I am trying to build a symmetrical linear power supply to drive some audio op amps (LM833N) which require +/-15V. I found this schematic diagram online:

enter image description here

The description of this circuit states:

The main supply source is a double – winding transformer. D1, D2, D3 and D4 provide full wave rectification. C1 and C2 are charged at about 1.4 * Vin (where Vin, is the voltage provided at each winding). For 78xx and 79xx regulators, the input voltage must always be higher than the output voltage by some minimum amount (typically 2volts). This means that the condition 1.4Vin>XX+2 must always hold true, to ensure normal operation. Choosing Vin to be exact equal to XX, we ensure that for XX>5V, the above condition holds true. We could choose Vin to be much higher but it is better to keep 1.4Vin as low as possible to minimize heat losses. At this point it is good to remember that in any linear regulator, the power loss due to heating is the current times the voltage dropped across the regulator (Heat loss in Watts = (V input – V output)* current.

So for my use case, XX must be 15V, but I have a few questions:

  1. The designer states that "the main supply voltage is a double-winding transformer", but the schematic only shows one primary and one secondary winding. The annotation underneath the symbol says "2X", are there supposed to be 2 transformers?
  2. The designer frequently refers to Vin ("the voltage provided at each winding"). Am I correct to assume that this is the voltage present on the secondary winding of the transformer?
  3. I understand that the voltage entering the linear regulators must be at least 2V higher than their rated output voltage, why does the designer state that the condition "1.4Vin > XX+2" and not just "Vin > XX+2"?
  4. All of these questions are to help me understand which transformer would be appropriate for this application. With my current understanding, I would need 120V (I live in Canada) on the primary being stepped down to at least 17V on the secondary, rated for 2A. Would this be adequate assuming the circuit it is driving does not require more than 2A of current?

1 Answer 1

  1. It means a transformer output that is center-tapped like in the picture, or has two secondaries which can be connected into center-tapped configuration.

  2. Yes, but just for clarity, it means XX volts AC, in volts RMS. Not the peak amplitude.

  3. Because XX volts AC has peak voltage of about 1.4 times XX

  4. It depends. It is a delicate balance of max current you draw, capacitor value, and transformer output voltage. The peak charged to cap is about 1.4× rated VAC output, the cap is charged to that voltage 120 times per second so it has 8ms time to diacharge from peak voltage to 17V at current you draw so that determines the capacitor value. Plus margins and tolerances.

  • \$\begingroup\$ I understand that it depends on the current being drawn, and capacitor values. The capacitor values are given in the schematic, and I will certainly not exceed 2A, and most likely less than 1A. Excluding margins and tolerances, could you provide me with an example of a suitable transformer? \$\endgroup\$
    – 00728M
    Commented Jan 28, 2022 at 3:16

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