On Sat, Apr 07, 2012 at 01:26:36AM -0700, Keith Lofstrom wrote: > The most likely failure for the system power supply is the > electrolytic capacitors at the output - cheap ones fail, > and the output can spike. If the power supply is dusty,
BTW - How this fails, as an aid to understanding: All computers contain "switching" power supplies - the line power is rectified, stored in large capacitors, then made into a medium frequency (around 100 KHz) "rectangular wave", banging back and forth between positive and negative. The voltages are around 400V - lethal! By changing the time ratio between the positive and negative half of the pulse, a regulator circuit changes average values of output to compensate for variations in input voltage, as well as the droop of the voltage stored in the capacitors between the 2 times 60Hz charging pulses. By using a high frequency, switching power supplies can use smaller transformers to turn the 400V into the 5 and 12 volts that the supplies make. The signals on the output are "AC", centered around zero, and the positive end of the signal is selected with a rectifier (or in more sophisticated power supplies, a transistor switch). the output of the switch bumps up and down, when the signal is present or absent. After the switch or rectifier, we need to make a steady DC voltage out of that bumpy one. We filter it with capacitors and inductors. Inductors rarely fail, but the capacitors are getting hammered with big current spikes in, followed by the current drain of the disk or motherboard - for disks, the current drain is also highly bumpy. And that stresses and heats the capacitors. Cheap capacitors fail over time; they heat up and the magical electrolytic juice inside either evaporates or is chemically transformed into sludge. The capacitors no longer store as much energy, and their output bumps up and down. The valleys cause electronics to forget - the peaks can fry electronic devices. These peaks and valleys are happening thousands of times a second; you won't see them with a voltmeter, and need an oscilloscope to really see what is going on. First-rate server grade components wear out more slowly, and have designed in spare capacity, so they will take longer to fail. Advertising claims and reputation manipulation don't make a system server grade; those of us who aren't experienced power supply designers (including me) will have a hard time detecting second rate or counterfeit capacitors and other components. We want the most trustworthy power supply we can afford - if only we could figure out what was trustworthy. With outsourcing and rebranding, your Antec Green Watt 800XYZ power supply might be coming from factory A in Thailand one week, and factory B in Shenzhen the next, and the components inside the supplies will have a similar varied history. You are relying on Antec to police the supply chain, and they are up against some very clever people happy to make shoddy substitutes in return for kickbacks from vendors. I met a reliability engineer who works at one of Apple's iPad assembly plants in China. The iPads are made from components sourced from more than thirty countries. The assembly plant has over 500 in-line automated test stations. Ex-pat engineers watch every step. Apple's walled garden no-add-ons "our way or go away" attitude limits choice but also removes uncertainty. Still, garbage can leak through (starting with stupid decisions in Cupertino). The only way I can see that would establish trustworthy systems is a completely transparent supply chain. You should be able to go online with the model and serial number and trace all the components back through the supply chain, identifying every supplier and material source. Armed with that, and field failure history, you (or your quality measurement software) could identify the incompetent or corrupt manufacturers, managers, even line workers, on the spot, in the store, and buy the slightly more trustworthy unit out of all the supposedly identical units. Once that got going, it would rapidly push back through the chain, rewarding quality and punishing the crapmeisters. But then, very few people care about this stuff, and buy any highly advertised piece of junk they imagine will cause their neighbors to envy them. See: http://www.youtube.com/watch?v=8AyVh1_vWYQ (rated R) Keith -- Keith Lofstrom [email protected] Voice (503)-520-1993 KLIC --- Keith Lofstrom Integrated Circuits --- "Your Ideas in Silicon" Design Contracting in Bipolar and CMOS - Analog, Digital, and Scan ICs _______________________________________________ PLUG mailing list [email protected] http://lists.pdxlinux.org/mailman/listinfo/plug
