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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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PC PSU shutdown condition with bad mobo caps
When a mobo has bad caps the PSU shuts down, but what triggers it? What does the PSU see from its point of view? Peak current spikes? -- Reply in group, but if emailing add one more zero, and remove the last word. |
#2
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PC PSU shutdown condition with bad mobo caps
Tom Del Rosso wrote: When a mobo has bad caps the PSU shuts down, but what triggers it? What does the PSU see from its point of view? Peak current spikes? Excessive ripple keeps the 'Power good signal' set to false. |
#3
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PC PSU shutdown condition with bad mobo caps
Michael A. Terrell wrote: Tom Del Rosso wrote: When a mobo has bad caps the PSU shuts down, but what triggers it? What does the PSU see from its point of view? Peak current spikes? Excessive ripple keeps the 'Power good signal' set to false. But whether or not that signal is involved in the chain of events (normally it drives reset and maybe not all designs necessarily use it that way) what is sensed on the PSU outputs is what I'm thinking about. Would it be voltage ripple and not just current all the way back in the PSU even if the PSU caps are good? And are PSU caps likely to be damaged too after use with a bad mobo? -- Reply in group, but if emailing add one more zero, and remove the last word. |
#4
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PC PSU shutdown condition with bad mobo caps
Tom Del Rosso wrote: Michael A. Terrell wrote: Tom Del Rosso wrote: When a mobo has bad caps the PSU shuts down, but what triggers it? What does the PSU see from its point of view? Peak current spikes? Excessive ripple keeps the 'Power good signal' set to false. But whether or not that signal is involved in the chain of events (normally it drives reset and maybe not all designs necessarily use it that way) what is sensed on the PSU outputs is what I'm thinking about. Would it be voltage ripple and not just current all the way back in the PSU even if the PSU caps are good? And are PSU caps likely to be damaged too after use with a bad mobo? No. If they are good, the PS will just shut down. The CPU PS generates its own control signals. They no longer run from 3.3V. The voltage is lower, and at a lot higher currents. That's why the quality of the electrolytic caps is so critical. I see people complain about the caps being too close to the CPU, but 1.8V @ 30A doesn't allow for long traces. |
#5
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PC PSU shutdown condition with bad mobo caps
On Thu, 26 Jul 2012 22:57:25 -0400, "Tom Del Rosso"
wrote: When a mobo has bad caps the PSU shuts down, but what triggers it? What does the PSU see from its point of view? Peak current spikes? The purpose of the caps that are close to the CPU is to reduce the ripple on the processor power line to tolerable levels. The processor can go from zero to 40 amps at anywhere from zero (ground bounce) to GHz rates. The capacitors have to smooth all that out. Basically, they form an energy storage system to deliver power during the high current spikes to the power hungry CPU. When the ESR (equivalent series resistance) of the caps increases due to electrolyte loss caused by overheating, less energy is available to the CPU during high current peaks. The result is lots of ripple and noise on the power supply line. Eventually, this gets the attention of the "power good" line to the power supply, which shuts down to protect the MB and CPU. http://en.wikipedia.org/wiki/Power_Good_Signal One irritating problem is the dominoes effect on multiple MB filter capacitors failing. When the ripple voltage is low across the capacitors, there is little ripple current through the capacitors. Capacitors only conduct during the time the voltage changes across them. When the voltage is pure DC, no current is drawn. They also only get warm when there's ripple current through the capacitor. When one capacitor (usually nearest the CPU) fails, the ripple current trhough the others increases slightly. That means that the internal heat generated in the other caps increases. When the next capacitor fails, the ripple current and internal heating of the remaining capacitors again increases, result in even more heating. Eventually, the few remaining caps can't handle the self-heating, overheat, bulge, and die. This is why you want to replace ALL the capacitors in a parallel string when one of them fails. Measuring the ESR of such a parallel string is impossible. Note that better MB's use polymer capacitors instead of electrolytics. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
#6
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PC PSU shutdown condition with bad mobo caps
On Jul 27, 9:22*am, Jeff Liebermann wrote:
On Thu, 26 Jul 2012 22:57:25 -0400, "Tom Del Rosso" wrote: When a mobo has bad caps the PSU shuts down, but what triggers it? *What does the PSU see from its point of view? *Peak current spikes? The purpose of the caps that are close to the CPU is to reduce the ripple on the processor power line to tolerable levels. *The processor can go from zero to 40 amps at anywhere from zero (ground bounce) to GHz rates. *The capacitors have to smooth all that out. *Basically, they form an energy storage system to deliver power during the high current spikes to the power hungry CPU. I'm going to expose my ignorance he are there not also small value capacitors in parallel, to take care of the GHz spikes? |
