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did this fuse blow funny?

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

Reply to
acannell
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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.

Reply to
John Popelish

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.

Reply to
acannell

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.

Reply to
John Popelish

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

Reply to
mpm

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.

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[8~{} Uncle Monster
Reply to
unclemon

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