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Transformer question

Discussion in 'Hobby Electronics' started by Matthew, Mar 20, 2005.

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  1. Matthew

    Matthew Guest

    I have a 14V centre tap transformer which was connected to an
    amplifier board in parallel. I connected the two AC outputs together
    so I could use it with another project but I couldn't get a reading on
    the multimeter. Do these only work in parallel? Why would it only give
    me a reading when measuring the two AC outs seperate?

    I am also thinking about getting a 9-0-9V toroidal from Altronics but
    want to use it was a single voltage supply. Can I join the two 9V
    outputs together to make 18V?

    Thank you.
  2. Phil Allison

    Phil Allison Guest


    ** Bet there were a couple of diodes involves in that example.

    ** That is just what happens.

    ** Because the two AC voltages have opposite polarity.

    At each point in time, the voltage on one terminal is of the opposite sign
    to the other.

    ** You do NOT do that - the two 9 volt wires will measure 18 volts from
    one to the other.

    The situation is no more complicated than with a couple of 1.5 volt cells
    in a torch handle - the transformer centre tap is similar to the spot
    where the top of one cell touches the base of the other.

    ............... Phil
  3. Kevin Ettery

    Kevin Ettery Guest

    The outputs of a centre tapped transformer cannot normally be connected
    together - there is usually only one seconday winding on the transformer,
    with a connection to the center of this winding (called the center tap), not
    two separate secondary windings. So essentially, in your case (assuming
    from your description), the transformer has a 28V secondary winding with a
    tap in the middle (at 14 V). Connecting two transformer outputs in parallel
    could be a good way of stuffing it. From your description you sound like
    you're connecting the outputs of some interposing circuitry - what's
    probably happening is some protective circuitry is killing the output to
    protect against the dead short on the outputs (hence no volts when connected
    together, but volts when separate).
    If its a center tap (which it seems to be) you don't need to connect the two
    9V outputs - just use the +9 & -9 V ends as they are the two ends of the 18V


  4. Matthew

    Matthew Guest

    Ok. What if I first pass both AC outputs through diodes so it it then
    DC, and then connect the two DC outputs together. That would be OK
    wouldn't it?

  5. Alan

    Alan Guest

    If you want to get 18v from a 9-0-9 transformer then connect one of
    the 9 v leads to ground and the other 9v lead will be actually 18v
    (and the 0v lead/centre tap will be at 9v).

    You can also put a bridge rectifer across the two 9v leads (take one ~
    or AC terminal of the bridge to one of the 9v leads and take the other
    ~ or AC terminal to the other 9v lead - ignore and insulate the 0v


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