Can Low Voltage kill a power supply?

Discussion in 'DIY PC' started by n33ck0@gmail.com, Aug 19, 2006.

  1. Guest

    Hi, everybody. 2 weeks ago ther was a case of low voltage in the
    neighborhood. After calling the utility co. it was determined there was
    too much demand on the grid. not anything specific in the house. So
    here is my question can a low voltage situation kill a computer power
    supply. Taking in account it is not a cheap one: I paid over $75.00US
    when new its rated @ 500w and it never gave me a problem before. Also
    its strange because the PS on a compaq desktop I also have didn't even
    flinch. Nothing else got burned or failed in my house. I know all
    components in that particular PC are good 'cause I tested it with
    another PS. Thanks Also is there any way I can check if is still good?
     
    , Aug 19, 2006
    #1
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  2. Rod Speed Guest

    A properly designed power supply should shut down in that situation.

    A badly designed power supply can be killed in that situation.
    The price doesnt mean a lot on that basic design question.
    Yeah, a properly designed power supply wont turn a hair.
    Thats all the evidence you need that its dead.
    You just did.
     
    Rod Speed, Aug 19, 2006
    #2
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  3. Guest

    Thank You Sir!. Suspicions confirmed.
     
    , Aug 19, 2006
    #3
  4. Just a bit curious as to how you think a power supply can be damaged by a
    low voltage situation?

    Since all the regulation in a PS is designed to convert AC to DC and then
    regulate those output voltages down to useable levels within a specified
    tolerance I don't see how a low voltage could result in a dead PS. I DO see
    how it can cause flakiness on the low voltage side since all outputs are
    based upon a properly regulated input voltage tolerance, but all that should
    result from a low input voltage is proportionally low output voltages.

    Personally I would think that most of the damage would have been caused by a
    backlash of higher voltage that can often occur after low voltage
    situations. This higher than normal voltage inrush could happen faster than
    the regulators are prepared to handle and cause a very quick spike to get
    through the filters and fry the lower voltage side of the regulation circuit
    perhaps.
     
    Ray Cassick \(Home\), Aug 19, 2006
    #4
  5. Sjouke Burry Guest

    A switching supply will draw a bigger current from the
    mains to compensate for less voltage, the output voltage
    wont drop ,power wont drop, so you need more current.
    When input is low enough ,and current becomes big enough,
    either the safety cuts in, or the supply blows.
    And that depends on the quality of the supply.
     
    Sjouke Burry, Aug 20, 2006
    #5
  6. Rod Speed Guest

    Basically its unlikely that it was just a clean sag in the
    mains voltage, its likely that there was a considerable
    over voltage as well as the excess load was removed
    or disconnected itself in the case of high motor loads etc.

    The failure may be something as basic as
    the internal fuse thats on the mains active.
    See above.
    That shouldnt happen, the ATX specs require that the supply
    shuts down cleanly when the output rails end up out of spec.
    Nope, because the ATX spec requires that the supply must
    shut down cleanly when the output rails go out of spec.
    Sure, thats what is likely what killed the badly designed
    supply. It couldnt handle that over voltage without dying.
    Its unlikely to be that, much more likely to just be the higher
    voltage that killed it if it didnt just blow the internal fuse.
    There is no regulation to handle inrush current, it
    just goes thru the usual filter and gets absorbed
    by the main caps that are used to rectify the mains.
    That would normally fry the chopper etc. The output diodes
    are generally pretty brutal devices that arent easily killed by
    that sort of thing that would have to get thru the transformer.
     
    Rod Speed, Aug 20, 2006
    #6
  7. Rod Speed Guest

    That increased current isnt what kills a badly designed supply.

    The only current that goes up is from the mains and the diodes
    that rectify the mains arent that marginal current capacity wise.

    What actually kills a poorly designed power supply in that
    situation is the inevitable associated mains surges as loads
    trip out due to the low mains voltage, particularly motor loads.
     
    Rod Speed, Aug 20, 2006
    #7
  8. John Doe Guest

    If you are concerned about poor house current, consider buying a
    voltage regulator (line conditioner). I've got a cheap one and it
    seems to do a good job. I don't need battery backup. I think a
    voltage regulator keeps things as stable as possible until it
    completely shuts down. That way your power supply doesn't see low
    voltages, it's all or nothing.

    Good luck.
     
    John Doe, Aug 20, 2006
    #8
  9. Very well can.
    The problem is the flyback regulator. Low voltage on the filter caps means
    an increased PWM duty cycle to compensate, stressing everything: drive
    transistors, flyback transformer, flyback diodes, etc.

    Of course, that shouldn't make it past the input suppressors and filter caps.
     
    David Maynard, Aug 20, 2006
    #9
  10. Rod Speed Guest

    Nope, not when it didnt drop enough to cause the other system to turn a hair.
    That shouldnt kill anything in a properly designed power supply.
    Shouldnt and didnt are too entirely separate matters.

