App note: Transient suppression devices and principles

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App note from Littlefuse on how do transient suppression devices protect electronic circuits. Link here (PDF)

There are two major categories of transient suppressors: a) those that attenuate transients, thus preventing their propagation into the sensitive circuit; and b) those that divert transients away from sensitive loads and so limit the residual voltages.

Attenuating a transient, that is, keeping it from propagating away from its source or keeping it from impinging on a sensitive loadis accomplished with filters inserted in series within a circuit. The filter, generally of the low-pass type, attenuates the transient (highfrequency) and allows the signal or power flow (low-frequency) to continue undisturbed.

Diverting a transient can be accomplished with a voltage-clamping type device or with a “crowbar” type device. The designs of these two types, as well as their operation and application, are different enough to warrant a brief discussion of each in general terms.

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  1. I have a major problem here. “Common wisdom” as well as Wikipedia (with a citation linked) hold that MOVs DO degrade as they absorb surges, whereas the linked Littelfuse source is insisting that they absolutely DO NOT (“It has been stated that a varistor’s V-I characteristic changes every time high surge current or energy is subjected to it. That is not the case.”, page 10-108). Either way, someone is blatantly, pompously and flat-out wrong. So which is it, and how should one decide who to believe (is this another “battery memory effect” that “everyone knows about” but doesn’t actually exist as stated) ?

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