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Using a UPS to Isolate and Test a Homebrew Offline smps

Discussion in 'Electronic Design' started by D from BC, Feb 20, 2007.

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  1. D from BC

    D from BC Guest

    Today, a UPS with a dead battery was given to me..

    A label underneath reads:
    The output of this device is not sinusoidal. It has a THD of 68% and
    max. harmonic of 57%.

    Crazy Question: Can I make use of this UPS to create an isolated
    120VAC or 170VDC source to safely test my homebrew offline smps with
    an earth grounded oscilloscope?
    The output power rating is ok.
    I'm assuming the UPS uses a lead-acid 12V batt..
    I have a car battery. Arrfff Arrfff... :)
    To hack or not to hack?

    Maybe I can dodge ordering an isolation transformer from
    Digikey...(About $45.00CAD)

    My local surplus electrical junk shop had a pile of transformers but
    no isolation types...strange...The owner looked too....nothing.

    Too poor to get an isolated Tektronics TDS digital phosphor scope :(
    D from BC
  2. Gibbo

    Gibbo Guest

    The peak voltage will be somewhat lower and obviosuly there will be
    masses of harmonics. As you are presumeably rectifying first, the
    harmonics won't be much of an issue. The lower peak voltage might be.

    Also many UPSs are only rated for *very* short run times. Say five or
    ten mins max. That also may not be an issue.
  3. PeterD

    PeterD Guest

    *most* UPSs use 24 volts (two 12V or four 6 volt batteries) not 12

    I'd suggest the isolation transformer is the best route to go.
  4. Tim

    Tim Guest

    All of the APC ones I have dissected had 12 volt Gel Cells in 'em. One
    cell per unit. I have salvaged parts from 4 different models of APC.

    - Tim -
  5. Henry Kiefer

    Henry Kiefer Guest

    | All of the APC ones I have dissected had 12 volt Gel Cells in 'em. One
    | cell per unit. I have salvaged parts from 4 different models of APC.

    At least the APC types have sinus output. Connect to 12 volts batteries in series and provide sufficient cooling.
    You can connect a 24 volts power supply (lower power) for the batteries instead. Or solar panels...

  6. Rich Grise

    Rich Grise Guest

    Well, you don't need to "isolate" it from anything - just set it on
    a board so it doesn't short to the concrete floor, and voila! Isolation!
    Of course, unplug the input cord.

    As far as using it as an input to your SMPS, it depends on what your
    SMPS input wants - it could be a good test of "What kind of line crap
    can my new design take?"
    DO NOT USE A "CAR" BATTERY!!! They are not designed for this kind
    of application - they're designed to start car engines. Use a marine,
    RV, golf cart, etc. "Deep Cycle" battery.

    And be careful to use the right number of them - as others have said,
    a lot of UPSs use 24V.
    Yeah, as long as your DUT can take the waveform.
    Should be no need - but it might be interesting to get a big ol'
    tranny and an oil cap and make a parallel-resonant circuit at 60 Hz
    across the output. ;-)
    No worries - like I said, the UPS is already isolated. (as long as it's
    not plugged into the wall, which is what the batteries are for. ;-) )

    Good Luck!
  7. D from BC

    D from BC Guest

    I opened up the UPS.. 12V 4.5Ahr battery.
    It's a Minuteman UPS..

    From posts, I'm thinking this idea is a PITA...

    UPS Noise problem:
    I could hook up a line filter.
    (Resonator was mentioned...)
    Could be ok..

    UPS Undervoltage problem:
    The outlet voltage is 124VAC today..
    The smps label has 115VAC output. This might be ok if I'm just
    tracking down transients and ringing on the homebrew smps.
    Hopefully not drowned in noise from the UPS.

    UPS time limited operation problem:
    I could put a fan on the UPS switcher.
    The UPS pcb looks loaded with control electronics... Maybe it'll auto
    cut off even though the battery is still fully charged.. But, I can
    get my testing done within 5min..

    Replacement battery problem:
    Somebody didn't like the car batt idea. So, battery shopping
    required.... I might use the UPS for my computer.. However, the power
    is great here .. Only 2 brownouts in one year..I don't need a UPS.

    Of course, UPS's make alert sounds so...piece of tape on the piezo :)

    Groannn :(
    Ordering that isotransformer from Digikey is looking better :)
    D from BC
  8. PeterD

    PeterD Guest

    Last one I did (destroy...) had a pair of 12V batteries.

    The one I use for my server farm has... ten 12V batteries (yes, in
    series, 120 vdc!) but it is a monster UPS--3.5 KVA, with waveform
    correction and a bunch of other stuff... <g>

    Personally were I to need to isolate soemthing and wanted to go that
    way, I'd get a 12v to 120v inverter, better utility: done with the
    project you can use it in your car or truck for general purpose AC!
  9. ehsjr

    ehsjr Guest

    Maybe you can get another UPS? and put
    the xformers secondary to secondary.

  10. D from BC

    D from BC Guest

    Actually got 2 UPS..same model..

    When I took the UPS apart, I was surprised to find a rather large line
    transformer. Almost accounts for half the weight of the unit.
    Just for charging the battery?
    (The battery inside is a 12V 4.5Ah batt.)
    I guess the battery couldn't be charged by a smps for some reason...
    The 115VAC output is done by switching.

    I guess you mean pull out the 60Hz power transformers and put
    secondary to secondary.. Toss the UPS smps pcb in the junk box..

    I dunno....
    I need 130Watt capability and less than 10% V drop from loading..
    I've melted the bobbins on 2 transformers doing that with underrated
    50W transformers..
    D from BC
  11. Gibbo

    Gibbo Guest

    No, that's the main transformer. It's probably used for the inverter and
    I don't think it is.
  12. ehsjr

    ehsjr Guest

    Yes. With the PCB and the batteries removed, there's enough
    room to add the transformer from the second UPS, and you
    already have the line cord and receptacles mounted to the
    UPS case, so with just a little work you ought to be
    good to go.

    I don't _know_, either, not having done it. Based on the
    weight of those transformers, my guess is they will do it.
    I figure it's worth looking into, for a 0 dollar investment.

  13. joseph2k

    joseph2k Guest

    Just get the isolation transformer, that is a good price.
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