Nick Veys wrote:

> 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...
>
> 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.
>
> - Are they reliable?
> - Are they safe to have in a commercial product?
> - 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)
> - Do they really protect?  Current has to sneak through in order for the
> temperature increase, so it seems risky.
>
> 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!
>
> Nick Veys | nick@veys.com

I don't design things professionally so I will have to leave most of the
questions you pose to the pros, but as an example...

They are part of the circuit that protects my Jeep's power windows when
they're at end-of-travel and someone is still mashing on the up/down
button -- that should be a good indication of how widely accepted they
are these days.

They "blow" because the current draw exceeds normal window movement
requirements and then reset shortly thereafter.  (There's more to the
circuit than that, but that's the general idea.  There's a physical
relay involved in this particular circuit also... click, click.)

Bourns and other manufacturers have good datasheets and whitepapers on
them and their use.

(Disclaimer: A family member of mine sells Bourns' products, so I won't
typically use any other type.  Bourns calls theirs MultiFuse.  They
provide a checklist document for choosing the correct part at
http://www.bourns.com/2/pdfs/mf_selection_worksheet.pdf)

The larger surface-mount ones are nice in that they go nicely on a board
and don't require the end-user to replace a blown fuse, if the circuit
is designed correctly according to the limitations on the datasheet...
yes, they dissipate the excess energy as heat, and they can be damaged
so they're less effective in the future.

Definitely a "neat toy" for a hobbiest like me.

--
Nate Duehr, nate@natetech.com

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