Another gotcha that their voltage limitation
( across the PTC component ) is less then the
maximal voltage across "normal" fuse.

It's difficult to find variety of SMT PTC's
for voltage range above 120V DC for example.



Alexandre GuimarGes wrote:
>
> Hi,
>
> > I'm curious, this is my first introduction to poly fuses...  I found a
> > few from Littelfuse, and they look interesting, but not enough info to
> > satisfy me...
>
>     I will try to help. I use them in some of the products I have designed.
>
> > The ones on Littlefuse seem to have a huge temperature coefficient so
> > when they get shorted through they heat up and become a highly-resistive
> > pretty-much-open-circuit kind of thing.
>
>     I guess all of them work the same way.
>
> > - Are they reliable?
>
>     I never replaced any and I have a few thousands boards in the streets
> with them for 2 years.
>
> > - Are they safe to have in a commercial product?
>
>     I hope so :)
>
> > - Are they more resistive in the non-open state than a normal
> > glass/blade/whatever fuse? (i.e. am I gonna waste batteries with these)
>
>     Yes. They are a little more resistive. You can check how much in the
> datasheets. How would you  waste batteries because of that ???
>
> > - Do they really protect?  Current has to sneak through in order for the
> > temperature increase, so it seems risky.
>
>     They protect better than a regular fuse in almost all instances. They
> are not fast enough to protect your semiconductors, so you have to limit
> current in some way before they act. I use them, usually in series with a
> power resistor to limit the max current.
>
> > Curious who's used them and where, they seem like a perfect solution to
> > most things I would use a fuse for, there's gotta be a catch!
>
>     Yes there is... They are more expensive than a regular fuse :-) You just
> have to be carefull because they are not magical in any way. They have a
> little resistance, will not act imediately and will keep current flowing to
> some extent. You have to be prepared to design with those factors in mind.
>
> Best regards,
> Alexandre Guimaraes
>
> > Nick Veys | nick@veys.com
> >
> > > Poly fuses reset themselves.
> > >
> > > Jesse Lackey wrote:
> > >
> > > > Unfortunately reverse battery situation can happen easily and
> > > > repeatedly, having a fuse blow or a short across the battery is
> > > > unacceptable.  Tx anyway though.
> > > >
> > > > J
> > > >
> > > > Dal Wheeler wrote:
> > > >
> > > >> along that line, put a fuse or a poly fuse in it and use
> > > the parallel
> > > >> diode as a fuse killer in the event of a reverse hookup.
> >
> > --
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> >
>
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