Fuse Amp Ratings
May 21, 2013 9:25 AM
Is there a method or device that will determine the amp rating of a fuse? I know that most fuses are marked with their specific amp rating, but I really need a way to verify. These are standard 5mm and 6.3mm fuses used in consumer level audio equipment. TIA
The way you do it is to make it blow under controlled measured circumstances. There's no way to check it without destroying the fuse.
posted by Chocolate Pickle at 9:32 AM on May 21, 2013
posted by Chocolate Pickle at 9:32 AM on May 21, 2013
If you have a huge bag of them, you can get an idea of what they'd protect by testing enough of them to characterize at +25C. There are a lot of different types, including slow blow, fast blow, very fast blow. Testing at the fuse factory uses pulses to establish the melting point of the fuse link, not continuous current, at least according to Littlefuse.
These things are engineered components, so guessing the rating from a single use scenario is going to be a less sane protection method that determining what fault current you want to interrupt, how quickly, at what temperature, and what open circuit voltages you'll see once the thing is blown, and then choosing the right one from a catalog. Fuses are pretty cheap. Unless there's a lot of value represented by the bag of fuses you have, you should contrast that with the value of the item you are trying to protect. They are a safety device and safety is not a synonym for frugal.
Are there ANY markings on the fuses, at all?
posted by FauxScot at 10:00 AM on May 21, 2013
These things are engineered components, so guessing the rating from a single use scenario is going to be a less sane protection method that determining what fault current you want to interrupt, how quickly, at what temperature, and what open circuit voltages you'll see once the thing is blown, and then choosing the right one from a catalog. Fuses are pretty cheap. Unless there's a lot of value represented by the bag of fuses you have, you should contrast that with the value of the item you are trying to protect. They are a safety device and safety is not a synonym for frugal.
Are there ANY markings on the fuses, at all?
posted by FauxScot at 10:00 AM on May 21, 2013
The best you'll be able to do is characterize the fuses you have by blowing them as FauxScot describes. Typically fuses have a small margin above the stated current, but you'll have to discover what it is for yourself. If you want it to blow at a very precise current, that's going to be tricky.
You might wire a bunch of them up in series (end-to-end) so that the quickest of the series will blow, but it's hard to say what you really want based on the question.
posted by wnissen at 10:00 AM on May 21, 2013
You might wire a bunch of them up in series (end-to-end) so that the quickest of the series will blow, but it's hard to say what you really want based on the question.
posted by wnissen at 10:00 AM on May 21, 2013
What I have are after-market fuses across a wide sampling of amperage that are blowing when they shouldn't be, in different pieces of audio equipment from different manufacturers. The stock fuses do not blow.
posted by Brocktoon at 10:19 AM on May 21, 2013
posted by Brocktoon at 10:19 AM on May 21, 2013
You may have nuisance blowing. It's usually because you have some brief transient conditions that exceed the rating of the fuse, but not the damage threshold of the protected device. Some fuses are rated to blow in minutes at 200% load and hours at 125% load. Others blow in milliseconds.
What you have is by definition, unknown. All you know is that the things blow. Why is the question.
The manufacturer of the device you are protecting should have a recommended replacement. If I were trying to solve this dilemma, I'd find out what that replacement is, look up the equivalent Littlefuse, buy it and test it.... from a reputable component seller (Digikey, Newark, Mouser, etc.)
BTW, fuses are a pain. Nuisance trips are a pain. Smoked hardware is usually a bigger pain. I know of nothing commercial for fuse testing, but I could build one for you. I suspect it's cheaper just to buy good fuses, though as I am not a cheap date!
posted by FauxScot at 10:30 AM on May 21, 2013
What you have is by definition, unknown. All you know is that the things blow. Why is the question.
The manufacturer of the device you are protecting should have a recommended replacement. If I were trying to solve this dilemma, I'd find out what that replacement is, look up the equivalent Littlefuse, buy it and test it.... from a reputable component seller (Digikey, Newark, Mouser, etc.)
