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Choosing power transformer voltage

Discussion in 'd.i.y.' started by Mark Harder, Feb 26, 2019.

  1. Mark Harder

    Mark Harder New Member

    I'm a newbie here, so I'm puzzled by contradictory advice as to the secondary voltage required of a power transformer when building an audio power supply. I'm building a +/- 40V DC supply and the BOM for one design specifies a "42 - 45 V, 2A CT" transformer. Now, I'm pretty sure that, for example, a 22-0-22V transformer won't cut it since 22*1.414 is only ~30V AC, pp. So, I'm assuming that the authors meant a 44-0-44V transformer in this case. However, another plan recommends that I pick a transformer with a secondary voltage giving 1.414*V AC= ~5 - 10 V AC in excess of the required DC voltage. In this case, I would choose a transformer with a 32V to 35 V AC dual secondary to make a +/- 40 V DC power supply (45/1.414 = 32V & 50/1.414 = 35V). What is the true rule (of thumb or whatever) for picking transformer secondary voltages?
  2. Pete MB&D

    Pete MB&D Pete Maddex, the one and only!

    It depends on if its a regulated supply, if it is you need more than the voltage drop across the regulators on top of the supply voltage.

  3. a.palfreyman

    a.palfreyman pfm Member

    So many variables. Larger transformers (higher VA rating) have better regulation, typically 5-10% which means off-load you get that amount of over-voltage so that at rated output (AC, not rectified to DC) you get the specified voltage. Smaller traffos typically have 15-30% regulation. My power amp runs off a pair of 300VA 25-0-25 traffos, one per channel giving 38Vdc after rectification, off load. You will also need at least twice the power rating of the circuit being fed, so if you are consuming 2A at 40Vdc, that is 80VA so you would need a circa 200VA rating traffo. 27-0-27 would give you about 42Vdc +/- a bit. The extra VA is needed because rectifying to DC means that the transformer is only delivering current when the output is above the DC voltage plus the voltage across the two rectifier diodes and so is only delivering current at the peaks of the waveform and therefore for a smaller period, typically 25% of the time and so has to deliver short bursts at high current output to charge the capacitors.
    Arkless Electronics likes this.

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