Bob, Thanks, lots of good information here. At 11:30 AM 2/11/03 -0800, Robert Smith wrote:
-BobYour requirements are not a great deal unlike modern automotive power source protection requirements."Handle anything, anyone could possibly imagine before or after the fact" Fred makes some great points. This approached can be enhanced by using an automotive type "load dump tolerant" regulator. They simply detach from the power source when te input voltage exceeds some (usually pretty high) level. No problems with burning fuses, wires, etc or with pissing off UL types. See TI for these, they have been doing a lot of work in this area. If you feel the need for a fuse, I also suggest the Raychem PolyFuse devices. I have had excellent luck with them in several applications. Just remember that their guaranteed hold/release ratings cover about a 2:1 range. Their RXE-250 devices, for example, are guaranteed (At 20 deg C) to hold at 2.5 A and trip at 5.0 A. And this is also a function of temperature. The trip/hold points vary as a function of the ambient temperature. To protect a 5.0 V bus, I prefer 6.2 V Zeners. Within tolerance they will clamp at 5.89 V (comfortably above 5.0 V + 10%) to 6.51 V (comfotably below the usual 7.0 V Absolute Maximum rating for 5.0 V parts). As to power rating, just get out your slide-rule and data sheets. A 6.2 Zener will dissipate about 15.5 W at 2.0 Amps, but will have to handle up to 32 W at 5.0 A. Common axial lead zeners from On-Semi are rated 5 W continous but handle 180 W surges up to 8.3 ms. This is ample time for a PolyFuse to trip. See RamChem for some of their (very good) applications data on these characteristics of PolyFuses. I think DeClan's concern about sucking power near the zener voltage is not a concern. I think he is thinking more about MOV type devices which do have a very sloppy breakdown curve. Ok for TV sets, but not for this application. The Zener "knee" is very sharp by comparison, leakage is in the uA level up to the point of conduction. Best wishes, Bob Smith
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Robert Ritchey
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