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#21
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ThePunisher wrote:
Regal wrote: I read on some website that the capacitors in a PC's PSU can hold a charge for long after they have been switched off and that the charge could be fatal. Is this really so? Surely that is exaggerating? You sould check the PSU with a stroboscope before opening it. tee-hee |
#22
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On Thu, 15 Apr 2004 17:30:32 +0100, Piotr Makley wrote:
Thinkingof power supplies ... if a faster processor needing extra power was installed (say, it needs an extra 30 W) then would that noticeably reduce the life of the power supply? Generally no, it would be a progressively shorter lifespan but even so there are other factors that could decrease lifespan or cause failure before that became significant. On the other hand, if the power supply can't maintain the additional load it may cause a lot of additional ripple which is harder on the motherboard. Rather than looking at the incremental wattage increase a determination should be made of the total system power usage... not necessarily an exact figure but in the ballpark, then using a quality name-brand unit so at least there's some assurance it's capable of wattage stamped on it's label. |
#23
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On Thu, 15 Apr 2004 18:43:06 +0100, Michael Salem
wrote: VWWall wrote: The capacitors in the input circuit of a PC PSU are charged to ~320 V DC. This can, indeed, be lethal, but they are shunted by bleeder resistors which will reduce the voltage to a safe value in a minute or two. By the time you get the unit out of the computer case and remove it's cover, the voltage is probably safe. To be sure, wait five minutes after unplugging the PSU before touching anything inside. This mustn't be relied on. From my own personal experience (with a monitor): the PSU failed and the monitor died. The cause of the failure was an open-circuit high-value resistor (10 megohms?). This allowed a large capacitor to charge with no discharge path other than leakage. The effect was utterly dead-looking equipment holding a large charge even when switched off. One difference would be that an ATX power supply is going to continue supplying 5VSB, that being another drain. |
#24
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On Thu, 15 Apr 2004 18:38:30 +0100, Tim Auton
tim.auton@uton.[groupSexWithoutTheY] wrote: You feel free to advise people to do whatever you like, but some people prefer to advise people of undetermined ability to err on the side of caution. Do I wait? No. But then I know what I'm doing. It's beyond overkill to advise waiting 24 hours. Even if you didn't know what you were doing you should've known that there's another very obvious way an ATX power supply drains besides the bleeder resistors. |
#25
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Piotr Makley wrote:
VWWall wrote: Regal wrote: I read on some website that the capacitors in a PC's PSU can hold a charge for long after they have been switched off and that the charge could be fatal. Is this really so? Surely that is exaggerating? The capacitors in the input circuit of a PC PSU are charged to ~320 V DC. This can, indeed, be lethal, but they are shunted by bleeder resistors which will reduce the voltage to a safe value in a minute or two. By the time you get the unit out of the computer case and remove it's cover, the voltage is probably safe. To be sure, wait five minutes after unplugging the PSU before touching anything inside. A PC monitor uses high voltage, (up to 25,000 V), on the CRT. The tube glass envelope is used as a capacitor, and can hold a charge for some time. Because the energy content is quite low, contact with this very high voltage is usually not deadly, but may result in serious injury from muscle reaction. Don't remove the housing from a CRT monitor unless you understand how to safely discharge this voltage! Thinkingof power supplies ... if a faster processor needing extra power was installed (say, it needs an extra 30 W) then would that noticeably reduce the life of the power supply? The main enemy of any electronics is heat. 30W additional output means ~10 W additional heat produced in the PSU. Some marginal units can't support even their label wattage. It all depends on how close to the limit you're pushing the supply. Also the PSU fan takes its input air from within the case. This air will be hotter due to the 30 additional CPU watts, and without good case ventilation will raise the internal temperature of the PS still more. Virg Wall -- It is vain to do with more what can be done with fewer. William of Occam. |
#26
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Kevin Lawton wrote:
Timothy Daniels wrote: | "Regal" wrote: || I read on some website that the capacitors in a PC's PSU can hold a || charge for long after they have been switched off and that the || charge could be fatal. || || Is this really so? Surely that is exaggerating? | | A well-designed power supply has bleed resistors across the | big capacitors (usually the filtering electrolytics) and the charge | should be essentially dissipated within seconds, certainly a minute, | of shutdown. As for a "fatal" charge, what's the maximum voltage | used in a PC - 12 volts for the fans? Bottom line - have you ever | seen a "Danger! High Voltage" warning on a PC case? You do sometimes see that warning on the PSU ! A switched-mode power supply works by 'pumping up' capacitors to a high voltage with sudden bursts of energy and then regulating the output down to the required level, switching the current off and on as needed. Draw more current and it uses bigger bursts of energy in each 'pump'. That's why it is so efficient in size terms. If you were using a linear power supply to supply the currents used inside a modern