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1 second UPS

Discussion in 'Electronic Design' started by stefanv, Dec 6, 2005.

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  1. stefanv

    stefanv Guest

    Hi, I’m new to this forum, impressed to see there are still a lot o
    people out there who love to play with electronics and do not mind sharin
    some good ideas. So here’s my situation;

    Dealing with a cabin with a lot of power outages. I have a trace inverte
    system providing DC from 12 volts batteries. I also have a deskto
    computer. When the grid power falls (or comes back) the inverter switches
    Most often not fast enough and the computer reboots :( I tried a simple UP
    (after the inverter) but it doesn’t recognized the modified sine wave fro
    the Trace as “clean” power, so it just runs its battery dead after abou
    20 minutes without consider the Trace is there take over.

    My search is to find an “UPS” that that will bridge the half a second
    need for the Trace inverter to kick in. Time being so short, I figure
    don’t need clean power, both voltage and frequency don’t need to b
    stable, just something half a second to fool my computer’s power supply A
    is still on.

    I was thinking along following lines:
    - feed power to computer using a NO contacts on a DPDT relay 110v AC tha
    goes on when AC (Trace) is present.
    - charge (before this relay)through a rectifier a 680uf 200v cap up.
    - create a 20ma 12v power using a rc bridge and a zenner over the cap t
    run a 555 at about 60hz
    - feed this frequency straight to an n-mosfet 200v 15amps switching th
    capacitors charge to the NC contacts of the relay .
    - put a resistor in series with the relay to run it at about 80V (so i
    still goes on at regular 110 supply, but falls faster when the powe
    starts going down.)
    - so when the power falls, the relay switches the line AC off and feeds
    straight square wave at 60 hz for as long as the cap will discharge.

    Again this is ugly power, but I only need it for half a second or less
    When the Trace inverter kicks in the relay will switch back on and regula
    power restored.

    Figuring charge in a cap is CV2/2 in watts/sec, a 680 uf cap should hold
    watts/sec. A computer power supply being approx 250 watts/hr, this is 0.0
    watts/sec. No I’m not expecting my cap to give me a full minute, tha
    would likely make something explode :)

    Comments? Better ideas? I like the challenge of doing this without
    transformer!

    Sorry for the long post.

    StefanV
     
  2. Noway2

    Noway2 Guest

    You may be able to find a transfer switch that will detect the power
    loss and transfer to an alternate source in a short enough period of
    time to keep your computer (or other loads) alive. The impression I
    have recieved is that computer power supplies have changed over the
    years, lengthening the time period that they withstand a power loss
    condition. The reason behind these changes, being to reduce the
    complexity and hence cost of the upstream transfer switches.

    I would suggest starting your search at the Asco website. I believe
    that they make relatively inexepensive devices, though the term
    inexpensive may be subjective..
     
  3. Dan Hollands

    Dan Hollands Guest

    If you still have the "simple" UPS system that works, why not connect your
    big battery in place of the battery in the UPS - assuming the voltages are
    compatible.

    --
    Dan Hollands
    1120 S Creek Dr
    Webster NY 14580
    585-872-2606

    www.QuickScoreRace.com
     
  4. w_tom

    w_tom Guest

    Once upon a time, some power supplies were made with an
    internal battery (sealed lead acid) so that power supply could
    continue to run for minutes after a power loss. Did not sell
    well because consumers need to see the big and separate UPS.
    Don't know if these power supplies are still available. But
    it is what you seek.

    Computer power supplies will normally maintain power output
    for a few tens of milliseconds of power loss. You need it to
    maintain power for hundreds of milliseconds. That typically
    means some type of battery as part of the computer's power
    supply. It was not a big battery - maybe about the size of
    six size D batteries. Once designed a 'PC' wall controller
    with that battery inside so that power loss left the PC
    working. They were becoming harder to find even back then.

    Otherwise the UPS needs power from some big ass line filter
    so that UPS does not confuse dirty power with a power loss.
    Such line filters will weight tens of pounds and cost a few
    $hundred. But it will (probably) solve the problem of using
    external UPS until the backup power kicks in. Have seen them
    used in factories where equipment had defective internal power
    supply; was crashing system for the most trivial of electrical
    problems.
     
