Difference between UPS and Inverter.

Discussion in 'General Electronics Chat' started by cjdelphi, Jan 19, 2010.

  1. cjdelphi

    Thread Starter New Member

    Mar 26, 2009
    I once used to have a 350 watt modified sign wave inverter, but when i hooked it up to the PC, the CRT monitor went all crazy and ghosting madly, also the PSU seemed to have a high pitch wine coming from it, so i quickly turned off.

    about 2 years ago, i bought a 350 watt pure sine wave inverter, works wonderfully, Monitor and PC love it, so I assumed that a modified sine wave are no good for computers and true sine wave is what's needed.

    So how come I got a UPS on bargain today for like $60 (650VA 7a/h 12v battery) , and I was thinking one day i'll wire it up to a deep cycle/car battery to run the computer on for hours not minutes)

    It works just how it should great stuff.... hang on?... Modified Wave (Square Wave) what? I bought a True Sine wave because i thought modified signwave is useless for a computer, so here we are running the computer and speakers with no interference no nothing, perfectly...

    What's going on here, home come there's low interference going on from an inverter but much much higher from a standalone inverter?...

    Is there huge filtering going on or something? I'd have just bought a UPS instead of the pure sine wave inverter if i'd have known.... something's different from the inverted wave form and leaving out as 240volts....
  2. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    Most computer gear uses switch mode power supplies. The first thing power goes to in these (after noise filters) is a bridge rectifier feeding smoothing capacitors.

    These will work on sine wave, square wave or plain DC at the correct voltage.

    That original inverter may have been over-rated by the maker, or it could have been sensitive to power factor - simple switched-mode supplies have a terrible power factor. (Note; recent & higher quality ones may have power factor correction).

    The waveform is more relevent to things like transformers and induction motors, they need something near a sine wave.
  3. JDT

    Well-Known Member

    Feb 12, 2009
    A UPS is not designed to work from the batteries for very long. Just long enough to properly back things up and shut down the computer. Most of the time it is powering you computer direct from the main supply. It will not have sufficient cooling for long term operation at full power. It will not give out a pure sine wave. In fact, expect it to give out an almost square waveform.

    Don't know why your computer works with this UPS and not the other "modified square wave" inverter.
  4. 3ldon

    Active Member

    Jan 9, 2010
    What did you power the 350 watt modified sine wave inverter from?
    Also, did it have voltage isolation between teh 12 volt side and teh 120 vac side? many do not.
    Most pure sine wave inverters do have voltage isolation, however there are plenty of people who have grounded the "ground" of said pure sine inverter (their 12 volt side was grounded) and it resulted in flames.
    another issue is the "ground" of most cheap and some expensive pure sine inverters is actually floating halfway between 120 and 0, with the "neutral" being grounded.

    funny thing is the cheap 12 volt lead acid based UPS's are often more robust than the more expensive inverters, they just aren't as efficient. you can't count on being able to connect the negative battery post to the "ground" however, every manufacturer is different.

    In any case, an isolation transformer takes care of the above issues.

    Most cheap UPS run the transformer at 110% of design voltage, and most 12 volt units use a pushpull topology, using 9-13 turns of 2, 12 or 14 gauge wires in parallel, the secondary being 140-170 turns of 18 gauge wire.
    It is trivial and I have done it several times (there is often plenty of space) to add one or two extra turns of wire to both sides of the "primary", and 10 more turns to the secondary, and add a fan or bigger heatsinks to the fets, and this will turn a standard off the shelf 80% efficinet UPS to one that is 85-87%, and it will run 2x as long before overheating. At the cost of voltage regulation, as the core is no longer saturated, its not going to be as "good"