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Discussion in 'Electronic Design' started by vk2kaw, Jan 28, 2005.

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  1. vk2kaw

    vk2kaw Guest

    Hi Folks
    I have a small 300w ups that runs on 12v
    and was wondering if someone could tell me how to start it without the
    mains connected to use as a converter 12v to mains (240v in Aus)
    Thanks Alan
  2. Guest

    What are you trying to power with it ? UPS's don't provide much power
    for long before the batteries die - especially little ones.
  3. vk2kaw

    vk2kaw Guest

    yes the internal battery is to small but I wanted to run in in my truck
    The ups is 12v on the input but will not start on 12v I was hoping to
    use it to power the laptop and charger for the controls
  4. About a year ago, I did a bit of reverse engineering on a small UPS
    and here is what I think I remember about how it worked:

    The input line is connected to the output by a relay that is powered
    by the incoming AC. The inverter transformer secondary was connected
    to the output continuously. As long as the relay is on, it has a
    contact that keeps the inverter controls shut down. Since the relay
    connects the input AC to the output and the transformer high side is
    continuously connected to the output, the input AC feeds backwards
    through the transformer, charging the battery through the fet switch
    body diodes. The inverter controls are not powered directly from the
    battery, but from DC rectified from the transformer low side by
    another set of diodes, whether because the high side is connected to
    AC or because the inverter is running.

    Once the input AC disappears, the relay drops out, first disconnecting
    the input line from the output and then telling the controls that the
    inverter is needed (break before make). The inverter controls have to
    get the inverter going before a small storage cap across the control
    supply bus fades out.

    When input AC is restored, the relay first tells the inverter logic to
    turn the inverter off, and then connects the input AC to the output.

    If your unit is something like the one I opened, you might be able to
    jump the battery to inverter logic (look for those two small diodes
    from the low voltage side of the transformer that connect to logic
    chip power pins or a low voltage regulator) through the on off switch,
    so that it will run the inverter from a cold start.

    Remember that you are dealing with very lethal voltages once the
    inverter case is opened.
  5. Gary J. Tait

    Gary J. Tait Guest

    why not get an inverter? They aren't that expensive now.
  6. jsmith

    jsmith Guest

    The inverter requires a start up pulse and can often be achieved by adding a
    push button switch that momentarily connects the 12VDC to the base of one of
    the inverter power transistors through an appropriate resistor.
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