Transformer question

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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.

Re: Transformer question

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

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**  That is just what happens.

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**  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.

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**  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

Re: Transformer question

On Sun, 20 Mar 2005 20:46:53 +1100, "Phil Allison"

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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?


Re: Transformer question

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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


Jenal Communications
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Re: Transformer question

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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).

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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



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