1st question: Functions of a Varistor?

A varistor is a type of resistor with a significantly non-ohmic current-voltage characteristic. The name is a portmanteau of variable resistor which is misleading since it is not continuously user-variable like a potentiometer or rheostat and is not a resistor but in fact a capacitor. Varistors are often used to protect circuits against excessive voltage by acting as a spark gap.

by incorporating them into the circuit in such a way that when triggered they will shunt the current created by the high voltage away from the sensitive components. A varistor is also known as Voltage Dependent Resistor or VDR.

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Thank you and good night

A spark gap is not a varistor, and a varistor is not a capacitor. The most common form, MOV (metal oxide varistors) are more like a zener diode - they operate by avalanching when voltage exceeds a threshold.

Typical uses are lightning protection, protection from errant power supplies, etc., - anywhere you want to protect a circuit against high voltage glitches.

Google "tranzorb", which is a trade name.

MOV's are specified by avalanche (or clamping) voltage, and an energy rating (in joules). The energy rating indicates how big a spike they can dissipate.

Another important spec is response time - the quicker the better, since you're trying to discharge a potentially dangerous voltage spike.

In a modern power supply, they might be used on either the input or output. For instance, I use them on the input to guard against lightning and other ESD, and against spikes on the input voltage, over which I have no control. On the output, they serve to protect the circuit my power supply is driving, in the event that my design goes kerflooey.

Metal oxide rectifier stacks were used in early protection circuits they have the advantage of being self healing after oxide layer puncture. I believe they were only practical for low voltage circuits due to the low breakdown voltage of each cell they start to get rather large for high volts.

I am a great believer of varistors, I build them into all of my socket extension strips with the hope they might just catch the spike that could mess up my electronics .

Below the turn-on (conduction starting) voltage, a metal-oxide varistor is an almost pure capacitor with current leading voltage by 90 degrees. The starting voltage is measured with a power frequency voltage and is defined in surge arrester standards as the voltage at which the resistive grading current through the varistor equals 1.0 milliamp crest. As the test voltage is increased from zero, the grading current phase shifts from capacitive to resistive and is completely resistive at full conduction.

Another important characteristic of a MOV is its TOV, or temporary overvoltage capability. TOV defines power frequency energy absorption capability and is a finite time at a given per unit overvoltage above the MOV's maximum continuous power frequency operating voltage (MCOV).

Metal-oxide varistors are non-linear and are designed to discharge impulse overvoltages only. Those overvoltages are of unidirectional polarity and typically result from lightning or switching surges. If subjected to power frequency or DC voltages that exceed their TOV, they will rapidly fail.

Oh dear Oh dear WRONG.!!!!!!!

The order the replies came in is the wrong order it should have been in reply to snake mike not bhankii. Bhankii got it right. snakemike go back to square one. read your text books again. bhankii get a gold star.

I blame wikipedia.

some one stuffed up the gen did they?

I noticed that Snakemike didn't give Wikipedia credit for his answer, the part coming from Wikipedia being the only correct part of it.

another function of varistor is thermal varistor.it depend on operating temperature to change the resistance of the electric circuit (as known by name NTC) and put in input of the power supply usually.

You are thinking of a thermistor and the come in both NTC and PTC. You might use and NTC device to limit inrush current as its resistance starts high and lowers when hot. There are polymer PTC's that can be used as circuit interrupters. Also, both NTC and PTC devices can be used as temperature sensors but I believe NTC is more commonly used and are readily available to meet standard response curves. Varistors are voltage dependent devices as indicated in other posts and though may have some temperature dependence are not used the same way.

Interestingly, AVX Transguards have an almost linear temperature - capacitance curve, although I've never heard of anyone utilizing that aspect.

I wish you'd explain your avatar.

It's a Morse key.

Duh. He's asking about the "No Alabaster Vixens" icon above.

Here are some app notes on Vishay's Transzorbs:

http://www.vishay.com/docs/88436/whatis.pdf

http://www.vishay.com/docs/88438/tvs_desc.pdf

http://www.vishay.com/docs/88439/typical_.pdf


Here is some info on AVX's Transguards MOV, which offers bidirectional protection:

http://www.avxcorp.com/docs/masterpubs/transgrd.pdf