#7
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PC PSU shutdown condition with bad mobo caps
On Fri, 27 Jul 2012 09:48:48 -0700 (PDT), spamtrap1888
wrote: On Jul 27, 9:22*am, Jeff Liebermann wrote: On Thu, 26 Jul 2012 22:57:25 -0400, "Tom Del Rosso" wrote: When a mobo has bad caps the PSU shuts down, but what triggers it? *What does the PSU see from its point of view? *Peak current spikes? The purpose of the caps that are close to the CPU is to reduce the ripple on the processor power line to tolerable levels. *The processor can go from zero to 40 amps at anywhere from zero (ground bounce) to GHz rates. *The capacitors have to smooth all that out. *Basically, they form an energy storage system to deliver power during the high current spikes to the power hungry CPU. I'm going to expose my ignorance he are there not also small value capacitors in parallel, to take care of the GHz spikes? Yes. There are ceramic chip caps found on the bottom of the PCB, directly under the CPU. Short lead length required for high frequencies. None of the external CPU pins actually see GHz range signals as they are divided down before they hit the various buses. However, the rise times are well into the GHz (picosecond) ranges, which can have components in the GHz range, and must be bypassed lest they radiate RF and attract the attention of the FCC. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
#8
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PC PSU shutdown condition with bad mobo caps
Tom Del Rosso wrote:
When a mobo has bad caps the PSU shuts down, but what triggers it? What does the PSU see from its point of view? Peak current spikes? PG wire? Jamie |
#9
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PC PSU shutdown condition with bad mobo caps
Jeff Liebermann wrote: On Thu, 26 Jul 2012 22:57:25 -0400, "Tom Del Rosso" wrote: When a mobo has bad caps the PSU shuts down, but what triggers it? What does the PSU see from its point of view? Peak current spikes? The purpose of the caps that are close to the CPU is to reduce the ripple on the processor power line to tolerable levels. The processor can go from zero to 40 amps at anywhere from zero (ground bounce) to GHz rates. The capacitors have to smooth all that out. Basically, they form an energy storage system to deliver power during the high current spikes to the power hungry CPU. When the ESR (equivalent series resistance) of the caps increases due to electrolyte loss caused by overheating, less energy is available to the CPU during high current peaks. The result is lots of ripple and noise on the power supply line. Eventually, this gets the attention of the "power good" line to the power supply, which shuts down to protect the MB and CPU. That's the point of my question. So it sees increased ripple voltage and not just ripple current. That means the PSU caps are overwhelmed and might suffer some damage too. http://en.wikipedia.org/wiki/Power_Good_Signal I've known the power-good signal since the XT, but it used to be just for releasing RESET after the voltages came up. It isn't a very sensitive detector of excessive ripple current since the latter has to be worse than the load regulation spec to trigger it. In old designs it probably wouldn't have detected ripple at all since it had its own filtering to create a delay after power came up. Note that better MB's use polymer capacitors instead of electrolytics. But you can't substitute them for electrolytics, can you? -- Reply in group, but if emailing add one more zero, and remove the last word. |
#10
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PC PSU shutdown condition with bad mobo caps
Tom Del Rosso wrote: Jeff Liebermann wrote: On Thu, 26 Jul 2012 22:57:25 -0400, "Tom Del Rosso" wrote: When a mobo has bad caps the PSU shuts down, but what triggers it? What does the PSU see from its point of view? Peak current spikes? The purpose of the caps that are close to the CPU is to reduce the ripple on the processor power line to tolerable levels. The processor can go from zero to 40 amps at anywhere from zero (ground bounce) to GHz rates. The capacitors have to smooth all that out. Basically, they form an energy storage system to deliver power during the high current spikes to the power hungry CPU. When the ESR (equivalent series resistance) of the caps increases due to electrolyte loss caused by overheating, less energy is available to the CPU during high current peaks. The result is lots of ripple and noise on the power supply line. Eventually, this gets the attention of the "power good" line to the power supply, which shuts down to protect the MB and CPU. That's the point of my question. So it sees increased ripple voltage and not just ripple current. That means the PSU caps are overwhelmed and might suffer some damage too. The computer's power supply is on the other side of the CPU supply, but a ittle ripple can get into the 3.3V or 5 V rails and cause erratic operation. If the power supply