Are most "home" ups's really standby power supplies?

Discussion in 'DIY Computers' started by dave, Feb 16, 2012.

  1. dave

    dave Guest

    As I understand it, a UPS runs the inverter from the battery all the
    time - and just ensures the battery is charged from the mains. When
    the mains fails the battery continues to power the inverter (& hence
    the devices) until such time as the battery runs flat or the mains are
    restored. Is this correct?

    I cannot seem to mind this aspect described in any UPS spec sheets!
    Specifically does anyone know if the APC Smart-UPS 1000 works as I
    describe above
    dave, Feb 16, 2012
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  2. Yep.

    It wouldn't be a UPS if it didn't have a battery. The 1000 is the
    nominal watt-hours battery capacity at sane loading.

    Cheers - Jaimie
    Jaimie Vandenbergh, Feb 16, 2012
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  3. dave

    dave Guest

    Thanks Jaimie. Just that I was reading about some kit that runs off
    the mains then switches over to the inverter at power-fail time. i.e.
    The inverter only runs when the mains is not present. Trouble is of
    course there is a short-term power glitch due to the change over time
    - which kind of defeats the object of the thing.

    We are getting some shiort-term power outs here - just enough to lose
    the energy left in the PC PSU and so brings the machine down. These
    times are less than 0.5 second (guesstimate) - but that is quite long
    in power terms I suppose.
    dave, Feb 16, 2012
  4. dave

    GB Guest

    How big does a smoothing capacitor have to be to hold say 500 watts? At say
    12v, is that about 50 Farads? Am I right that a 50 Farad capacitor is kind
    of massive? So, you can't protect against temporary power glitches that way?

    My physics has become rather hazy over the years.
    GB, Feb 16, 2012
  5. dave

    GB Guest

    Partly to answer my own question
    GB, Feb 16, 2012
  6. Ah, right - sorry, I did misunderstand what you were asking, though
    the answer is pretty much the same anyhow.

    The APC kit definitely doesn't do that, the power runs through the APC
    system all the time and has no outage, although it does take a moment
    to cutover to batteries - but that is covered by internal capacitors,
    as well as those in the PSU of whatever devices are plugged in.

    I can't speak for other makes of UPS, but I'd assume anything that
    mentions line conditioning will be doing the same.
    It is - sub-10 microsecond cutover times are normal for a UPS.

    You can generally find the user manual/tech specs of any UPS online
    somewhere, and "transfer time" or "cutover time" is what you should
    look for. My APC 1400VA states 4ms.

    Cheers - Jaimie
    Jaimie Vandenbergh, Feb 16, 2012
  7. dave

    dave Guest

    A 50F capacitor, with 12 VDc supply has energy of approx. 3600 Joule.
    Dissipating this in just one second would result in develping
    0.06kWatts i.e. 60watts. I hope my calcs are correct anyway. Yes, a
    50F capacitor though would be about of the size of the Empire State
    building! Well maybe a bit less :)
    dave, Feb 16, 2012
  8. dave

    dave Guest

    Ah I see - thanks again. 4ms would be fine I'm sure - faster than 1/4
    of one mains cycle.
    dave, Feb 16, 2012
  9. dave

    Ian Guest

    That seems about right. All the low-end APC UPSs I've used (700VA and
    1500VA) switch from mains passthrough to inverter on a power fail,
    with a clunk, whirr and buzz - so probably a relay involved.

    There are no "capacitors in the UPS" to smooth over the transition,
    though capacitors in the PSU of the supplied equipment should hold
    up for the brief glitch, however this is worth testing: In one case
    we were running a TV and games console off a UPS. When the mains
    went off, the screen went blank. "S$$t!". A few seconds later the
    picture came back as if nothing had happened. What had happened was
    the cheap POS PSU on an HDMI switch hadn't held up, resetting the
    switch and losing the video while it renegotiated. Never had any
    trouble with computers with decent PSUs.

    One other thing is if you're suffering intermittent mains dips
    (rather than full-off power cuts) the UPS will tend to switch to
    inverter mode frequently, then back to mains a few seconds later.
    This is annoying (2 x clunk, buzz, whirr every few minutes) and
    knackers the battery. APC were decent enough to replace the
    battery under warranty though, so +1 for APC.
    Ian, Feb 17, 2012
  10. dave

    dave Guest

    Ah useful. It seems I need a "double converstion online UPS" (=
    expensive) - but these "home" ones (several hundred VA) may *not* be
    of that type. I see the Smart-UPS 1000 is a "line interractive"
    (=standby + autotransformer), which is not really what I need here.
    dave, Feb 17, 2012
  11. If its switchover time is short enough, it is.

    Cheers - Jaimie
    Jaimie Vandenbergh, Feb 17, 2012
  12. dave

    dave Guest

    Well I meant they are no so good for frequent very short dropouts as
    not the best usage mode for the batteries. But just to ask, with your
    APC unit, does it have any (audio) noise when in use (cooling fans
    etc). I presume there is no inverter buzz as the inverter is not
    running(?). In short - is it completely quite when on standby? I do a
    lot of musical editing and noise is an issue. I could "remote" a ups
    but that's a bit of hassle here.
    dave, Feb 17, 2012
  13. dave

    Daniel James Guest

    That's what's known as an "off-line UPS". They do work, but they're not
    as good (or as expensive) as an on-line UPS (which is what you first

    Google will find you pros and cons of the two, now you know the terms.

    Daniel James, Feb 17, 2012
  14. Hardly matters - a half second dropout will barely touch them, they
    stay at optimum charge the rest of the time. But since they're nasty
    lead-acids you need to replace them every few years anyway.
    Mine aren't directly comparable as they're 2U rackmount jobs. They
    have a very small 50Hz buzz which would be wildly annoying (I'm very
    sensitive to 50Hz buzz, and I've no doubt you are too being in music
    editing!), so I keep them in the basement.

    Running an extension cord (or better, wiring up a UPS-drive wallmount
    power socket) is well worthwhile.

    Cheers - Jaimie
    Jaimie Vandenbergh, Feb 17, 2012
  15. dave

    Joe Kotroczo Guest

    APC do have a "Smart-UPS Online" line of products:

    Joe Kotroczo, Feb 17, 2012
  16. Nach längere Bedenkzeit hat dave geschrieben:
    IIRC, the battery life expectation of a "double conversion" UPS is
    significantly shorter compared to the switch-over type.
    With both types you'll have the problem with degradation over the years.

    I remember one case where we had a power failure and the UPS went down
    after about 5 minutes, while it should have lasted 30 minutes.

    Helmut_Meukel, Feb 18, 2012
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