| Electronics > Beginners |
| 500mA fuses blow at 1A, am I testing them incorrectly? |
| << < (3/5) > >> |
| Macbeth:
--- Quote from: amyk on April 22, 2018, 04:21:29 pm ---Fuses are inherently not precision devices, because they're based on physical processes which are approximate. If you want precision overcurrent shutoff, you'll need active circuitry. --- End quote --- Don't forget to protect the active circuitry with a fuse. |
| Specmaster:
--- 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] |
| John_doe:
Thanks again, everyone. I'll do some tests with 250mA fuses and if they are insufficient, I'll change my design. I'm really glad I asked this question, because before I did I just assumed that the current rating of fuses is the current in which they are guaranteed to break (or within a reasonable margin, something like +20/-20 %). I don't even know why I assumed that, it's been like that for as long as I can remember (30+ years). |
| Brumby:
With a lack of background knowledge, it is not uncommon for a simplified understanding to be taken as a sound basis for everyday living. I mean, it seems to work, doesn't it? But the fact is that fuses are fickle beasts. While you can rely on them acting, you can't be particularly certain as to when they will act. This characteristic is demonstrated in household power protection in Australia - well, NSW at least. Our standard power point circuits that were protected by 15A wire fuses could have those fuses replaced by 20A circuit breakers - with absolutely no wiring rework. The reason being that the point where the wire fuses would blow was so varied that they added a safety margin in the gauge of the wiring. However, with circuit breakers, the breaking current was much more predictable, so they allowed them to be higher rated, using up some of that safety margin - but without impacting safety. |
| IanMacdonald:
House supply MCBs also require about 2x rated current for a rapid trip. Did some test years ago with a single turn secondary transformer and a clamp meter. They are more accurate than open wire fuses, though. The logic behind this is that the cable takes time to heat up, and so can tolerate some overcurrent provided it's not maintained for too long. If you run a fuse or MCB at over the rated current it will eventually trip, though it make take some considerable time. In low power electronics served by a small mains transformer, fuses are not particularly good at preventing overload/overhating because the relatively high impedance of the transformer prevents enough current flowing for a clean blow. If the fuse were rated low enough to prevent the transformer burning out, it would cause nuisance blows. This is why most wall warts and the like use a thermal trip attached to the primary winding. |
| Navigation |
| Message Index |
| Next page |
| Previous page |