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Sorry for the confusing question, but for PTC's or Resettable Fuses such as something like this: http://www.sparkfun.com/products/8357

Is there a "limit" on how many......I guess "resets" it can handle? Im assuming it becomes more and more inefficient each time it "resets" after cooling back down? Is there some sort of limit someone should give these fuses? or are do they pretty much have an indefinite lifespan?

Thanks

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Trip cycle or trip endurance are the measures that you are looking for. The first is the number of times you can trip and assume that damage to the part has not occurred, the second is the length of time the trip state can be active for before the part is damaged.

Many factors will also determine whether the PTC becomes damaged or not, heat, humidity, age, etc. and it is impossible to determine when the fuse will fail. Normally the point of putting one of these in a system is that the replacement cost of the PTC is less than the replacement cost of the components it is protecting. Think of every trip as one get-out-of-jail-free over traditional fuses.

These are a last line of defence and don't assume that the fuse will always trip to protect your circuit. I hate to think about the number of times a $100 tweeter in my stereo has blown to save the $0.10 protection fuse.

spearson
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    Most fuses are not designed so much to protect circuitry as to prevent fires. In some cases they may serve to protect devices which can withstand overloads of significant but not unlimited duration, but in general the goal is for the fuse to react before things ignite, not necessarily before they are damaged. – supercat Mar 07 '12 at 18:59
  • This makes sense when i look at it that way! –  Mar 07 '12 at 19:10