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August 04, 2009
Re-usable Fuses?

When I was in college, I was attending a party when suddenly the stereo speakers stopped working.  The party host was bummed as he thought that he had "blown" his speakers by playing the music way too loud.  A few days later his roomate turned on the stereo and ta-da...the speakers worked again!! 
What he didn’t know then but I know now is that almost all speaker manufacturers use resettable fuses to help protect the stereo speakers from college idiots who host loud parties.  
Several years ago, Raychem (now Tyco) helped to develop and patent a fuse that actually opened when a over-current problem occurred (short-circuit or over-load...as in case of the speaker story).  Basically, when a resettable fuse gets hot...the fuse quickly opens...and when the fuse cools down...the fuse resets itself (please read tomorrow’s application blog when I talk about the physics behind how a resettable fuse works). 
Resettables - Radial 
Resettable fuses are used in a variety of applications including automotive circuit boards, home electronics (like stereo speakers), industrial control circuits, instrumentation, communications and telecom, computers and battery packs.
Resettable fuses are available in several different configuration such as radial leaded, surface mount (from 2920 to 0805 packages), and axial leaded (straps).
Radial leaded resettable fuses are generally defined by their voltage.  Resettable fuse voltages range from 16 VDC to 600 VDC...but here’s the "rub"...the higher the voltage...the less current range is available.  Therefore, although we do manufacture a 600 VDC resettable, the maximum current is only 160mA whereas the 16VDC resettable fuse has a maximum current of 14A.
Resettable - 2920
The OptiFuse part numbering is very simple.  If it’s a radial leaded fuse, then the prefix is "RS" and then the voltage followed by the current.  For example RS60-250...this is a radial leaded, 60VDC, 2.5A resettable fuse.  If it’s a surface mount fuse, then it’s "R" plus the package size and then the amperage.  For example R1206-100...this is a surface mount 1206 package and 1A.  (btw...did you notice that resettable amperages are expressed a bit differently than other types of fuses?) 
Resettables - Straps
One of the most popular applications of a resettable fuse is the protection of rechargeable battery packs.  Each battery pack is required by law to be protected.  The resettable battery strap is the way to accomplish the protection needed.
Lastly, the competition out there is fierce with Raychem (Tyco), Bourns, and Littelfuse all jockeying for print position.  Unfortunately, this also means that OptiFuse is less likely to be half the cost of similar products like the electronic or automotive fuses.  OptiFuse does manufacturer a very high quality product at a competitive price.
A complete cross reference can be found on the OptiFuse website by using the following link:
Remember...tomorrow’s blog will be devoted to the how and why’s of resettable fuses so stay tuned...
Thank you so much very much for all your support of OptiFuse as we attempt to "reset" (or even upset) the power protection industry

Jim Kalb

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