I've got a few designs with them in it. Yes they work, but do have SOME
drawbacks.

1. If what you are trying to do is to protect a catastrophic problem, be
aware that in case of a short circuit the poly will heat rapidly
momentarily, so if the polyfuse leads are short and not bent over, the poly
might unsolder itself. What I am saying is that there needs to be a "heat
loss" loop in the polyfuse leads to prevent it from unsoldering.

2. If you are trying to prevent a simple long-term overcurrent or high
temp, they work better than anything.

3. IN MY EXPERIENCE, like slo-blo fuses, they don't react fast enough to
protect most semis, such as PICs, etc. The PIC will valiantly sacrifice
itself to keeep the poly from "blowing".

--Bob

At 03:03 AM 8/10/2003 -0600, you wrote:
>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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