---Original Message----- From: Mike To: PICLIST@MITVMA.MIT.EDU Date: Tuesday, 12 August 1997 02:07 Subject: Reliability thinking and risk assesment >At 05:54 PM 8/11/97 +0930, MikeS wrote: > >>But do you turn off the following when leaving your house - VCR, clock >>radio, TV, microwave, hifi, fridge, freezer(ouch)? These all operate whilst >>in 'non-attended' mode. Some would be a PITA to turn off every time you >>left for a short duration - some not really possible (deep freeze) They all >>pose a risk though. > >Yes. I turn off everything that has a switchmode supply in it which is >not a brand name product by the biggest companies. So therefore, I use Do you pay attention to things like UL ratings? >smoke detectors with interlock to the mains power so it turns off the >power points within 10 seconds of being tripped (AND still leaves the >light circuit active so I can see at night in case of alarm). Nice idea that. Unfortunately, in some cases, by the time a sufficient volume of smoke makes its way to the smoke sensor, its a little late - you've got a self-sustaining fire. But it would stop electricity from becoming involved further. Its a good argument for metal enclosures for PC's too. That would reduce risk, compared to the nice, contoured ones many (incl Compaq) are producing these days. > >I leave the clock radio on since it has one of those very nice conventional >transformers AND a PTC in series with it. And if I go away for more than >2 days - I turn off the freezer - since I wouldn't risk the bacterial >contamination in case the utility turned off the supply while I was gone >and just happened to turn it back on a couple of hours before I got back, >I know of one person who got sick for this very reason ! Um, not nice. Be easy to make a cheap PIC cct that monitored freezer temp, and wrote info in NV memory to indicate dangerous temperatures. Good app for the 8pin device. > >Risk assesment is an area which I have some philosophical interest both >in terms of hardware design and software methodology. > >I am of the opinion that the 1 in 10 million is a bit optimistic when it >comes to PC switchmode power supplies - UNLESS they are from the top brand >name suppliers like HP, COMPAQ etc - then you *might* be close. > >In Australia we get lots of PC power supplies made by Chinese sweat shops >and I've seen how they treat integrated circuits and switchmode transistors >in respect of static - I am NOT impressed by what I have seen at ALL ! But Mike, if they're made in humid conditions, then that implies low static risk! > >Faith and electronics reliability (to me) are mutually exclusive concepts. Not implying faith, more pragmatism... You can never reach 100% reliability - so at what level do you declare a product ready to ship? > >A faithful switchmode power supply is 'almost' an oxymoron ! > Yet you put your faith (sorry, that word again) in linear power supplies? They use semis too, and a fault in them could cause the transformer to cook off - and cause a fire... PTC's only work up to a certain (power) level. And not all manufacturers fuse them adequately. MikeS