# Polarized fuse



## ekim68 (Jul 8, 2003)

I just ran into this on a board repair...A fuse that's like a diode....Apparently with two different power supply's, batteries and adapter, there has to be a fail-safe in case one or the other fails...And, the other lives on...Typical surface mount..


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## Elvandil (Aug 1, 2003)

Is there a question?


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## JohnWill (Oct 19, 2002)

A diode with a limited current rating will act as a polarized fuse, kind of...


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## ekim68 (Jul 8, 2003)

Elvandil said:


> Is there a question?


No. Just bringing it to light. However, I'm having a heck of a time finding a replacement.


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## Elvandil (Aug 1, 2003)

If you find a link to anything about this, please share it.


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## ekim68 (Jul 8, 2003)

I picked it up from this conversation. Scroll down to solution#2...

http://www.fixya.com/support/t370962-nec_versa_m500_laptop_main_power_defects

(The board I'm working on is in a portable dvd unit.)


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## cwwozniak (Nov 29, 2005)

JohnWill said:


> A diode with a limited current rating will act as a polarized fuse, kind of...


Murphy's Law: _Any semiconductor device connected in series with a fast acting fuse will blow first, thereby protecting the fuse._


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## Elvandil (Aug 1, 2003)

If you've decided that the fuse is definitely (or probably) the thing you need to replace, I doubt you're going to find one easily or at all. You may as well replace it with a fuse and diode in series. That should work just as well (barring any space problems, but it can't be *that* tight, can it?).

Any markings on the component or board to indicate polarity direction?


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## ekim68 (Jul 8, 2003)

Actually Elvandil it's an interesting looking thing... I'll take a picture and try to post it tomorrow.....
But the problem is resolved....Seems the fuse rested on several traces and did a couple of things. 
One of them was charging the battery with a continuous voltage of 5.5....The other trace carried the 9 volts to the main operation...The battery pack was bad, so it might have had an effect on the fuse to cause it to blow. I removed the cable from the battery terminal because the user said they didn't use the battery at all.
I installed a pico-fuse, sp., with the correct rating and everything is working....
(BTW, I don't usually do board repairs anymore because the surface mounted components are so small and just the presence of a soldering iron will make them shrink in terror.) 
But, this was a friend's machine and I had to take a look....


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## Elvandil (Aug 1, 2003)

LOL. Good work. Surface-mount is not fun to work on. I was modding a scanner and had a resistor stick to my iron. I had to use a sewing needle to get it off and was sure it was burned out, but put it on the board, anyway. It worked, but that is the last time I ever attempted work on a $700 device with my gigantic (seemingly) hands.


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## JohnWill (Oct 19, 2002)

With the correct tools, most surface mount products are not that hard to work on. The only stuff I don't tackle anymore are the really fine pitch components like processors and support chips. There are just too many pins that are too close together.  Resistors and caps are pretty easy as a rule, and small logic chips only slightly more difficult.


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