  5. Ted Edwards

    Ted Edwards Guest

    The only 100% reliable system I've ever built for this sort of thing
    used a regulated switching supply to float charge a SLA 12V battery
    which powered an inverter which ran the computer. Since the first
    supply is regulated, the batteries are just sitting there being floated
    until the power fails. When this happens, The secondary inverter simply
    keeps going off the batteries. No relays, no switches, no glitches. I
    built that system back in the '70s.

    Ted
     
  6. Stefan,

    Guess you have to refresh your basics on electricity. The energy stored in a
    capacitor is CV/2 in J(oules) or W(att)s(econds). The energy required to
    power the PC for half a second is 250 * 0.5 = 125Ws, which is much, much
    more then your capacitor can hold.

    petrus bitbyter
     
  7. Mark

    Mark Guest

    another problem with relays...

    I tried something like this for a sump pump...

    the pump connected to the DPDT relay wipers,

    the power line to one set of contacts

    an inverter to the other set of contacts...

    I first tested this with a lamp load and when the relay switched, my
    inverter blew up...

    I suspect the relay contacts.... when opening at the peak of a cycle
    .....,can arc for a short while and the arc actually bridges the
    inverter to the power line which promply destroys it...

    I decided I would need complex time delays and two relays to absolulty
    prevent this...
    In my case I didn't care if the pump was off for a few seconds during
    the switch over, but I didn't like my inverter blowing up when the
    realys switched...

    So in your case, a realy may not do the trick....

    I agree with the other poster, get a big set of batteries for the UPS
    and just use it directly... it's already desgined to do the
    switchover... thats what I ended up doing for the sump pump...

    Mark
     
  8. Jim Thompson

    Jim Thompson Guest

    [snip]

    A lot of UPS's (APS for example) have an output that signals "Your
    computer must shut down in x minutes".

    Why not use said signal to start an inverter?

    Here, in Arizona, I've contemplated such a signal system to start a
    motor-generator set (I have a very expensive reef tank that wouldn't
    do well with any sustained power outage :)

    ...Jim Thompson
     
  9. Ken Taylor

    Ken Taylor Guest

    You need to make sure you use a make-before-break relay for that
    application. There are actual mains cut-over contactors which are made
    for the job.

    A satellite station I worked on previously had a big cut-over switch for
    the mains to generator. In auto-mode it was guaranteed to cut across
    within a cycle of the mains supply. Apparently the *really* big versions
    of this (ours was only 20kVA) would break your arm if you got in the way!

    Cheers.

    Ken
     
  10. Guest

    UPS are commonly availble for FREE with bad batteries, which are
    frequently 12 volts.

    So just put your computer on a surplus UPS, go oversize, and use
    seperate battery/s

    I say go oversize because the design run time isnt just battery
    capacity, its also about component heating.

    Oversize is cheap and effective....
     
  11. Mark

    Mark Guest

    make before break will connect the inverter to the power line and blow
    it up....

    even if you use a break before make relay like I did, the arc will jump
    across sometimes and still blow up the inveter


    Mark
     
  12. Ken Taylor

    Ken Taylor Guest

    I'm clearly losing it - I made double-sure I wrote the right thing coz I
    suspected I'd get it arse-about.

    Ken
     
  13. Your electric bill is in kilowatt-hours, not kilowatts per second.

    Your 250 watt computer would run for 1600 hours (not seconds) to equal
    your 400 kilowatt hour electric bill.

    Watts are joules per second. No hours involved.

    Joules are watt-seconds, not watts per second.
     
  14. stefanv

    stefanv Guest

    I see your point and my mix-up between watts-second and watts per second.
    will have to go back to my school books and refresh.

    Re my schematic at http://www.maximasa.com/cnc/1secUPS.jpg could I brin
    up the voltage using an H-bridge?

