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Two Different Full-wave Rectified Voltages From One Center-tapped Transformer and Bridge Rectifier

Two voltages, one twice the other, require only a center-tapped transformer and bridge rectifier

  1. crutschow
    The schematic below shows a simple way to get two full-wave rectified voltages from one center-tapped transformer winding and one bridge rectifier.
    Adding a capacitor filter at the center-tap gives roughly half the DC voltage from the center-tap output to ground as compared to the bridge output to ground.

    This can be useful for example, if you want to build a power supply with two different output voltages, such as 5v, along with a higher voltage, such as 10-12V. (The LTspice simulation is shown for a 12Vrms center-tapped transformer).
    This minimizes dissipation in the linear regulators and reduces the transformer current requirements.

    This works because all 4 diodes are acting as a full-wave bridge for the outer transformer connections while the center-tap is using the two bridge diodes to common as a full-wave rectifier.
    The two bridge rectifiers connected to common are, in effect, doing double duty.

    It may look a little confusing since normally a full-wave rectifier is shown with the diodes in the outer paths to the output with the transformer center-tap grounded, but it works just as well with the diodes in the ground path and the output taken from the center-tap.

    Of course it should be apparent that the transformer and bridge rectifier must be rated for the sum of the currents from the two outputs.
    Typically the transformer should be rated at ≈2.4 times the bridge DC current plus ≈1.6 times the center-tap DC current.
    This is due to the high RMS value for the current pulses charging the output capacitors (true of all rectifier-capacitor supplies).
    For example, the LTspice simulation gives an RMS current in the transformer of 702mA for a full-wave bridge DC output of 298mA (not shown below).
    Note the increased current in the ground diodes of the bridge (ID4 shown) due to it having to carry the sum of the two output currents.


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