Lesson learned about mini fuses

Thread Starter

Gibson486

Joined Jul 20, 2012
327
So, I thought those Littelfuse OMNI BLOK fuses were the best thing since sliced bread. So small and convenient.

I have a high current circuit (7 Amps) that was supposed to trip on over voltage crow bar circuit with a triac. So, I thought, well, I can just get an 8A Omni block fuse. Well, I had an over voltage event and the fuse never blew. Instead, it just sat in the tripped triac mode for a few hours and eventually burned. So, I went nuts trying to see why this did not work. After about 2 hours of doubting my circuit, I eventually went to the fuses data sheet (which is separate from the fuse holder and fuse datasheet).

1575663188886.png

Turns out if I want to fast blow this fuse, I need to go double the rating. Unfortunately, the power supply is rated to 10 amps....

Important lesson learned. Never assume anything and if stuff is very convenient, scrutinize the hell out of it!
 

Thread Starter

Gibson486

Joined Jul 20, 2012
327
I guess i never really thought of a fuse rating like that.....I have always just used 2 Amps or lower fuses, and I guess it makes sense because when things short, you are gonna be well over 2 amps anyways. Thanks!
 

Reloadron

Joined Jan 15, 2015
5,646
Yeah, lesson learned. Fuses have become a major study in what has become Fuseology of sorts. When selecting a fuse we always want to look at the Time-Current Characteristics charts for a given type fuse. The days of just grabbing a 10 amp slo blow or fast blo are long over.

Actually isn't Omni-Blok just the name of a fuse holder type? Just for example 354 Series OMNI-BLOK® Molded Base Fuse Block for 3AB/3AG Fuses. Which is for fuse types 3AB or 3AG fuses? Been quite a few years since I had to spec out fuses for a design.

Ron
 

MisterBill2

Joined Jan 23, 2018
7,050
So, I thought those Littelfuse OMNI BLOK fuses were the best thing since sliced bread. So small and convenient.

I have a high current circuit (7 Amps) that was supposed to trip on over voltage crow bar circuit with a triac. So, I thought, well, I can just get an 8A Omni block fuse. Well, I had an over voltage event and the fuse never blew. Instead, it just sat in the tripped triac mode for a few hours and eventually burned. So, I went nuts trying to see why this did not work. After about 2 hours of doubting my circuit, I eventually went to the fuses data sheet (which is separate from the fuse holder and fuse datasheet).

View attachment 193919

Turns out if I want to fast blow this fuse, I need to go double the rating. Unfortunately, the power supply is rated to 10 amps....

Important lesson learned. Never assume anything and if stuff is very convenient, scrutinize the hell out of it!
I have used some different mini-fuses to isolate sensor power of short circuited sensors on crash vehicles. They were in the 5 volt supply line, and they had to clear within 2 milliseconds if there was a short circuit. The fuses that I found would clear the line in less than 2 milliseconds, so it was OK. But they were not cheap, about $2.50 USD each.
 
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