PTC wrong resistance

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#1
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Hey freaks...

 

I was analyzing a bad board trying to find something wrong. 

Something that puzzles me. There is 4 PTC in the board, 2 of them shown the resistance that datasheets set as maximum, 6 Ohms, but other 2 shown 27 Ohms.

 

Then, can a old or blown PTC increase its resistance? 

I had never experience that...

This topic has a solution.

Regards,

Bruno Muswieck

Last Edited: Mon. Oct 5, 2015 - 11:34 AM
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They can fail if the specification for power dissipation is exceeded. 

It all starts with a mental vision.

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PTC is a fairly broad description. There are two main technologies - ceramic and polymer. The polymer ones have a wearout mechanism- they go high resistance.

It sounds like you have polyfuses (polymer ptc). These are good for occasional overloads, but not being operated a number of times - like current limiting the charge of a battery.

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Thaís is the guy: Surface Mountable PTC Resettable Fuse: FSMD 014-R
Freaks, i don't know if make my self clear, the resistance 27 Ohms when there is no power in the board. For me it is strange...

Suppose that the current is 100 mA on the PTC, then: Vptc = 100mA x 27 ohms = 2,7V.

Regards,

Bruno Muswieck

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Yes, you were perfectly clear. I described the failure mechanism - they wear out. From what you gave described, the device has failed. Replace it.

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Bruno, I think that Kartman is "suggesting" that the 27 ohm ones have already been over stressed and failed and stayed at their high resistance value (unless I have misunderstood Kartman's comment).

Ross McKenzie ValuSoft Melbourne Australia

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The polymer fuses do 'wear out' after a while. I have had this pleasure. :)

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Ohhh, thanks guys...  That's new information for me.. 

 

I'll replace it...

 

Regards,

Bruno Muswieck