At 14.35 2008.05.28, you wrote: >Hi Mario, > >One thing which is usually done in cases where power supply >malfunction can damage things is to use a crowbar circuit. This >consists of an SCR and a triggering circuit which turns on the SCR to >short the power supply to ground in the event of the voltage going >over a certain amount. Sure! I hadn't thought of it. A micropower comparator and an SCR, and a fuse to blow. That shall work just fine! >If you google for crowbar circuit, you'll find >it. I'm not sure but I think that this can fairly easily be made to >draw only a few uA. You would also want to add a real fuse to the >input of the regulator so that the SCR would actually cause it to >blow. Yup, useful also as a flag/indicator. ;P Thanks, Mario > >Sean > > >On Wed, May 28, 2008 at 9:48 AM, Electron wrote: >> At 13.16 2008.05.28, you wrote: >>>At 07:02 AM 5/28/2008, you wrote: >>> >>>>Hello, >>>> >>>>It looks like there's no suitable 5V Transil/Tranzorb when you want >to protect >>>>a circuit that has to sleep most of the time (say draw no more than >>>>few uA's on >>>>its own when sleeping). >>> >>>You need to protect what (input, output, power supply?) , against >>>what (voltage, available current) , exactly? >> >> Power to several expensive and delicate +5V IC's. >> >> I already had a voltage regulator fail because of a stupid hardware >bug of one >> of the above IC's, which made it draw around 190mA in some particular moments >> (bug then acknoledged in the errata): the TPS71550 voltage regulator >fried and >> passed +20V to the rest of the board, frying everything. >> >> Because of this experience, I thought it would be wise to put a >simple zener-like >> device to protect the expensive chips from possible future failures >of the voltage >> regulator. But even if I knew zeners wouldn't do it, now I discover that even >> transils <10V have a lot of leakage current (those >=10V have indeed >very little >> leakage current, and in fact having worked with them I had the >memory that they >> had very little leakage current and would be a good solution to my >problem). I >> cannot offer more than few uA of leakage current due to the >circuit's very low >> consumption necessity. But it would still be nice to protect the >precious chips. >> >> >>>>A 5.6V Zener already draws too much current at 3.0V, go figure at 5.0V! >>>> >>>>5V Transils/Tranzorbs rarely are seen, and now I know why: if >reverse leakage >>>>current is in the urder of uA's on common transils/tranzorbs, on >>>>very low voltage >>>>ones instead it's about 800uA! So they aren't usable for micropower >circuits: >>>>http://www.farnell.com/datasheets/35080.pdf >>> >>>Zeners in general have softer "knees" at lower voltages. 3.3 and 2.4V zeners >>>are practically useless, so we usually use more complex circuits, >but sometimes >>>you can use the Vf of an LED. >> >> To reach 5V would require a white LED, and I'm afraid I'd have to select the >> right diode among many, since LEDs aren't characterized for this application. >> >>> >>>>In short, there's no way to protect with a diode a micropower 5V line? >>> >>>Is it an input, an output, a power supply, what? >>> >>>>Ok, "good designs never break", but my experience is different. >>>> >>>>I am now using a 50mA polyswitch as a solution (cannot see smaller >>>>ones), I just >>>>hope it reacts quick enough to eventual overcurrents. >>> >>>They are thermal in nature so they don't react particularly quickly, the >>>trip current is temperature sensitive, so you need to allow a lot of margin >>>for higher temperatures and tolerance, and their capacity for interrupting >>>current, and voltage rating, are often unimpressive. Voltage drop might be >>>an issue too. Other than that, they're great. ;-) >> >> i.e. almost nothing remains ;) >> >> Cheers, >> Mario >> >> >>> >>>>Best regards, >>> >>>Spehro Pefhany --"it's the network..." "The Journey is the reward" >>>speff@interlog.com Info for manufacturers: http://www.trexon.com >>>Embedded software/hardware/analog Info for designers: http://www.speff.com >>> >>> >>> >>>-- >>>http://www.piclist.com PIC/SX FAQ & list archive >>>View/change your membership options at >>>http://mailman.mit.edu/mailman/listinfo/piclist >> >> -- >> http://www.piclist.com PIC/SX FAQ & list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist >> >-- >http://www.piclist.com PIC/SX FAQ & list archive >View/change your membership options at >http://mailman.mit.edu/mailman/listinfo/piclist -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist