Slow Blow vs. Fast Blow Fuses

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eddiejg
Senior tube assistant
Posts: 163
(12/3/03 10:53 am)
 Slow Blow vs. Fast Blow Fuses
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 The 1st amp I built was from a kit that came with a fast blow fuse.
I've always used them in the amps I've built, but I've also noticed that a lot of others use slow blow fuses.
Pros & cons for each?
Thanks.
-Ed
 
cactustubes
Junior tube assistant
Posts: 18
(12/3/03 12:33 pm)
 Re: Slow Blow vs. Fast Blow Fuses
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 Thanks for that fuse info Ritchie.  

Edited by: cactustubes at: 12/4/03 11:27 am
 
ernie jr
Junior tube assistant
Posts: 41
(12/3/03 2:41 pm)
 fuses
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 fast acting fuses can blow when you flip the stand by switch or get any current surge when the fuse is near its limits. slo blo's wont blow as easily with a little surge
ernie
 
bnwitt
Senior tube assistant
Posts: 140
(12/3/03 3:10 pm)
 Re: fuses
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 As ernie says, slo blo fuses are better at withstanding start up conditions of guitar amps. This is because of the initial in-rush current associated with inductive loads which transformers are. Standard fuses are better suited for purely resistive loads. In my field (HVAC) you will find standard fuses on things like heating elements and slo blo fuses on motor circuits (inductive loads). If your amp (and it all depends on the amp's design) doesn't repetitively blow standard fuses (of the proper amp rating) then you may(and I stress may) get more instantaneous protection and suffer less collateral component damage in the event of a short in the circuit. You therefore, may wish to use them for that reason. On the other hand, if your amp blows standard fuses frequently, you would be better off using a properly rated slow blow fuse than putting in a higher amp standard fuse to keep the old girl running as that could be mucho bad for the amp in the event of a short or other over current situation.

Edited by: bnwitt at: 12/3/03 3:11 pm
 
Ritchie200
Senior tube assistant
Posts: 131
(12/3/03 11:37 pm)
 Re: fuses
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 Finally! Something technical that I can comment on with a reasonable degree of accuracy! I used to work for Bussmann Fuse. You all hit the nail on the head with the exception of Cactus' comment on the spring. You may see coils (for lack of a better description) on many types of fuses. Some could be for heat sink purposes to somewhat buffer current spikes(slo-blow). On a "fast acting" fuse it pulls the element apart at overcurrent to prevent any possibility of a reconnect due to heat expansion. Ever tap on a burnt out light bulb to get it to work again? Same principle, only for circuit protection you must make absolutely sure that when it blows - the circuit will remain dead. THe coil (spring) has absolutely no effect on how fast the fuse blows. Believe it or don't, the testing that goes on for these little 1/4 inch fuses is mind boggling! They must blow within established times by milliseconds, at certain percentages of the rated current load to be considered acceptable. There is an actual science to some of the crazy shaped elements of the larger amp sizes. Pretty cool stuff. I actually designed the stamping dies for the 1/4 inch and 5mm end caps and many of the elements. Hey Barry, the Class J, Tron, CC, were my elements - I'm sure you have seen a few of those! It was a neat job until they crated everything up and sent it over to China - which is why I am an ex-employee.
Jim
 
bnwitt
Senior tube assistant
Posts: 149
(12/4/03 6:17 pm)
 Re: fuses
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 Jim,
you are indeed correct, and mostly Buss Tron's. A slew of them. Probably not quite as many as you though. Too bad about the china thing. Bill Clinton opened the door with "favored nation status" and the move is still going.
Barry

Edited by: bnwitt at: 12/4/03 6:17 pm
 
Ritchie200
Senior tube assistant
Posts: 132
(12/11/03 2:41 pm)
 More boring info....
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 I just wanted to throw this out as more general info on fuses. I was just thinking back to a program we had called Fusology and some of the questions that were constantly being asked by end users. A better understanding might help determine problems and determine proper protection for our amps.

One misconception is that any current over the rated value of the fuse will cause the fuse to blow. Yes, and No! Yes, it will eventually open - but it may take some time. It takes about 200% over-current (to rating) for the fuse to blow instantaneously. Let's say your amp is drawing about 30% over-current. It would take an hour for the fuse to open. The performance graph ramps up fairly quickly, but there is a time factor. Does not matter if the fuse is a fast blow or slo blow, constant over-current performance is identical. Bottom line - if you blow a fuse, you have a problem! Check it out. I have seen very, very, few 1/4" fuses with quality issues that would cause it to blow under the rating. They are all individually tested before they get packed.

What does the voltage rating mean? When an element is vaporized, it can leave a carbon trail and pieces of the element still connected to the caps. The voltage listed is what the fuse can handle without secondary arcing after the element has failed. I can not remember how they figure the amp/voltage rating for this. A call to the Application Engineers can clear that up if you are interested.

These little guys are probably one of the cheapest components in your amp. Used properly they can tell you a lot about what is going on inside. Make sure you have plenty of spares of the correct rating. Going up is not an option (unless you determine there is legitimate increase in draw) and could be very expensive! You would not believe some of the stories/lawsuits!

Ok, you can wake up now...
Jim
 
Voxbox
Junior tube assistant
Posts: 57
(12/14/03 3:47 pm)
 more boring info......
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 Ace info Richie!

As a guy who has all sorts of fixed stuff for over 20 years, no piece of kit in my experience ever stopped working just because of the fuse! (only one time it was the fuseholder).
There is ALWAYS a reason.

The time taken to blow a fuse is critical in some applications, like sensitive electronics with those "chip" things in them. Therefore the faster blowing types should be used.

Cheers

Voxbox
 
Bradster
Junior tube assistant
Posts: 2
(3/1/04 1:18 pm)
 B+ fuses and catalog voltage ratings
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 So let me get this right in my mind, if one needs a 500mA fuse on a B+ line, and a fuse chart lists the fuses at 250v, wouldn't I need to extrapolate to hit the 500mA exactly at my B+ voltage? or is the current rating of a fuse dis-associated with the voltage
(don't see how that could be, but...)


For instance say an amplifier specifies a 500mA (1/2 Amp) fuse on the B+, but the B+ is like 400vdc so looking at the mouser catalog I see they list all the fuses rated like: "Amps@250v"

So would I extrapolate to get the rating at 400v, like say:

400/250 = 1.6

500mA x 1.6 = 800mA

and I then checking the catalog I see that Littlefuse lists an 8/10-Amp/250v (Littlefuse Part#313.8) fuse, would this then actually give me my 1/2 amp protection at the 400v B+ voltage?
 
 
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