| ??? 05/27/06 03:47 Modified: 05/27/06 03:48 Read: times |
#117167 - Fuses Responding to: ???'s previous message |
David said:
Kai, just picked up your comment implying fuses MUST be in the secondary of the transformer winding. What justification? In 1996 we were informed by our distributor of mains transfomers, that manufacturer will change the recommendations about using fuses with their mains transformers, becoming valid in 1997. We were told, that for our transformers providing two secondary windings, primary fuses are no longer recommended and that secondary fuses will become mandatory. Then, we contacted several manufacturers of mains transformers and told them, that to us this change wouldn't make any sense, because we standardly want to put fuses in the primary winding, to be able not only to fuse an overload at secondary winding of transformer, but also an accidental short internally of primary winding. We were told then, that with the introduce of "Niederspannnungs-Richtlinie" (sorry, don't know the english expression) in 1997, as part of the new CE-standards, new standards for mains transformer become valid. Concretely spoken, when shorting an output of mains transformer the involved fuse is allowed to take a certain period of time to blow, which is in the range of dozens of seconds (don't remember the exact value). With the new standards this delay has been reduced, so that a primary fuse will no longer fullfill the new standards. Why that? A primary fuse must be rated some higher than the maximum current rating of transformer to allow the inrush current to flow, when the core becomes magnetized when turning on the mains voltage. But with this headroom the primary fuse takes a too long time to blow, when a secondary winding is shorted. Only the use of secondary fuses will fullfill the new standards, because they do not suffer from the inrush current and can more tightly match with the current rating of transformer, resulting in a faster blow when shorting an output winding. At that time almost all manufacturers of mains transformers demanded the use of these secondary fuses and dropped the primary fuse. In the meantime you can see some manufacturers again recommending primary fuses. Maybe they have modified the recipe for their transformers a bit... Kai |
| Topic | Author | Date |
| smps/linear ps. | 01/01/70 00:00 | |
| the moon is in the 3rd quarter | 01/01/70 00:00 | |
| Post circuit diagram | 01/01/70 00:00 | |
| Break of GND line? | 01/01/70 00:00 | |
| Maybe he's forgotten the caps? | 01/01/70 00:00 | |
| Any connection to Phone line ? | 01/01/70 00:00 | |
| Have you examined the linear PS? | 01/01/70 00:00 | |
| one caveat | 01/01/70 00:00 | |
| granted, but that's a misapplication. | 01/01/70 00:00 | |
| his question is why does it fail with 78 | 01/01/70 00:00 | |
| True enough, but until he does the work | 01/01/70 00:00 | |
| as usual | 01/01/70 00:00 | |
| Natural enemy | 01/01/70 00:00 | |
| Quite true, Kai | 01/01/70 00:00 | |
| You are experienced, I see... | 01/01/70 00:00 | |
| I just wonder | 01/01/70 00:00 | |
| When I need a clean signal | 01/01/70 00:00 | |
| but then you have to clean up | 01/01/70 00:00 | |
| Fuse in secondary circuit. | 01/01/70 00:00 | |
| Fuses | 01/01/70 00:00 | |
| Slow-blo? | 01/01/70 00:00 | |
Yes, but... | 01/01/70 00:00 | |
| schematic | 01/01/70 00:00 | |
| no schematic here ... ??? | 01/01/70 00:00 |