12 volt fuses

Thread Starter

nickmms

Joined Jan 2, 2014
40
Hi all
I have been testing some of our equipment and found that the fuses do not blow at correct amps. I am running a 12Volt PSU with an in line fuse on +12V... but the 5 amp fuses do not blow to about 10 amps. I have tried 2 different brands both are glass fast blow rating at 5 amps 250v. I searched on internet and from what I can gather even though it is rated at 250V it still should blow at 5 amps.
thanks
 

KJ6EAD

Joined Apr 30, 2011
1,581
The major fuse manufacturers all publish helpful documents explaining fusing time/current curves for their products. Bertus linked you to one.

Fuses don't open the instant a load current slightly exceeds their nominal rating. In most cases, to do so would cause nuisance opening. Typical specifications will include a maximum time allowed to open at 200% of nominal rating and it is far from instantaneous!
 

Thread Starter

nickmms

Joined Jan 2, 2014
40
The major fuse manufacturers all publish helpful documents explaining fusing time/current curves for their products. Bertus linked you to one.

Fuses don't open the instant a load current slightly exceeds their nominal rating. In most cases, to do so would cause nuisance opening. Typical specifications will include a maximum time allowed to open at 200% of nominal rating and it is far from instantaneous!
Thanks
I have been going through the stuff Bertus sent and other sites. I thought it might be about the 200% where it actually blows.
 

mcgyvr

Joined Oct 15, 2009
5,394
See the "time curve" in the datasheet for each specific fuse.
That tells you how fast and at what percent of the fuse rating a fuse should open up.
 

MaxHeadRoom

Joined Jul 18, 2013
28,619
If you want a fuse with a very fast sweep through, there is Rectifier fuses, intended principally to protect solid state equipment.
Apart from the normal slo-blo and fast blow rated types.
Max.
 

ian field

Joined Oct 27, 2012
6,536
HI bertus
had a look at the pdf ... but it all didnt not sink in
Still not sure why the 5 amp fuse would not blow till double the rating
Putting it another way what fuse type and rating would I need to blow at 5 amps for 12V DC?
thanks
Buried somewhere on one of the On-Semi power rectifier application handbooks is a few pages on fuse ratings - there's a graph plotting % overcurrent vs time for fast, medium & slow-blow fuses.

Apparently all 3 curves have a very shallow slope at 125% overload and the time axis went upto thousands of seconds.

I vaguely remember that one of the 3 curves intersected 1 second at 600% overload.
 

k7elp60

Joined Nov 4, 2008
562
Fuses also have a voltage rating. The fuses for type ATM and ATO or DC use are rated a 32V. This voltage rating is important as if the fuse is used in a voltage higher than 32V it may not open the circuit when it blows. AGC style are generally 250V, there are some exceptions of the slow-blow that are rated at 32V also. If a 250V fuse is used on a 12V circuit it may not open the circuit when it opens (blow). So it is important to use fuses in the right voltage range.
 

#12

Joined Nov 30, 2010
18,224
This resembles the fan speed thread in Projects right now. You can not buy a fuse rated for 5 amps that will blow today with 5.0001 amps going through it. Fuses are not intended to be precision current sources. They are designed to protect circuits from overt fault conditions. If you want to limit the circuit to 5.0000 amps you will have to design a circuit to do that.
 
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