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PreSapian 9th November 2012 08:51 AM

two transformers into one
 
i have 2 18-0 transformers that i want to use it like a 18-0-18 for myrefc amp. whats the best way to do this?

DF96 9th November 2012 10:02 AM

Connect the two secondaries together, the right way round: start of one winding to the end of the other. Use a voltmeter to check that you have 36V AC across the combined winding. If you get almost nothing then swap one of the secondaries over.

If the two transformers are not identical in every way then you will get some 50/60Hz ripple after a full-wave rectifier as well as the expected 100/120Hz ripple.

When playing with mains transformers etc. always do the initial power-up with a lamp limiter in circuit. That protects you, your house wiring and the circuit from your mistakes.

PreSapian 9th November 2012 10:21 AM

the two are the same model of the triad brand. am i correct in thinking that it's better to find a single transformer instead since the chances are slim that the two trafos are perfectly identical?

MarianB 9th November 2012 04:12 PM

It is as simple as connecting the 2 transformers in series, theyr windings are not part of the same magnetic core so there is no special way to connect them, there is no phase to worry about, just connect the in series and then check the voltage from theyr common point to the rails and make sure you have the correct voltage either side, it does not have to be identical voltage, even simetricall transformer seldom have identical voltages, it is verry hard to achieve that, but it is also not needed, even with difference of 1V or more it would not be any disaster cus the bridge rectifier along with filter caps will equalise the voltages, if you want you can also use some rezistors paralelled with the filter caps for better simmetry, let say 100k, or something like that.

DF96 9th November 2012 04:38 PM

Quote:

Originally Posted by MarianB
there is no phase to worry about, just connect the in series and then check the voltage from theyr common point to the rails and make sure you have the correct voltage either side

No. The phase has to be correct. Checking the voltage from the common point does not confirm that you have wired them correctly.

Quote:

even simetricall transformer seldom have identical voltages, it is verry hard to achieve that, but it is also not needed, even with difference of 1V or more it would not be any disaster cus the bridge rectifier along with filter caps will equalise the voltages,
No. The bridge will not equalise the voltages, If not balanced you would get 50/60Hz ripple as I said. Not a disaster, of course, but unwanted.

Quote:

if you want you can also use some rezistors paralelled with the filter caps for better simmetry, let say 100k, or something like that.
No. In the event of assymetry adding these resistors will make no difference at all.

I think you need to learn a bit more about PSUs, then you will spread less misinformation.

DF96 9th November 2012 04:40 PM

Quote:

Originally Posted by PreSapian
the two are the same model of the triad brand. am i correct in thinking that it's better to find a single transformer instead since the chances are slim that the two trafos are perfectly identical?

If they are nominally identical then they should be close enough for this purpose.

Once you have the secondary phase correct, then of course you must maintain the same primary wiring too, as swapping either will invert the phase.

MarianB 9th November 2012 04:45 PM

@DF96 i think you should take the time to learn about people better before you suppose anything, and further more characterise them.

Ofcourse the bridge rectifier and filter caps equalise the voltages, and ofcourse adding paralelled resistors adds to the symmetry, remember ATX PSU's? remember the imput stage? remember the resistors on the filter caps? i think maybe it is you that should learn more about the concept.

As for the power transformers, as i sayd as log as the windings are not of the same magnetic core there is no phase to worry about, and if you get about 18V from common point to either side then ofcourse it is ok, it is not direct current to worry about polarity.

Quote:

Originally Posted by DF96 (Post 3234905)
Once you have the secondary phase correct, then of course you must maintain the same primary wiring too, as swapping either will invert the phase.

That is ridicoulus, what possible effect could the way the primary windings are connected the the power line, uppon the seccondaryes? sorry but i think it is you that spreads missinformation.

BZed 9th November 2012 06:09 PM

DF 96 is correct Phase does matter. 40 years in electronics, I've been though this one.

metalsculptor 9th November 2012 06:30 PM

Quote:

Originally Posted by MarianB (Post 3234910)
That is ridicoulus, what possible effect could the way the primary windings are connected the the power line, uppon the seccondaryes? sorry but i think it is you that spreads missinformation.

Same as with DC reversing the wiring reverses the polarity, If you were to view it with an oscilloscope triggered in a manner which preserves the phase relationship then you would see the secondary waveform invert when the primary wires were swapped. Once AC is rectified the phase information is lost.

Rather than bother with this it is simpler to treat it as a 180 degree phase shift. With AC systems phase is very important, look up phasor diagram, this is the key to understanding many of the characteristics of an AC system.

PreSapian 10th November 2012 06:28 AM

hi df96,

are you telling me to connect them
18-0 18-0
or 18-0 0-18?

thx

also individual trafos are actually 9v dual secondaries


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