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| 500mA fuses blow at 1A, am I testing them incorrectly? |
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| Zero999:
--- Quote from: Specmaster on April 23, 2018, 05:18:37 pm --- --- Quote from: Psi on April 22, 2018, 12:28:21 pm ---That's 100% normal, it takes twice its rated current to immediately blow a fuse. A 500mA fuse will handle 500mA pretty much 24/7, though it might blow after a year or two since you are running it right on the edge of its rating. A 500mA fuse would blow at 750mA but it might take hours. A 500mA fuse doesn't magically know when you reach 501mA. Its all based on heat and getting the fuse wire hot enough to melt and that takes time. The fuse manufacture doesn't know what ambient temp is or how well the fuse is heatsinked to its holder, so there has to be a massive amount of added error margin in fuse design. The general rule of thumb in the industry is that a fuse blows at twice its rating. So your 500mA fuse blowing right away when you apply 1A is normal --- End quote --- You are spot on. Because fuses are so coarse and are capable of carrying far more than their rated current, MCBs were introduced to provide closer protection. This is a technical discussion in its own right and is far to deep for this medium. Most decent equipment designers will have rated their fuses so that at full power they were already creeping up to the point of blowing, ie the circuit at full chat might be taking 750ma and by rating the fuses at 500ma means that fuse will blow that much sooner as it only has to see a 33% increase in current to blow instantly instead of 100%. From mobile device so predictive text might have struck again [emoji83] --- End quote --- The problem with that is the fuse might blow, even though there's no overload present, especially at higher temperatures and after the fuse has aged somewhat. The correct approach is to use a 750mA fuse and overrate the circuit, giving plenty of safety margin, but that will incur an additional cost. |
| CJay:
There are several 'electronic' fuse projects out there which with a few components will give predictable and repeatable results but it's more complexity and could fail in a way that causes damage. |
| james_s:
The fuse is generally there to prevent a fire, it's sized to protect the smallest wiring in the chain, it's not going to protect a semiconductor in most cases. If a semiconductor does fail shorted that's when you need the fuse, to prevent the power cord from catching fire. Ideally you want some sort of active overload protection built into the device, and then you want a fuse there to prevent a major fault from cascading into a catastrophe. |
| Zero999:
--- Quote from: CJay on April 25, 2018, 02:36:25 pm ---There are several 'electronic' fuse projects out there which with a few components will give predictable and repeatable results but it's more complexity and could fail in a way that causes damage. --- End quote --- The correct solution is to use an electronic fuse, in addition to a real fuse, which will blow in the event it fails. The same is true with a transformer powering an LM317 circuit: never rely on the regulator's thermal/overcurrent for protection against fire. Always use a real fuse for that! |
| CJay:
--- Quote from: Hero999 on April 25, 2018, 06:31:27 pm --- --- Quote from: CJay on April 25, 2018, 02:36:25 pm ---There are several 'electronic' fuse projects out there which with a few components will give predictable and repeatable results but it's more complexity and could fail in a way that causes damage. --- End quote --- The correct solution is to use an electronic fuse, in addition to a real fuse, which will blow in the event it fails. The same is true with a transformer powering an LM317 circuit: never rely on the regulator's thermal/overcurrent for protection against fire. Always use a real fuse for that! --- End quote --- It did occur to me on my drive home that I should have added that but you beat me to it :) |
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