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Question about diodes

  1. Nov 30, 2012 #1

    I want to make an simple linear power supply, using an 12V 50A transformer rectifier and a capacitor.

    What kind of rectifier/diodes are best to avoid an excessive voltage drop from the transformer? Because using common diodes the voltage drops to 10,5V without load.

    Many thanks.
  2. jcsd
  3. Nov 30, 2012 #2


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    Staff: Mentor

    At those kinds of currents, even Schottky rectifier diodes will still show about 0.7V to 0.8V forward drop:


    Why do you need 50A?
  4. Nov 30, 2012 #3
    You know if you use bridge rectifier on a transformer that has 12V ac output, you get higher DC voltage. So it is not 12V-2 diode drop= 10.5V!!! For no load, you get 12X1.4-1.5V≈15V!!! This is because you get the peak voltage, not the RMS voltage. Now the trick is "NO LOAD". Under load, voltage drop depending on the internal resistance of the transformer and the size of the cap.

    There is no way to avoid voltage drop across the diode. As Berkman suggested, Schottky has less voltage drop but still significant. To minimize the drop, not only use the biggest diode, use more in parallel so they share the current and reduce the voltage drop a little. It is not that efficient as the Vbe drop of each diode is a little different and end up one carry more than the other. But that still will help.
  5. Dec 1, 2012 #4
    Many thanks

    Sorry I wanted to say 5A not 50.

    It may be used for an audio amp but it might be used for powering a halogen lamp too it doesn't have a defined use yet.

    Thanks yungman, I didn't knew that rectified and filtered voltage was higher than the AC output! (I have little knowledge about AC electricity) I will try the parallel Schottky diode setup and see if voltage under load is ~12V. I will update after finushing building it.
  6. Dec 1, 2012 #5
    5A is a different world from 50A!!!, You can find one single diode to do the job. Just get the diodes and make sure you use BRIDGE rectifier configuration. Put a load and draw 5A and measure the voltage. I don't know your transformer, it might drop voltage under full load. There is only one way to find out.........try it.
  7. Dec 2, 2012 #6


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    This can be quite a problem once you start to take current from your supply. The unloaded voltage will be near the peak AC volts (less some diode drop) and will stay there if there is a smoothing capacitor. Once you start taking current, there will be ripple and the average voltage will drop noticeably. The actual amount will depend upon the load, the value of the capacitor and the source resistance of the transformer.
    Audio amp supplies will usually have a series regulator to take out the ripple and voltage 'sag' and halogen lamps do not need DC (so why bother?).

    A bridge rectifier will reduce the ripple significantly but, of course, there will be two diode voltage drops involved. Most PSUs use a bridge, these days.
    This wiki article is worth reading.
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