceramic metal-oxide materials which exhibit directional behavior only on a microscopic scale. This type is commonly known as the metal-oxide varistor (MOV). Varistors are used as control or compensation elements in circuits either to provide optimal operating conditions or to protect against excessive transient voltages. When used as protection devices, they shunt the current created by the excessive voltage away from sensitive components when triggered. The name varistor is a portmanteau of varying resistor. The term is only used for non-ohmic varying resistors. Variable resistors, such as the potentiometer and the rheostat, have ohmic characteristics.

History The development of the varistor, in form of a new type of rectifier based on a cuprous oxide (Cu2O) layer on copper, originated in the work by L.O. Grondahl and P.H. Geiger in 1927. The copper-oxide varistor exhibited a varying resistance in dependence on the polarity and magnitude of applied voltage. It was constructed from a small copper disk, on one side of which, a layer of cuprous oxide was formed. This arrangement provides low resistance to current flowing from the semiconducting oxide to the copper side, but a high resistance to current in the opposite direction, with the instantaneous resistance varying continuously with the voltage applied. In the 1930s, small multiple-varistor assemblies of a maximum dimension of less than one inch and apparently indefinite useful lifetime found application in replacing bulky electron tube circuits as modulators and demodulators in carrier current systems for telephonic transmission. Other applications for varistors in the telephone plant included protection of circuits from voltage spikes and noise, as well as click suppression on receiver (ear-piece) elements to protect users' ears from popping noises when switching circuits. These varistors were constructed by layering an even number of rectifier disks in a stack and connecting the terminal ends and the center in an anti-parallel configuration, as shown in the photo of a Western Electric Type 3B varistor of June 1952 (below).

The Western Electric type 500 telephone set of 1949 introduced a dynamic loop equalization circuit using varistors that shunted relatively high levels of loop current on short central office loops to adjust the transmission and receiving signal levels automatically. On long loops, the varistors maintained a relatively high resistance and did not alter the signals significantly. Another type of varistor was made from silicon carbide (SiC) by R. O. Grisdale in the early 1930s. It was used to guard telephone lines from lightning. In the early 1970s, Japanese researchers recognized the semiconducting electronic properties of zinc oxide (ZnO) as being useful as a new varistor type in a ceramic sintering process, which exhibited a voltage-current function similar to that of a pair of back-to-back Zener diodes. This type of device became the preferred method for protecting circuits from power surges and other destructive electric disturbances, and became known generally as the metal-oxide varistor (MOV). The randomness of orientation of ZnO grains in the bulk of this material provided the same voltage-current characteristics for both directions of current flow.

Composition, properties, and operation of the metal-oxide varistor

The most common modern type of varistor is the metal-oxide varistor (MOV). This type contains a ceramic mass of zinc oxide (ZnO) grains, in a matrix of other metal oxides, such as small amounts of bismuth, cobalt, manganese oxides, sandwiched between two metal plates, which constitute the electrodes of the device. The boundary between each grain and a neighbor forms a diode junction, which allows current to flow in only one direction. The accumulation 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 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 junction breaks down due to a combination of thermionic emission and electron tunneling, resulting in a large current flow. The result of this behavior is a nonlinear current-voltage characteristic, in which the MOV has a high resistance at low voltages and a low resistance at high voltages.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Varistor

Start with the simplest possible case. Write down what Varistor claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Varistor before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Varistor ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Varistor

In research: Varistor appears in science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Varistor in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Varistor is common in secondary-school and first-year university syllabi. It links to neighbouring topics Electrical components, Resistive components, so understanding it makes those chapters shorter.
In everyday life: Look for Varistor outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Varistor in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Varistor means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Varistor out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Varistor in simple terms?

A varistor (a.k.a. voltage-dependent resistor (VDR)) is a surge protecting electronic component with an electrical resistance that varies with the applied voltage. It has a nonlinear, non-ohmic current–voltage characteristic that is similar to that of a diode.

Why does Varistor matter?

Because it connects several science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Varistor?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Varistor.