Electronics notes/Varistors

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This is for beginners and very much by a beginner / hobbyist.

It's intended to get an intuitive overview for hobbyist needs. It may get you started, but to be able to do anything remotely clever, follow a proper course or read a good book.


Some basics and reference: Volts, amps, energy, power · batteries · resistors · transistors · fuses · diodes · capacitors · inductors and transformers · ground

Slightly less basic: amplifier notes · varistors · changing voltage · baluns · frequency generation · Transmission lines · skin effect


And some more applied stuff:

IO: Input and output pins · wired local IO · wired local-ish IO · ·  Various wireless · 802.11 (WiFi) · cell phone

Sensors: General sensor notes, voltage and current sensing · Knobs and dials · Pressure sensing · Temperature sensing · humidity sensing · Light sensing · Movement sensing · Capacitive sensing · Touch screen notes

Actuators: General actuator notes, circuit protection · Motors and servos · Solenoids

Noise stuff: Stray signals and noise · sound-related noise names · electronic non-coupled noise names · electronic coupled noise · ground loop · strategies to avoid coupled noise · Sampling, reproduction, and transmission distortions

Audio notes: See avnotes


Platform specific

Arduino and AVR notes · (Ethernet)
Microcontroller and computer platforms ··· ESP series notes · STM32 series notes


Less sorted: Ground · device voltage and impedance (+ audio-specific) · electricity and humans · power supply considerations · Common terms, useful basics, soldering · landline phones · pulse modulation · signal reflection · Project boxes · resource metering · SDR · PLL · vacuum tubes · Multimeter notes Unsorted stuff

Some stuff I've messed with: Avrusb500v2 · GPS · Hilo GPRS · JY-MCU · DMX · Thermal printer ·

See also Category:Electronics.

This article/section is a stub — probably a pile of half-sorted notes and is probably a first version, is not well-checked, so may have incorrect bits. (Feel free to ignore, or tell me)

A varistor is a short name for a Voltage Dependent Resistor (VDR), (a.k.a. VARIable resiSTOR).


They act not unlike zener diodes, and have applications similar to polyfuses: often used to protect a load against relatively transient high voltages, wasting that energy as heat.


Varistors are often specifically a metal-oxide varistor (MOV), to the point MOV and varistor are near-synonyms to most people.


A MOV responds to voltage: high resistance at low voltages, lowering resistance at high voltages.

But not linearly - find a varistor I-V curve, and you'll note they present a steepish curve over a specific threshold (much like diodes, because its workings are like a diode's, though MOVs are bidirectional so can be used in AC), the MOV's rated voltage is basically that curve's knee, where it starts conducting noticeable current.


This characteristic makes them useful in parallel with a sensitive load, to protect that load against high transient voltages / small surges, effectively shunting them away and wasting them as a heat.

Which is also why they cannot deal with sustained over-voltages; they would basically become the load, heat up, and probably burn - so MOVs used as protection are often combined with other protective elements .