is failing, it's usually just a coincidence unless it was a really crappy design. http://en.wikipedia.org/wiki/Power_Good_Signal I've known the power-good signal since the XT, but it used to be just for releasing RESET after the voltages came up. It isn't a very sensitive detector of excessive ripple current since the latter has to be worse than the load regulation spec to trigger it. In old designs it probably wouldn't have detected ripple at all since it had its own filtering to create a delay after power came up. Note that better MB's use polymer capacitors instead of electrolytics. But you can't substitute them for electrolytics, can you? They are also called solid polymer electrolytics. Substitution will depend on the max & min ESR allowed in the circuit. http://thecapking.com/plg1c821mdo1.html is one example. http://www.thecapking.com/whatwestock.html is a list of common low ESR caps used on motherboards. |
#11
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PC PSU shutdown condition with bad mobo caps
On Sat, 28 Jul 2012 01:18:15 -0400, "Tom Del Rosso"
wrote: So it sees increased ripple voltage and not just ripple current. Sorta. If any part of the noise or ripple on any of the power supply lines goes below the defined threshold, the power good line will drop. http://www.formfactors.org/developer%5Cspecs%5CATX12V_PSDG_2_2_public_br2.pdf See Fig 7 on Pg 25 and 3.3.1 on Pg 26. That means the PSU caps are overwhelmed and might suffer some damage too. Nope. There's quite a bit of DC resistance between the power supply and the ripple source, which is the CPU. The ripple is there not because the power supply is producing the ripple or noise, but because the CPU is NOT a constant DC load. The power consumption of the CPU varies radically what's being computed, power saving features, data transfer speeds, etc. The filter caps on the motherboard are actually protecting the power supply from too much ripple produced by the CPU. http://en.wikipedia.org/wiki/Power_Good_Signal I've known the power-good signal since the XT, but it used to be just for releasing RESET after the voltages came up. Things have changed a bit. See the timing specs in the ATX12V Power Supply Design Guide in the first URL. It isn't a very sensitive detector of excessive ripple current since the latter has to be worse than the load regulation spec to trigger it. If it meets the ATX design specs, *ANY* voltage excursion below the allowable design tolerances on any of the voltage lines, should initiate a shutdown. In general, the voltage tolerances at +/-5%. See 3.2.1. In old designs it probably wouldn't have detected ripple at all since it had its own filtering to create a delay after power came up. Yep. The old designs also didn't sense excursions of the 117VAC power supply input. Note that better MB's use polymer capacitors instead of electrolytics. But you can't substitute them for electrolytics, can you? Yes, you can, but only if the ESR is low enough. Many motherboards have a mix of solid polymer and electrolytic caps with the polymer handling the high ripple current and high temperature filtering, and the electrolytics handling the less critical filtering. The issue is primarily cost. Solid polymer caps are about 5-10 times as expensive as electrolytic. http://capacitorlab.com/capacitor-types-polymer/ -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
#12
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PC PSU shutdown condition with bad mobo caps
On Thursday, July 26, 2012 11:12:57 PM UTC-7, Tom Del Rosso wrote:
When a mobo has bad caps the PSU shuts down, but what triggers it? What does the PSU see from its point of view? Peak current spikes? I don't know if the Power_Good signal from the PSU is just a power-on reset signal for the motherboard or if the mobo is designed to shut down if that signal goes false. OTOH the mobo doesn't drive the Power_Good signal; the only thing it does drive is the Power_On signal on the green wire. Motherboards have several voltage regulators -- CPU, AGP or PCI-E, and memory, and I'm guessing they're designed to shut down the power in case of overload because it could be too dangerous not to (tons of amps for creating arcs and fires); I can't say for sure because I haven't looked for this protection on many mobos, but my ancient PC-2007 had a "U" shaped bar to sense CPU current. I don't know if PSU caps are damaged by bad caps on the mobo, but I doubt it's due to voltage but only current, and some reviewers have seen PSU caps and other PSU components fail when loaded to the PSU's full rated power with a testing machine -- see JonnyGuru.com , HardOCP.com , HardwareSecrets.com, which disect what they review, failed or not. |
#13
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PC PSU shutdown condition with bad mobo caps
En el artículo , Tom Del Rosso
escribió: And are PSU caps likely to be damaged too after use with a bad mobo? Yes. -- (\_/) (='.'=) (")_(") |
#14
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PC PSU shutdown condition with bad mobo caps
En el artículo , Tom Del Rosso
escribió: That means the PSU caps are overwhelmed and might suffer some damage too. Yes. All motherboards I've seen with failed caps in the processor voltage regulator cct have also had failing caps in the PSU. Replacing the PSU as well as recapping the board is what makes repair uneconomic. -- (\_/) (='.'=) (")_(") |
