did this fuse blow funny?

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i have a 1 amp , glass, fast blow, 1 1/4" fuse on the hot wire going
into a 65W SMPS. It blew on several units being tested, but its not
clear to me why. And the way the fuse "blew" was strange. Its seems
more like it "broke" off at the "weld" inside one of the metal cups.
You cant see where the fuse blew, but if you shake the fuse the
element bounces oscillates inside like a tuning fork. Its still
connected to one of the metal cups inside. Anyone ever see something
like this? There is no blackening of the glass or any other indication
the fuse blew.
And its not clear why a 1A fuse would blow on a power supply whos max
input current is supposed to be 72W. (60A inrush)

Its the LPS53-M by Astec


Re: did this fuse blow funny?


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My guess is that the input side of the supply is a rectifier
and filter capacitor, with little to limit the inrush
current that charges the cap very fast if the power is
applied at the peak of the line cycle.  The pulse does not
last long enough to melt the fuse element, but gives it a
large mechanical sag-and-retighten because of the rapid
temperature change.  This repetitive bending and tensioning
at the wire ends, eventually fatigues the metal and it lets go.

Most such supplies require a slow blow fuse, possibly one
with a spring mechanism inside, that absorbs the expansion
and contraction of the element with a material with a high
fatigue resistance.

Re: did this fuse blow funny?


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I keep reading and getting told by Astec that I should use a fast blow
fuse. Whats the deal here? "slow blow does not adequately protect a
SMPS". Is this true? I would think that with the inrush current specd
at 60A, that a fast blow would be too sensitive, and a slow blow more
appropriate.


Re: did this fuse blow funny?


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If Astec is telling you that the fuse is there to protect
the supply, they are admitting that the supply is poorly
designed.  Fuses are normally used to protect the wiring
from overload after the supply has failed, shorted, and to
protect the surroundings from fire caused by supply failure.
  Evidently, Astec is having lots of trouble with this model
failing and they are trying to improve its chances of
survival by using a twitchy fuse, instead of adding the
proper inrush limiting system.

Sounds pretty futile, to me.

Re: did this fuse blow funny?


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In case anybody "cares", you do know you can unsolder the outside end
caps on those glass cartridge type fuses and remove the remains of the
fuse element wire....

I "invented" this fuse repair job on a site so remote, that if it
wasn't already on the truck, you really didn't NEED it....

-mpm


Re: did this fuse blow funny?


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Just a thought. If there is a poor connection on one end
of the fuse, it could overheat on that end and blow it.
I have an inexpensive IR thermometer that I use all the
time to check equipment with. I can find an overheating
fuse, breaker or connection quickly with this little jewel.
I paid around $20.00 for it.

http://tinyurl.com/ytzov3

[8~{} Uncle Monster


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