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Box "o" speaker line matching transformers

Discussion in 'Electronic Repair' started by klem kedidelhopper, Dec 19, 2012.

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  1. I have a job to run about 35 speakers including cameras and security
    through a building. The building is a sprawling 200 year old Behemoth
    of a place that I don't want to try to fish wire through. The building
    was part of a now defunct college and as luck would have it almost
    every room is home run wired back to a closet with at least 4 CAT5
    wires. This closet seems like a good place to install our equipment
    and so we plan to run the alarm system as well as the cameras off
    these CAT 5's. In addition there will be a background music system.
    The music will be very soft and so I would like to go with 70V lines
    with speakers tapped at .25 to .50 W each. The runs would be no more
    than say 100 to 125 feet and so I don't think that the 24 gauge wire
    should be a problem with each CAT5 cable handling one or even two
    speakers apiece. So anyhow that's the job and I'd appreciate any
    comments on that but my other question is this: I have boxes of
    unidentified line to voice transformers. I don't know if they're 70 or
    25 volt units. Is there an easy way to determine this? I don't have an
    impedance bridge. Thanks, Lenny
     
  2. Sjouke Burry

    Sjouke Burry Guest

    Try to find the saturation voltage of the trannies.
    Put a variable 50(or 60) hz voltage on one, measuring the
    current.
    When that current suddenly starts to increase, you are over
    the saturation voltage.
    Subtract about 30 % to grade the trannie.
     
  3. Sjouke Burry

    Sjouke Burry Guest

    A small lightbulb in series(5-15W) should act as a suitable series
    limit resistor.
    And the current will go sharply up with only a few volt increase.
    The reason is, that the core becomes saturated, and the inductance
    of the primary will drop to almost zero, leaving only copper
    wire resistance as limiter.
    With the right bulb in series, you might not even need a
    current measurement, the bulb will uddenly start shining.
    Most transformers will be designed such, that at the design
    voltage, and low frequency, the core is almost saturated.
     
  4. So then assuming the UUT is in fact an unknown 70V transformer, I
    might have the supply cranked up to almost 100V before I see this
    increase in current? Can I feed this transformer off my bench variac?
    Will this current increase be a sudden or very gradual increase? Lenny
     
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