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Protect existing power supply with switch or diodes

Discussion in 'General Electronics' started by Tim, Oct 11, 2003.

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  1. Tim

    Tim Guest

    When you go to tie in the battery source of power, you may want to
    consider adding a switch to select between either the rectified power
    from the AC/DC converter or battery power.
    Alternately you can add two rectifying diodes, such as a 1N4001 or
    similar part. The rectifying diodes only allow current flow in one
    direction. So without needing to throw a switch, power to the speakers
    can come from the AC/DC or batteries.

    If a switching method similar one of the two mentioned above is not
    done, there is risk of damaging the output of the existing AC/DC
    rectifier if the rectification is done with 3-terminal voltage
    regulator such as the LM338. This device has a low output impedance
    (resistance). Current from the battery would flow to the speakers and
    into the output of the LM338 and potentially damage the part. Then
    you'd have to use the battery source all the time.

    Take a look an see if there is a 3-terminal device or just a resistor
    and zener diode used for the rectifying. If it's just a resistor and
    zener, then you'll just end up wasting a portion of the battery energy
    in the zener.

    Hopefully you can see better what I am talking about with the diagram
    "drawn" below in text.

    AC power-->AC/DC rectifier-->Anode-Cathode--\
    Battery power--------------->Anode-Cathode--/

    Adding a SPDT (single-pole double throw) switch of pair of rectifier
    diodes would require cutting the existing connection between the AC/DC
    rectifier and the speakers.

    Good luck,
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