#15
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PC PSU shutdown condition with bad mobo caps
Mike Tomlinson wrote: En el artículo , Tom Del Rosso escribió: That means the PSU caps are overwhelmed and might suffer some damage too. Yes. All motherboards I've seen with failed caps in the processor voltage regulator cct have also had failing caps in the PSU. Replacing the PSU as well as recapping the board is what makes repair uneconomic. I don't see that combination of failures in the PCs I rebuild. Power supplies are usually easier to recap than a motherboard. Good 105C low ESR caps are cheap in quantity. Jeff posted one link. Here are some others that I've bought from: http://www.thecapking.com/whatwestock.html I buy some Panasonic FM series in bulk from www.digikey.com |
#16
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PC PSU shutdown condition with bad mobo caps
On Sat, 28 Jul 2012 13:49:19 +0100, Mike Tomlinson
wrote: En el artículo , Tom Del Rosso escribió: That means the PSU caps are overwhelmed and might suffer some damage too. Yes. All motherboards I've seen with failed caps in the processor voltage regulator cct have also had failing caps in the PSU. That's not what I've been seeing. I don't do too much recapping, but replacing the MB caps usually solves the problem. I have seen bad power supplies, but MB cap failures seem more common. One common characteristic I see in failing ATX power supplies is that they tend to have only a few physically small capacitors in the DC output filter section. The ATX supplies that are crammed full of big electrolytics seem to survive nicely. Replacing the PSU as well as recapping the board is what makes repair uneconomic. Ummm... it's easier to recap a power supply than a mother board. The PS comes out easily, is easily disassembled, usually has a single sided low density PCB, and is easy to test after recapping. The MB requires removing lots of hardware, replacing many more small caps, is a multi-layer PCB, and takes more time to test. What makes MB recapping uneconomical is that rather large number of caps that usually need replacement. Testing the caps on the MB is problematic because many are in parallel. The only sure way is to replace anything that might fail. That's usually about 12-15 caps on a typical MB. I can frequently buy a used MB on eBay for less than what I charge for my time. I still do recapping because I detest creating eWaste, but I don't make much money on recapping. -- Jeff Liebermann 150 Felker St #D http://www.LearnByDestroying.com Santa Cruz CA 95060 http://802.11junk.com Skype: JeffLiebermann AE6KS 831-336-2558 |
#17
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PC PSU shutdown condition with bad mobo caps
On Saturday, July 28, 2012 5:49:19 AM UTC-7, Mike Tomlinson wrote:
En el artículo , Tom Del Rosso escribió: That means the PSU caps are overwhelmed and might suffer some damage too. Yes. All motherboards I've seen with failed caps in the processor voltage regulator cct have also had failing caps in the PSU. The OST brand caps in that voltage regulator in my ECS K7S5A Pro failed, as did the PC-Tur Elite caps (CEC International?) in that regulator in my ECS P4S5A, but the caps in their PSUs were fine, and all were Panasonic, Rubycon, or Nichicon. |
#18
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PC PSU shutdown condition with bad mobo caps
Jeff Liebermann wrote: On Sat, 28 Jul 2012 01:18:15 -0400, "Tom Del Rosso" wrote: So it sees increased ripple voltage and not just ripple current. Sorta. If any part of the noise or ripple on any of the power supply lines goes below the defined threshold, the power good line will drop. http://www.formfactors.org/developer%5Cspecs%5CATX12V_PSDG_2_2_public_br2.pdf See Fig 7 on Pg 25 and 3.3.1 on Pg 26. I don't see any mention of current so it isn't 'sorta' triggered by ripple voltage. That means the PSU caps are overwhelmed and might suffer some damage too. Nope. There's quite a bit of DC resistance between the power supply and the ripple source, which is the CPU. That would matter if the supply had sense lines, but PC supplies don't. The voltage is sensed at the supply's output. But I understand you to mean that although the caps in the PSU are working harder, the discharge is small enough that they won't be heated unless they were defective as well. -- Reply in group, but if emailing add one more zero, and remove the last word. |
#19
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PC PSU shutdown condition with bad mobo caps
On Fri, 27 Jul 2012 09:22:19 -0700 Jeff Liebermann
wrote in Message id: : The purpose of the caps that are close to the CPU is to reduce the ripple on the processor power line to tolerable levels. The processor can go from zero to 40 amps at anywhere from zero (ground bounce) to GHz rates. The capacitors have to smooth all that out. Basically, they form an energy storage system to deliver power during the high current spikes to the power hungry CPU. 40A? Try 110A! Take a look at page 82. http://www.intel.com/content/dam/www...-datasheet.pdf For the high end 130W Xeon over 200A: http://www.intel.com/content/dam/www...-datasheet.pdf Page 22. |
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