    MUCH more likely than your scenario killing the power supply.
     
    Rod Speed, Aug 20, 2006
    #10
  11. Guest

    Low AC voltage results in a higher duty cycle, which can cause
    inductor saturation. When an inductor saturates it stops
    opposing changes in current, and this can cause the current to
    increase beyond the capacities of the driving transistors.
     
    , Aug 20, 2006
    #11
  12. Thanks to all for the time and additional explanations.

    I never did get into all the intricacies of switching PS so it was all
    appreciated.
     
    Ray Cassick \(Home\), Aug 20, 2006
    #12
  13. According to that kind of logic it didn't fail at all, yet it did.

    In theory nothing should... but it failed.

    That's funny considering you've just been singing the "properly designed
    power supply" mantra.
    Frankly, no. Surge suppression of the type you're arguing is relatively
    simple and fairly static from design to design. I.E. the power mains don't
    'change' when one goes from, say, a 400W design to a 450W. The flyback
    circuit, however, is a whole different situation. Get that off so the core
    goes into saturation with an unpredicted scenario and you've got fried
    flyback. And it's no very hard to do, especially when plagiarizing,
    'revising', or designing to power line 'specs' (meaning normal tolerances).
     
    David Maynard, Aug 21, 2006
    #13
  14. Rod Speed Guest

    Wrong again. One PSU is properly designed and the other isnt.
    Wrong again. One PSU is properly designed and the other isnt.
    Yes, clearly the one that did die isnt properly designed.
    Fraid so.
    Yes, but you dont know that the badly designed power supply
    would have survived that surge even in the 400W version.

    Likely it never did the surge suppression properly.
    It is however trivial to design the supply so that that cant
    happen by monitoring the rectified mains voltage and shutting
    the supply down when its low enough to produce that result.
    It is however trivial to design the supply so that that cant
    happen by monitoring the rectified mains voltage and shutting
    the supply down when its low enough to produce that result.

    Not a shred of rocket science required at all.
     
    Rod Speed, Aug 21, 2006
    #14
  15. Yes, your statement was 'wrong' because, as you note below, one is not
    right so comparing to "the other system" says nothing about whether low
    voltage killed it.
    Do you ever *think* about what's said or do you just invent whatever seems
    convenient?

    The statement was perfectly true. In THEORY, nothing should fail in a
    properly designed power supply.
    Which is what I've said all along.
    Glad we agree on something so now you can stop singing "in a properly
    designed power supply."
    'Fraid not.

    True, I don't. But surge suppression is trivial and relatively constant
    from design to design while flyback design isn't so the likelihood of
    screwing up one or the other or both is not the same.
    You mean "likely" because you're in the middle of an argument and saying
    "likely" seems a good thing to say.
    That costs money. But, no, it isn't 'trivial' because it takes knowing at
    what line voltage core saturation would occur (as but one consideration).

    That costs money. But, no, it isn't 'trivial' because it takes knowing at
    what line voltage core saturation would occur (as but one consideration).

    An opinion that proves you've never designed one.

    And with that I am finished with this thread. You can not post how it's all
    trivial and pretend you 'won' something.
     
    David Maynard, Aug 23, 2006
    #15
  16. Rod Speed Guest

    Pathetic, really.
    I never said that the low voltage is what killed it.
    Have you ever actually managed to bullshit your way out of a wet paper bag ?
    Irrelevant to what killed that particular power supply.
    Liar. You claimed a particular failure mechanism without any
    evidence that that is what actually killed that particular supply.
    Or I could tell you to go and **** yourself instead.
    Fraid so.
    Irrelevant when you dont know that the supply
    that failed has adequate surge suppression.

    And it aint just what is trivial that matters, there are plenty of examples
    where stupid cost cutting sees inadequate surge suppression.
    You dont know that the design was screwed up and that it
    wasnt just cost cutting that produced the inadequate design.
    Wrong, as always. I said that because even with the lower powered
    versions of PSUs, anyone with a clue has noticed that some do have
    inadequate surge suppression and other gross design deficiencys
    like killing what is power from the PSU when the PSU fails.
    So does surge suppression.
    Even the most basic testing of the design would show that problem.

    So it is indeed trivial to have the supply shut down when the
    input voltage has dropped below what the design can handle.
    So does surge suppression.
    Even the most basic testing of the design would show that problem.

    So it is indeed trivial to have the supply shut down when the
    input voltage has dropped below what the design can handle.

    Wrong, as always. Its a fact that there isnt a shred of rocket
    science required at all to have the supply shut down when the
    input voltage is below what the design can handle.
    You have always been, and always will be, completely and utterly irrelevant.
    Only silly little children give a flying red **** about 'winning', child.
     
    Rod Speed, Aug 23, 2006
    #16
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