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Thread: Cheap if not free power supply... UPS

  1. #1
    Variable Bitrate mayhembdm666's Avatar
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    Cheap if not free power supply... UPS

    Hey there guys, I just thought that i might post this up for people who are trying to source a power supply for say an old pc that is sitting in the shed or for simply running a laptop.

    The very first project that i did with my old VK Commodore was using a UPS that i grabbed from work, Basically with a UPS just remove the internal battery and connect those two lines to your car battery. I would still advise to use a separate circuit/fuse link but generally you should be pretty safe even without depending on the UPS.

    Now as for the free part, UPS's these days are generally thrown away or kept aside for spare parts when the battery dies as the price of a new UPS can be pretty cheap and most companies will replace the UPS instead of just the battery. A lot of these have a rating of say 300watt or so.

    So just make a few phone calls to your local computer supplies/shops and see if they have any that have been traded in or given to them.

    Also just remove the buzzer on the UPS to stop the "no power" beep that will occur.

    Im not to sure on the energy loss/conversion but my guess is that these are more or less an inverter with a battery charging circuit built in.
    2004 Holden WL Caprice Auto GENIII
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  2. #2
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    Yeah, I too used this trick many years ago when doing remote broadcasts for the radio station where I worked at the time. Mounted an ancient industrial-grade TrippLite in the van, wired it to the aux battery stack, and used it to power the few pieces of gear we had that wouldn't run on 12 volts.

    A potential caveat- many (if not most) modern consumer-grade UPSes will not self-start. In other words, you can connect a healthy battery to them, press the front panel button, and absolutely nothing will happen unless they're plugged into the wall. For whatever reason, it seems that many of the units I've come across will only turn on the inverter if they are first connected to an active mains connection, turned on, and then the mains plug pulled from the wall.

    Older units (by which I mean at least 10-15 years old, at this point) don't seem to have this problem. It might also just be something unique to consumer-grade stuff, I've not exhaustively researched the issue.

  3. #3
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    That's correct joe_p, you need to find a UPS that expressly supports "Cold Start" or "Cold Boot".

    Most of the cheaper UPS units nowdays don't support that, so they rely on starting using mains power, then they will switch to battery when they detect mains power loss.

    The commercial units will almost without fail all have Cold Start functionality, but the cheaper domestic units wont.

    Also, the cheaper domestic grade units quite often to not have pure sine-wave inverters inside them, so you can quite often end up with noise on your audio like you would with a normal in car inverter.

  4. #4
    Raw Wave
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    And it needs to be 12V. Many use 24V or 48V. (And multi-kW versions use ~400V! )

    But inverters these days are so cheap....

    But a good sinewave on-line UPS....

  5. #5
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    Ahh good point too OldSpark, I recall the unit I picked up for my office is a 48V system, its a 2kW commercial UPS.

  6. #6
    Raw Wave
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    If you picked it up, I presume it isn't ferro-resonant....

    (LOL! I was somewhat involved in the local Aus industry when UPS & inverters "revolutionised" from dinosaur technology to hi-freq conversion, MODERN specifications etc. My peers couldn't understand what was wrong with the "current" stuff (good pun eh?!). Funny that they no longer recall such attitudes & comments!)

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