How fast are 'Fast Fuse' and how to identify?

bigpanda · 21st March 2005, 04:48 PM

Hi,

I have got some 5A fuse and my new project needs 5A 'Fast Fuse'. How to tell if mine are 'fast' and how fast is 'fast'? I cannot find any marks on the fuse expect the '5A'

Tks for input

richie00boy · 21st March 2005, 04:54 PM

It will say F5A is it's a fast one or T5A if slow. If unmarked assume slow.

AndrewT · 21st March 2005, 05:31 PM

Hi,
wired fuses come in 3 speeds. fast, standard & slow (or time delay), the markings would be F5A, 5A, T5A.
All these are fairly slow compared to an MCB and extremely slow compared to BJT output device. Fuses rely on downstream robustness to survive the short term overload before the fuse opens. Transformers have a large amount of thermal capacity and hence lag to help them meet this overload.
The real purpose of fuses is to stop the equipment or house wiring catching fire.

K-amps · 21st March 2005, 09:46 PM

If the fuse has a squiggly wire (not coiled or straight) it's probably fast

mpmarino · 21st March 2005, 09:56 PM

to test:

Place the fuse across the + and - terminals of a car battery. If it blows in under 1ns it is a fast fuse. Be sure to test all fuses in this manner before installing them.

Disclaimer: I'm just kidding of course

johnnyx · 21st March 2005, 10:27 PM

If a fast fuse is specified, then it's best to forget the ones you have, and get some F5A ones. Look in a parts catalogue.
BTW there are ultrafast fuses designed to protect semiconductors, marked FF5A. Some industrial ones are designed to protect thyristors etc. in inverters for motor control. I'm not that sure how effective they are though, I only get to see the ones that have failed.

K-amps · 21st March 2005, 11:01 PM

Quote:

Originally posted by johnnyx
If a fast fuse is specified, then it's best to forget the ones you have, and get some F5A ones. Look in a parts catalogue.
BTW there are ultrafast fuses designed to protect semiconductors, marked FF5A. Some industrial ones are designed to protect thyristors etc. in inverters for motor control. I'm not that sure how effective they are though, I only get to see the ones that have failed.

Hey Johnny X, are you saying with the FF fuses, BJT's no longer need to protect the fuse?

Jonathan Bright · 22nd March 2005, 10:00 PM

No, I think Murphy's Law (electronic component sub-set) is pretty rigid at this point. "A fast acting fuse will always be protected by the semi conductor blowing first")

21st March 2005, 04:48 PM	#1
bigpanda diyAudio Member Join Date: Jun 2004 Location: HKSAR	How fast are 'Fast Fuse' and how to identify? Hi, I have got some 5A fuse and my new project needs 5A 'Fast Fuse'. How to tell if mine are 'fast' and how fast is 'fast'? I cannot find any marks on the fuse expect the '5A' Tks for input

21st March 2005, 04:54 PM	#2
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	It will say F5A is it's a fast one or T5A if slow. If unmarked assume slow. __________________ www.readresearch.co.uk my website for UK diy audio people - designs, PCBs, kits and more

21st March 2005, 05:31 PM	#3
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, wired fuses come in 3 speeds. fast, standard & slow (or time delay), the markings would be F5A, 5A, T5A. All these are fairly slow compared to an MCB and extremely slow compared to BJT output device. Fuses rely on downstream robustness to survive the short term overload before the fuse opens. Transformers have a large amount of thermal capacity and hence lag to help them meet this overload. The real purpose of fuses is to stop the equipment or house wiring catching fire. __________________ regards Andrew T.

21st March 2005, 09:56 PM	#5
mpmarino diyAudio Member Join Date: Jun 2004 Location: Crunchville, where I don't fit in.	to test: Place the fuse across the + and - terminals of a car battery. If it blows in under 1ns it is a fast fuse. Be sure to test all fuses in this manner before installing them. Disclaimer: I'm just kidding of course __________________ Crazy Yankee.

22nd March 2005, 10:00 PM	#8
Jonathan Bright diyAudio Member Join Date: Feb 2004 Location: Queensland	No, I think Murphy's Law (electronic component sub-set) is pretty rigid at this point. "A fast acting fuse will always be protected by the semi conductor blowing first") __________________ "It was the Spring time of the year when aunt calls to aunt like mastodons across the frozen waste." P.G. Wodehouse.


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21st March 2005, 09:46 PM	#4
K-amps diyAudio Member Join Date: Nov 2003 Location: Indiana	If the fuse has a squiggly wire (not coiled or straight) it's probably fast

21st March 2005, 10:27 PM	#6
johnnyx diyAudio Member Join Date: Jan 2004 Location: manchester	If a fast fuse is specified, then it's best to forget the ones you have, and get some F5A ones. Look in a parts catalogue. BTW there are ultrafast fuses designed to protect semiconductors, marked FF5A. Some industrial ones are designed to protect thyristors etc. in inverters for motor control. I'm not that sure how effective they are though, I only get to see the ones that have failed.

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