BTW, fuses are a pain. Nuisance trips are a pain. Smoked hardware is usually a bigger pain. I know of nothing commercial for fuse testing, but I could build one for you. I suspect it's cheaper just to buy good fuses, though as I am not a cheap date!
posted by FauxScot at 10:30 AM on May 21, 2013
I think you could do this non-destructively with two basic items:
The difficulty might be that at very low resistances, it might be hard to get a consistent reading from even the same fuse at different times by using the standard technique of touching the ends of the fuse with the usual pointed probes, but you can get around this by wiring a fuse holder with leads directly to the ohmmeter (easy with older meters, at least) and measuring the resistance of each fuse in the holder.
You could try to interpolate numerical ratings linearly from measured resistances even if your unknown fuses have significantly different resistances from any of your known fuses, but since resistance varies linearly with wire cross-sectional area for a given length, and heat dissipation varies with wire surface area, and positive feedback arises from resistance increasing as the wire gets hotter, I think that's infeasible, but you'd still be able to rank amp ratings in terms of greater or lesser.
posted by jamjam at 11:18 AM on May 21, 2013
1. an ohmmeter capable of measuring very low resistancesMeasure the resistance of the known fuses and compare those to the resistances of the unknown.
2. some fuses with very similar geometry to the ones you want to measure, but with known ratings.
The difficulty might be that at very low resistances, it might be hard to get a consistent reading from even the same fuse at different times by using the standard technique of touching the ends of the fuse with the usual pointed probes, but you can get around this by wiring a fuse holder with leads directly to the ohmmeter (easy with older meters, at least) and measuring the resistance of each fuse in the holder.
You could try to interpolate numerical ratings linearly from measured resistances even if your unknown fuses have significantly different resistances from any of your known fuses, but since resistance varies linearly with wire cross-sectional area for a given length, and heat dissipation varies with wire surface area, and positive feedback arises from resistance increasing as the wire gets hotter, I think that's infeasible, but you'd still be able to rank amp ratings in terms of greater or lesser.
posted by jamjam at 11:18 AM on May 21, 2013
Instead of testing them, why not try using fuses rated at a lower amperage?
For example, if you want 20 amp protection, put in 18 amp fuses and see if they blow. If not, then surely you are protected for 20 amps.
You could always have the 20 amp fuses ready for backup in a concert situation.
posted by weapons-grade pandemonium at 11:52 AM on May 21, 2013
For example, if you want 20 amp protection, put in 18 amp fuses and see if they blow. If not, then surely you are protected for 20 amps.
You could always have the 20 amp fuses ready for backup in a concert situation.
posted by weapons-grade pandemonium at 11:52 AM on May 21, 2013
It isn'down to the speed rating of the fuses is it? Fast blows will blow if a slow blow is specified. Marked F for fast and T for slow.
posted by BenPens at 12:11 PM on May 21, 2013
posted by BenPens at 12:11 PM on May 21, 2013
I was thinking the same thing, BenPens— audio equipment might well be spec'd to have a slow-blow fuse to avoid blowing during turn-on inrush or some other transient but still protect against a prolonged problem.
I think you could do this non-destructively with two basic items...
I wonder if you could do it more reliably by precisely measuring the fuse's change in resistance as the fusible link heats and cools after current pulses of various sizes? You could characterize the temperature rise for a given current (which should let you estimate the point at which it blows?) as well as the thermal time constants (which should let you distinguish fast/slow/etc fuses). It seems like an interesting puzzle. It would certainly be more expensive and time-consuming to do this than to simply throw away the existing fuses and buy new ones, though.
posted by hattifattener at 11:16 PM on May 21, 2013
I think you could do this non-destructively with two basic items...
I wonder if you could do it more reliably by precisely measuring the fuse's change in resistance as the fusible link heats and cools after current pulses of various sizes? You could characterize the temperature rise for a given current (which should let you estimate the point at which it blows?) as well as the thermal time constants (which should let you distinguish fast/slow/etc fuses). It seems like an interesting puzzle. It would certainly be more expensive and time-consuming to do this than to simply throw away the existing fuses and buy new ones, though.
posted by hattifattener at 11:16 PM on May 21, 2013
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Get a sampling of fuses, test them, understand who sort of variation there is from your fuse supplier...
But verifying that a given fuse will definitively blow at a given current? I suspect there's no good way to go about that.
posted by straw at 9:32 AM on May 21, 2013