PC it would be far bigger and dissipate a lot more heat. Close, but no cigar! There is no "pumping" of the input capacitors. They are either charged to the peak line voltage from a 240 V line input, or they are charged with a voltage doubler rectifier circuit from a 120 V line input. The approximately 340 V DC derived is then switched at a rate of about 50,000 cycles and then transformed down to the desired output voltages. The time the "switch" is on determines the output voltages, and PWM (pulse width modulation) is used to regulate these voltages. The main reduction in size is due to the small core needed for the output transformer. In addition, the input transformer is eliminated completely. Under normal circumstances, the bleed resistors should do their job - but you wouldn't open up the PSU under normal circumstances would you ? Under fault conditions - what if a bleed resistor has failed ? The most frequent reason I have opened a PSU is to replace the fan. Even with a failed bleeder there is enough leakage current to discharge the capacitors in less than the "many hours" often quoted. If you're really worried, let the supply remain un-plugged overnight. Then be very careful not to cut yourself on the sharp metal edges in most PSUs! Virg Wall -- It is vain to do with more what can be done with fewer. William of Occam. |
#27
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kony wrote:
On Thu, 15 Apr 2004 18:43:06 +0100, Michael Salem wrote: VWWall wrote: The capacitors in the input circuit of a PC PSU are charged to ~320 V DC. This can, indeed, be lethal, but they are shunted by bleeder resistors which will reduce the voltage to a safe value in a minute or two. By the time you get the unit out of the computer case and remove it's cover, the voltage is probably safe. To be sure, wait five minutes after unplugging the PSU before touching anything inside. This mustn't be relied on. From my own personal experience (with a monitor): the PSU failed and the monitor died. The cause of the failure was an open-circuit high-value resistor (10 megohms?). This allowed a large capacitor to charge with no discharge path other than leakage. The effect was utterly dead-looking equipment holding a large charge even when switched off. One difference would be that an ATX power supply is going to continue supplying 5VSB, that being another drain. Also the main switching supply will continue to operate until the capacitors are down to ~ 250 V, still more drain. With the excption of those PSUs that have heat sinks at an input voltage potential, it's very hard to get even your finger tips on the full voltage. A little care is still advised. Virg Wall -- It is vain to do with more what can be done with fewer. William of Occam. |
#28
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kony wrote:
This mustn't be relied on. From my own personal experience (with a monitor): the PSU failed and the monitor died. The cause of the failure was an open-circuit high-value resistor (10 megohms?). This allowed a large capacitor to charge with no discharge path other than leakage. The effect was utterly dead-looking equipment holding a large charge even when switched off. One difference would be that an ATX power supply is going to continue supplying 5VSB, that being another drain. The +5vsb typically uses a small bias transformer and is unrelated to the +300vdc buss. To confirm, monitor the +5vsb while you unplug the AC cord or switch OFF the rear panel switch. The +5vsb goes away instantly (or as soon as the +5vsb caps discharge.) |
#29
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Timothy Daniels wrote:
I read on some website that the capacitors in a PC's PSU can hold a charge for long after they have been switched off and that the charge could be fatal. Is this really so? Surely that is exaggerating? A well-designed power supply has bleed resistors across the big capacitors (usually the filtering electrolytics) and the charge should be essentially dissipated within seconds, certainly a minute, of shutdown. As for a "fatal" charge, what's the maximum voltage used in a PC - 12 volts for the fans? That's a maximum *external* voltage. The OP asked about *internal* voltages. The 300vdc buss charges to the peak value of the AC input sine wave (times 2 in the U.S.) or 132 * 1.414 * 2 = 373vdc max (typically 330-340 volts.) That's enough to give a nasty jolt. The amount of this charge left over time depends on, of course, the size of the capacitors and the bleed resistors (if any) in parallel with them. |
#30
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On Thu, 15 Apr 2004 18:43:06 +0100, Michael Salem
put finger to keyboard and composed: VWWall wrote: The capacitors in the input circuit of a PC PSU are charged to ~320 V DC. This can, indeed, be lethal, but they are shunted by bleeder resistors which will reduce the voltage to a safe value in a minute or two. By the time you get the unit out of the computer case and remove it's cover, the voltage is probably safe. To be sure, wait five minutes after unplugging the PSU before touching anything inside. This mustn't be relied on. From my own personal experience (with a monitor): the PSU failed and the monitor died. The cause of the failure was an open-circuit high-value resistor (10 megohms?). This allowed a large capacitor to charge with no discharge path other than leakage. The effect was utterly dead-looking equipment holding a large charge even when switched off. Yes, that's a very common failure mode. These "startup" resistors are typically of the order of 100K or more. They feed 350V to the base or gate of the main chopper transistor. If this resistor goes open circuit, then the PSU doesn't start oscillating, and the main filter cap doesn't discharge. - Franc Zabkar -- Please remove one 's' from my address when replying by email. |
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