Is there such a thing as a inverse varistor?

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Discussion Overview

The discussion revolves around the concept of an "inverse" varistor, specifically whether there exists an electrical component that creates a short circuit when the voltage exceeds a specified threshold, regardless of the polarity of the applied voltage. The scope includes theoretical considerations and practical applications of various components that may exhibit similar behaviors.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants inquire about the existence of a component that behaves as an "inverse" varistor, questioning the definition and functionality of such a device.
  • Triacs are mentioned as components that can be triggered at any gate voltage and can conduct until the current is removed, suggesting a potential alternative.
  • A negative temperature coefficient thermistor is noted for its decreasing resistance with increasing temperature, but it does not create a short circuit.
  • Participants discuss metal-oxide varistors (MOVs) as components that conduct under overvoltage conditions, with some asserting that this behavior aligns with the concept of an "inverse" varistor.
  • One participant references a thyristor-like device that operates without a gate lead and can be bidirectional, relying on breakover voltage to conduct.
  • Crowbar circuits are described as protective devices that create a short circuit to ensure that fuses blow or circuit breakers trip, likening their function to a last-resort safety measure.
  • Some participants mention that MOVs are one-time use devices and discuss the characteristics of avalanche diodes and fusible resistors as alternatives that may meet similar criteria.
  • A gate turn-off device (GTO) is proposed as a potential component that could function in an "inverse" manner, depending on the interpretation of the term.

Areas of Agreement / Disagreement

Participants express differing views on the definition and existence of an "inverse" varistor, with no consensus reached on whether such a component exists or how it should be characterized. Multiple competing perspectives on related components and their functionalities are presented.

Contextual Notes

Some limitations include the ambiguity surrounding the term "inverse" and the specific operational characteristics required for the discussed components. The discussion also reflects varying interpretations of voltage and current behavior in different devices.

kmarinas86
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Is there such a thing as a "inverse" varistor?

Is there a type of electric component that will create a short circuit once the voltage exceeds some specified value which it has been rated at, regardless of what end the voltage appears at?
 
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Triacs can be triggered to turn on at any programmed gate voltage, with either polarity. They will not turn off until the current is removed. See http://en.wikipedia.org/wiki/TRIAC

Bob S
 


A negative temperature coefficient thermistor decreases in resistance as it's temperature increases. However the resistance never goes to zero.

There is one company that uses a capacitor that shorts when the voltage exceed the capacitor's rating.
 


kmarinas86 said:
Is there a type of electric component that will create a short circuit once the voltage exceeds some specified value which it has been rated at, regardless of what end the voltage appears at?

Why is that inverse? That's how MOVs work, AFAIK.
 


berkeman said:
Why is that inverse? That's how MOVs work, AFAIK.

I would agree with berkeman. There is a thyristor device that didn't gain a lot of popularity because MOVs took over. I forget what it is called, but it is like an SCR without the gate lead. I believe it can be made to be bidirectional. It relies on breakover voltage to conduct and will continue to conduct until the current through it approaches zero.
 


This type of circuit using SCRs was (probably still is) used in some applications as a "last ditch" protection device, the idea being that it puts a short circut across the input power supply, to guarantee that all the fuses blow and/or the circuit breakers trip.

They were usually called "crowbar" circuts, by analogy with jamming a big iron bar into a piece of machinery to stop it regardless of how much collateral damage occurs.
 


Why is that inverse? That's how MOVs work, AFAIK.

I agree with Berkeman.

MOV devices are used as lightning and other surge arrestors for exactly this purpose.
One point to not is they are 'one time' components.That is they have to be replaced after they have acted.

A straight forward avalanche diode also offers the desired characteristic and is reusable if the short circuit current is limited to a suitable value.

Also on the list might be fusible resistors. These are also one time devices, often employed in switching power supplies. These are really overcurrent devices, but Ohm's law enables that to be recast as overvoltage, with suitable circuit configurations.

If, of course you actually meant a device that tuns off rahter than on by inverse then how about a gate turn off device (GTO)?
 

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