Voltage across a capacitor; AC supply

Discussion in 'Homework Help' started by John4, Aug 29, 2015.

  1. John4

    Thread Starter New Member

    Aug 25, 2015
    3
    0
    Hi

    I have been told that the question below might be similar to potential university interview questions for physics/electronic engineering.

    In relation to this circuit diagram:

    Diagram - AC, diode & capacitor PD question.png

    Initially 240V rms mains voltage is connected between terminals A and B in the circuit above. You can assume that the capacitors and diodes are appropriately rated to work at this voltage.
    When the capacitors have ceased to charge, the mains voltage is disconnected.
    What voltage will then be measured between points C and D?

    To my mind the voltage between C and D will just be the sum of the voltages across the two capacitors but C1 will not charge so the 240V rms will be all across C2. This will result in the voltage between C and D to be 240V.

     
  2. MikeML

    AAC Fanatic!

    Oct 2, 2009
    5,450
    1,066
    Think superposition... There are really two separate circuits in the diagram...
     
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  3. MrAl

    Well-Known Member

    Jun 17, 2014
    2,433
    490
    Hi,

    Keep in mind that capacitors charged by AC sources through diodes charge to the peak voltage, not to the RMS voltage level. The peak is sqrt(2)*Vrms. That's a lot higher than the RMS voltage.
    Since each cap charges on different half cycles and assuming each one charges up to the full peak, you can easily guess what DC voltage will show up across the two caps.
    Make sense yet or no?
     
  4. John4

    Thread Starter New Member

    Aug 25, 2015
    3
    0
    Thanks for your help, very much appreciated.

    So approx 339V DC across each cap / 679V from points C to D?
     
  5. MrAl

    Well-Known Member

    Jun 17, 2014
    2,433
    490
    Hi,

    Yes :)
     
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  6. John4

    Thread Starter New Member

    Aug 25, 2015
    3
    0
    Thanks!
     
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