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Steering Diode

Discussion in 'Electronic Basics' started by Sir Charles W. Shults III, Feb 18, 2004.

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  1. The explanation on the page you supplied was quite clear and concise.
    If diode D2 is not forward biased, it cannot conduct. If 15 volt power is
    available, D2 is reverse biased and power from the battery cannot pass
    through it. Only when then 15 volts fails can D2 conduct battery power.

    Cheers!

    Chip Shults
     
  2. electricked wrote:
    (snip)
    Yes.
     
  3. Yes.

    At the plant where I work, the power house has a series of 135
    (nominal) DC buses that all the critical relay logic runs off, to make
    sure things work through a power blip. There is a big transformer
    supply (with cap bank storage) for each bus that is set by transformer
    taps to produce about 140 volts. There is a single backup transformer
    operated supply (with cap bank) that serves as secondary source for
    all these first line supplies. It is set to about 138 volts. Then
    there is a common battery bank and charger system that runs at about
    136 volts at no load as the third source for all systems. The main
    and backup supplies are diode connected same as your example. There
    is a current measurement on each supply, so we can tell if any
    backups are carrying load. A while back, we got current on both the
    main and backup supplies for several banks, and we found defective
    rectifiers on the first line supplies that increased their ripple
    voltage enough that the second line supply was supplying current
    during the dips.
     
  4. electricked

    electricked Guest

    Hello all,

    I have a question about the diagram and explanation given on the following
    page: http://ourworld.compuserve.com/homepages/g_knott/elect212.htm

    It says that while the power supply lasts the equipment will be powered by
    it and if it fails, the battery will start conducting and give power to the
    load.

    Now, I understand how this works, but what troubles me is that if both are
    in great operating order, why doesn't the battery conduct and the power
    supply wait until the battery is dead? Is it because the power supply
    provides a higher voltage? Please explain.

    Thank you!

    --Viktor
     
  5. electricked

    electricked Guest

    That's exactly what the page said. But why? Why can't the battery conduct to
    start with and not the power supply? Is it because the battery's voltage is
    lower than the power supply's? I don't get it.

    --Viktor
     
  6. electricked

    electricked Guest

    That's exactly what the page said. But why? Why can't the battery conduct
    to
    What if the power supply was 10V and the battery was 12V? Would the battery
    supply the power until it's finished and then the power supply take over?
    See what I mean?

    Thanks for speedy response!

    --Viktor
     
  7. electricked

    electricked Guest

    Thanks John! Makes perfect sense now.
     
  8. -----------------
    Yes, the PS voltage is higher, and that presents 14.4V on one side
    of the steering diode, and 12V on the other, reverse-biasing it,
    and reverse-biased it remains OFF.

    The battery might not even need the diode, that is, it might as well
    be recharged while the PS is on, if the battery's requirements were
    simple, or if it had a switch to turn off the battery positive when
    it was "full", as long as we don't exceed its charging maximums.

    But if 15V is left across a nominally 12V battery, it will indeed
    overcharge it and damage or overheat it.

    -Steve
     
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