Voltage Rails???

paroxymparoxym Toronto, Canada
edited July 2003 in Hardware
:scratch:Excuse my confusion in this matter but I need to get this straightened out once and for all. :wtf:
Where should my voltage rails be at for optimum stability? I'm speaking in terms of the -+12, -+5, -+3.3 volt rails. I have googled it up and waded through a bunch of info and all I've gathered is that they should be within +/-5% of the actual voltage. Does this include the -5v,-12v rails as well? I have only seen people referring to the + rails.
Thanks in advance

Comments

  • Straight_ManStraight_Man Geeky, in my own way Naples, FL Icrontian
    edited July 2003
    NP, took me a while to test this out one week way back when with a VOM and an oscilloscope.

    Well, technically not much is done amperage wise with the - voltage rails, AND normally they are polarity inverted and low amperage takeoffs from the "positive" rails. So, they will be off by as much as the postive rails and knowing the positive rails are within -+ 5% of true ideal means for most computer PSUs that the negative rails will be also if the PSU is not so overloaded that there IS no negative voltage available or the takeoff part of the circuitry is not trashed (hard to trash JUST that part that inverts without first trashing the "positive" part).

    HTH
  • SpinnerSpinner Birmingham, UK
    edited July 2003
    Ageek said
    NP, took me a while to test this out one week way back when with a VOM and an oscilloscope.

    Well, technically not much is done amperage wise with the - voltage rails, AND normally they are polarity inverted and low amperage takeoffs from the "positive" rails. So, they will be off by as much as the postive rails and knowing the positive rails are within -+ 5% of true ideal means for most computer PSUs that the negative rails will be also if the PSU is not so overloaded that there IS no negative voltage available or the takeoff part of the circuitry is not trashed (hard to trash JUST that part that inverts without first trashing the "positive" part).

    HTH

    :rolleyes: So am I correct in thinking that what you are saying is that, in an ideal situation the '-' voltage should mirror that of the '+', but in more instances than not, that isn't the case. Which in itself isn't anything really to worry about. ?

    Because if my negative volatages (or whatever) are anything to go by, having a 5% variance would be extremely optimistic, considering how completely off the mark they are.

    In my experience, having less than perfect negative voltages is perfectly fine. I say that, because my system hasn't got ideal negative rails, and it's as stable as a rock.

    SPINNER
  • paroxymparoxym Toronto, Canada
    edited July 2003
    Thanks for the clarification there Spinner.

    Thats where this question spawned from -- the -5v rail.
    Currently my voltages are:
    +5v = 5.06
    -5v = -5.39
    +12 = 11.86
    -12 = -12.27

    My -5v rail is well over the 5% mark (sometimes dipping to -5.44), and although I havent experienced any stability issues yet, I wanted to make sure everything was kosher. ;)
  • Straight_ManStraight_Man Geeky, in my own way Naples, FL Icrontian
    edited July 2003
    Well, the only times you need negative voltages are:

    when it is in a PM mode, or when it is being powered up. So, if you have a PSU with all power allocated you would have ZERO negative values, but if you have within 80 of true the circuits will switch right. When things are sleeping or are hiberating or are suspending (depends on what you think of wordwise, hibernate and suspend use different power mgt strategies to do about the same thing-- which is to conserve power as much as possible while ideally being awakenable). They are usually ATX and CPU minor signal usages.

    So, both right, but a PSU that is not under a load that might be marginal for that PSU will also have negative rails to spec. The reason for a slightly overcapacity PSU is not that you have to have ti to run, it is that the excess capacity is supported by bigger Capacitors which can bridge sags, or undervoltages, than a weaker PSU with smaller caps would drop power almost to zero for a short time with same sag. Also, it can boot more gear at once-- boot time provides greatest draw on PSU's capacity.

    If you have a very good UPS os a continuous-on-battery UPS you can get by with a slightly smaller PSU becasue it has no sags to bridge. Better PSU will let you use a slower reacting UPS which typically is cheaper.
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