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Simple question on Fuses

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Newbie level 2
Jun 23, 2015
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Hi all - I am new here - so please go easy on me.

I am trying to fix a PC ATX PSU which is not supposed to contain user repairable components.
But I thought I would take a look so opened it up and made sure the big capacitors were discharged.

Anyway - it looks like it was just a fuse blown (Motherboard was bad and so drew too much current).
So I feel safe it replacing the fuse.. the original was a F5L 250V which is a glass fuse with wires so it can be used as a through hole component on the board. It had a plastic sheath - presumably to contain the glass should it explode!!!

Anyway - my question.. am I safe to replace it with a "5A Axial FF Non-Resettable Wire Ended Fuse, 250 V ac" which is epoxy coated ?
if I understand - this is a very fast blow fuse (so better/safer that the fast blow spec of original?) Also, it is not glass, so should not have the possibility of shattering. On the face of it - it seems be better, but what makes me nervous - is that it is a cheap PSU, so I would have expected them to use one of these in preference to (what I assume) is a more expensive glass fuse.

So am I missing something? I don't need know that perhaps a ceramic may have been better.. I just need reassurance that it is "safe"..
Any help/guidance appreciated.
Thanks in advance for your time.

Hi there,

I can only suggest you maybe have a little read of these pdfs, or other accurate/reliable fuse selection guides, I have found them quite helpful to clear up doubts I had about power supply transformer primary fuses.

Another option is contacting a company who manufacture fuses and asking a direct technical question to the support department (but some don't have the time to answer every e-mail, sadly).

Hopefully some-one more qualified/informed will also answer you, but for the timebeing, I hope these fuse selection guides help you a little.

View attachment littelfuse_fuseology_selection_guide.pdfView attachment Guide_to_Fuse_Selection.pdf

- - - Updated - - -

P.S. ...a bad answer would have been: "Yeah, sure, from what you say - why not, sounds like any old similar rated fast fuse will be acceptable for a PSU", a better answer is the previous one, where you can use relevant knowledge to be more certain of your choice.

A FF fuse may be blown when switching on the power supply, because it doesn't withstand the inrush current.

Thanks Guys,
I cant see much to suggest it a bad alternative, perhaps the interrupt rating is better on the glass .. but surely 50A is enough on a 650W power supply, which when supplying max (which it never will) would only draw 2.7A and rated to blow at 5.
I was also using this to factor in the FF rating, I appreciate the FF will more susceptible to blow on turn on or power spike.. but again - at a max stable draw of 2.7A .. I was assuming even the spike at turn on would be OK (and FF do not seem THAT more sensitive than F if you look at the spec sheets you linked).
I am not fixing it to sell on or anything.. but was just interested.
If someone can give more definitive guidance, that would be great, but its all good learning for me, so thanks again :)

Hi there.
More bad advice (!)

I find fuses difficult to figure out, even with the selection guide information; the power supply transformer needs 200mA, based on the manufacturers recommendations, but in practice I have to use a 500mA slow/250V AC (the closest I have at home to 200mA x 1.3 = 260mA, which is too tight to be worth using a 250mA fuse) or it blew at power up every time with no load - inrush current, as FvM said - thus that correlates with sizing the fuse to be 1.3 times larger than the rated steady on current, plus a bit more for headroom, and slow blow for power supplies and circuits with occasional but harmless peaks , but not so high rated as to put your equipment at risk if something goes wrong.
I prefer glass cartridge fuses because I can see if the fuse wire is intact or not, but that's a very silly reason to choose them over ceramic types as a continuity tester is what counts.
Slow blow should fuse very quickly if sized more or less correctly, if I remember well it's still well under a second. I use the fast blow ones for circuits with ICs as I think their needs are more urgent in this respect. No doubt I could be wrong about a lot of this.

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