Do TVS diodes last (practically) forever?

That's totally an unknown.  "Our" application varies widely between layouts and operation, some may experience far more large transients than others.  The TVS will last indefinitely if it's not ever called on to dissipate a voltage transient over it's peak rating.  What kills them is a really big blast that overloads them.

I think every six months is overkill.  As for testing, it's very difficult to do without a special setup, and you really can't test them in circuit.  The good news is, the most common failure is shorted, you'll know right away if that happens!

I'm going to take a different tack. No they do not last forever. When they are exposed to a surge condition they do fail. You want them to fail, rather than the device they are intended to protect. WHEN they fail, you'll know it. The magic smoke gets released, and downstream devices lose power.

For optimum effectiveness, TVSS needs to be implemented in a layered architecture.

gunrunnerjohn posted:
The good news is, the most common failure is shorted, you'll know right away if that happens!

Ah, thanks, I think that’s the key!

I’m kind of disappointed in myself that I didn’t realize this. A long time ago I learned through experience that diodes tend to fail shorted, in sufficiently spectacular fashion that I should not have forgotten. 

One way to look at it is: if a TVS does experience (and suppress) a large transient that causes it to fail open (and, thus, unable to protect against further transients), your equipment at least had one transient less than it would have without a TVS in place.  And less is better (although none would be best).

For the interested, I found this discussion of TVS failure modes from the good folks at Vishay:

IT consultant by day, 3rd generation Lionel guy (raising a 4YO 4th generation Lionel Lil' Man) by night in the suburbs of the greatest city in the world - Chicago. Home of the ever-changing Illinois Concretus Ry.

A properly functioning TVS diode appears as an open circuit until it's breakdown voltage is reached.  Having them in parallel shouldn't be an issue with one exception. 

The capacitance of a TVS diode is significant in some circuits, especially if you have the higher wattage ratings.  The 36V models we frequently specify have around .001uf capacitance for the 1.5KW rating, that could easily affect some circuits.  Obviously, for multiple TVS diodes, you add them up in parallel.

The capacitive reactance of .001uf at 3.27 mhz is less than 50 ohms, a few of those across the DCS signal would likely impact the signal strength, something to consider.

Not sure where you'd be putting the choke, and any choke in series with the TVS would basically kill any surge suppression capability of the TVS anyway.  You just need to be aware of the capacitance of the TVS when you're considering their usage.

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Alan Mancus

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