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Regarding Transformer in the Application Note 1849.

Dileep Kumar
Prodigy 20 points

I have read " AN-1849 An Audio Amplifier Power Supply Design " and it says that the transformer used is dual primary and secondary  transformer of 300VA and output 24 volts. but the design says the rail voltages are +/- 72V. the design uses 2 transformers and in the schematic each transformer's dual secondary outputs are connected in series which will make the output go to +/- 48 volts. I need to know if i'm wrong and what is the correct explanation regarding my understanding, if i'm wrong.

  • 0
    TI__Guru**** 189401 points
    Hi, Dileep,

    Unfortunately, no further information is available beyond what is published in that app note.

    -d2
  • 0
    Prodigy 170 points
    A quick look shows that the transformer gives out 48 Volts RMS, through a bridge rectifier and smoothed the will give approx 1.414 times 48Volts which is 67.8 volts. Now the voltage on a transformer is specified at full load, off load it will be higher due to the coil resistance dropping it at full load, for a 300VA transformer the load regualtion, i.e the difference between on and off load voltage will be about 6% from memory for a 24 0 24 volt secondary, so 6% more than 67.8 volts is 71.9Volts.
  • 0 in reply to Russel Hughes
    TI__Mastermind 45055 points

    On must be careful about transformer specifications.  When the transformer is at rated load of 300VA, the output voltage will be 24-0-24 Vac.  At no load or minimal load the voltage will be higher.  This depends on the series impedance of the main circuit and the transformer.  With this understanding, the +/-72Vdc rail at no load is possible.  I do not know where the 6% comes from.  From my experience it is not the same on all mains power supplies.  It must be calculated based on the whole power supply circuit.  Plus, do not forget the ripple on the supply capacitors.  The amplifier will clip on the lowest voltage of the ripple.  

  • 0 in reply to Gregg Scott
    Prodigy 170 points
    If you look at the datasheet of a 25 0 25 volt 300VA transformer:

    http://www.farnell.com/datasheets/1661839.pdf

    300 VTX-146-300-125 230 2 × 25 6 6 60 60 × 120 2.3

    You will see that the load regulation is specified at 6% for that voltage and that power rating.

    BR

    Russ
  • 0
    Prodigy 20 points

    Thank you everyone for answering. I understood where I was wrong.

    my question has been answered.