    Re transfering AC to DC, back to AC and back to DC. Indeed, it sound
    crazy, but it's a combination of 2 things.
    First the chalenge of making an idea work once it's in your head!
    Next, I know of many people with the same problem who wouldn't mind havin
    one of these (if it ever works) The luxury of just plugging in a devic
    in-line and having the problem fixed, is worth more to them than openin
    up computers, changing power supplies, opening up UPS, hooking externa
    batteries or buying overtock items and rebuilding them to fit the purpose

    The value for them is in an easy, off-the shelf, plug and play device.
    Yes, if ever, I would probably build 50 of them and sell them just abov
    cost. I love building things and I like the idea of a compact in-lin
    device vs a bulky one.

    It's a chalenge, I appreciate your advice!

    StefanV
     
  15. Guest

    Maybe get yourself some nicads and run the LT rails off those during
    power down. Perhaps monitor the 5&12v rails, and when they get close to
    min V spec, switch on your nicads, using a solid state switch. This
    could sit in a spare pci slot, making it nice and neat. It would be a
    UPS as well as a glitch coverer.

    NT
     
  16. Pat Ford

    Pat Ford Guest

    What about getting a 12V computer power supply, running that off the
    battries and charger the batteries with a charger?

    Pat
     
  17. w_tom

    w_tom Guest

    Why is "converting dc to ac then back to dc again" crazy
    when that is exactly what a computer power supply does?
    Computer power supplies that contain a 12 volt battery use
    that 'converting' feature to integrate a rechargeable battery
    into a non-interruptable computer power supply.

    Capacitors could accomplish same. Then we temper the theory
    with the reality of numbers. Some manufacturers made computer
    power supplies that includes a rechargeable battery.

    Meanwhile, we also call that a laptop computer.
     
  18. Stefan,

    This is exactly why I advised to refresh your basics. Maybe the confusion
    comes from the word "power". AFAIK it's used meaning both "force" and
    "energy" which are two different things. If you have 110V on your outlet and
    a fuse of lets say 16A then you have 110*16=1760W available. But that's not
    what you pay for. (Unless you have a minimum contribution.) You have to pay
    for the part of that 1760W * the time you use it. That's what you find on
    your bill.

    400kWh = 400,000Wh = 1440,000,000 Ws. Which directly shows why they use kWh
    on the bills. (If they really put kw/hr on the bill they do definitly wrong.
    Maybe you can even sue them for it. I would not be very astonished if they
    do, as bills are often made by bookkeepers that may have some knowledge of
    money but no knowledge at all of the products they sell.)

    So if you need 250W for half a second, you need 125Ws. (What's that in kWh?
    :) That's not much energy compared to the amount on your bill. Even a small
    battery can produce it - but not within half a second. To store it in a
    capacitor on the other hand requires quite a huge capacitor. Calculate for
    your own, using the CV/2 rule.

    To summerize the lecture :)

    Energy = Force * Time -> [J] = [W] * (or [Ws] but not [W/s] !)

    petrus bitbyter
     
  19. Chris Jones

    Chris Jones Guest

    No, the energy is 0.5 times the capacitance times the voltage squared.
    or C*V*V/2

    It is practical to power a computer for a second from a large capacitor, but
    that capacitor would be perhaps somewhat larger than a normal PC power
    supply.

    Chris
     
  20. Chris Jones

    Chris Jones Guest


    I think if you open up the PC power supply and find the terminals of the
    main input capacitor, these terminals could be brought to a shrouded
    (finger proof) insulated connector on the back of the power supply. You
    could then build a grounded box containing a bank of capacitors which would
    be connected essentially in parallel with the internal capacitor of the PC
    power supply. The larger capacitor could keep the supply running for a
    second or more. In order to avoid very large surge currents when the PC is
    first turned on, a high-power series resistor should be inserted so that
    the external capacitor charges gently. A diode across the charging
    resistor would allow the fairly large current to flow out of the capacitor
    when powering the PC.

    5Amp 400V
    --------|<|-------
    | |
    .-----/\/\/------.
    | 10kOhm |
    | .-----------
    | + | + |
    ----- ------------- \
    ----- ------------- /
    | PC cap | external \ 100kOhm
    | | cap / Bleed resistor
    | | |
    -----------------.-----------

    There are numerous safety considerations when building something like this,
    and if you don't understand the risks involved then it is better to learn
    about the dangers before considering building it.

    Chris
     
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