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#11
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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. 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 ? Kevin. |
#12
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"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? The maximum voltage inside a PSU is the mains of course! They should have bleeder resistors, but that doesn't mean they all do. When giving advice to people of undetermined ability and experience on usenet I think it's safest to assume the PSU is badly designed and will potentially hold dangerous voltages for some time after it's unplugged. If someone is sufficiently lacking in knowledge to have to ask the question they should assume that *every* part of the inside of their PSU is lethal for 24 hours after it's unplugged. Tim -- Love is a travelator. |
#13
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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. ROFLMAO :-) |
#14
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"Tim Auton" tim.auton@uton.[groupSexWithoutTheY] wrote in message
... | "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? | | The maximum voltage inside a PSU is the mains of course! | | They should have bleeder resistors, but that doesn't mean they all do. | When giving advice to people of undetermined ability and experience on | usenet I think it's safest to assume the PSU is badly designed and | will potentially hold dangerous voltages for some time after it's | unplugged. | | If someone is sufficiently lacking in knowledge to have to ask the | question they should assume that *every* part of the inside of their | PSU is lethal for 24 hours after it's unplugged. | Bollox ! |
#16
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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? Not sure what voltages the caps in a PC PSU would reach, but in general those caps can be dangerous because, once charged, they can deliver very high currents for short durations - enough to get some spectacular fireworks, burn things to ashes etc. |
#17
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"DCA" wrote:
"Tim Auton" tim.auton@uton.[groupSexWithoutTheY] wrote in message .. . [snip] | The maximum voltage inside a PSU is the mains of course! | | They should have bleeder resistors, but that doesn't mean they all do. | When giving advice to people of undetermined ability and experience on | usenet I think it's safest to assume the PSU is badly designed and | will potentially hold dangerous voltages for some time after it's | unplugged. | | If someone is sufficiently lacking in knowledge to have to ask the | question they should assume that *every* part of the inside of their | PSU is lethal for 24 hours after it's unplugged. | Bollox ! 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. Tim -- Love is a travelator. |
#18
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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. 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! A common way of taking advantage of the extra-high-tension available in a television set was to make an arc to earth and use it to light cigarettes. This was in valve days; don't try it with semiconductor EHT rectifiers with low impedance, as it'll fry them. Best wishes, -- Michael Salem |
#19
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In article ,
Parish wrote: 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. ROFLMAO :-) You have to leave the 'scope attached for 24 hours to get a valid reading ;-) -- Al Dykes ----------- adykes at p a n i x . c o m |
#20
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In article , Martin Slaney
writes 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? Not sure what voltages the caps in a PC PSU would reach, but in general those caps can be dangerous because, once charged, they can deliver very high currents for short durations - enough to get some spectacular fireworks, burn things to ashes etc. It would seem to me that the safest thing to do would be to don wellies and hose the system down with water before touching anything. -- Roger Hunt |
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