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Simple capacitor question

Discussion in 'General Electronics Discussion' started by Mustwin351, Sep 12, 2014.

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  1. Mustwin351

    Mustwin351

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    Apr 10, 2013
    When you have a capacitor between your two inputs on an alternating current circuit (i.e. possibly a power supply) why does that not create a direct short?

    What am I missing? If a capacitor passes ac voltage why is that not like a line to line short?
     
  2. Gryd3

    Gryd3

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    Jun 25, 2014
    A capacitor can appear as anything from a short to a very high value resistor depending on the frequency and capacitance.
    There is a formula for it...

    Xc = 1 / (2*Pi*f*C)

    Xc = Capacitive reactance (Ohms)
    Pi = 3.14159.....
    f = Frequency (Hz)
    C = Capacitance (Farads)
     
  3. davenn

    davenn Moderator

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    Sep 5, 2009
    A capacitor doesn't pass AC as such .... as in NO current flows directly between the 2 plates
    Rather each plate alternately charges and discharges

    And as Gryd3 said, it will "act" as a short circuit dependant on the info he gave above
     
  4. Mustwin351

    Mustwin351

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    Apr 10, 2013
    Thanks guys I recall that now.

    For a power supply the capacitor goes on the "dc" side or the circuit and no matter the reactance of the cap you won't have a short. Instead it just "charges" the capacitor.

    It's been sometime since school. Thank you.
     
  5. davenn

    davenn Moderator

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    Sep 5, 2009
    The capacitor after the rectifier smoothens out the AC to something that resembles a DC voltage ( but with ripple (the remaining AC))

    Have a look here and ask about anything you have a problem understanding :)
    http://www.learnabout-electronics.org/PSU/psu12.php
    cheers
    Dave
     
    Gryd3 likes this.
  6. Mustwin351

    Mustwin351

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    Apr 10, 2013
    Great link Dave. Thanks!

    It states that in order to use a bridge rectifier for full wave rectification that a center tapped transformer must be used...but it then refers to figure 1.1.3 for this which does not have a center tapper transformer.

    Is this just a mistake in figure 1.1.3?

    http://www.learnabout-electronics.org/PSU/psu11.php
     
  7. Gryd3

    Gryd3

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    Jun 25, 2014
    This is certainly a mistake ;)
    The Label for each image is below the image, not above :p
    You are looking at figure 1.1.4 The Bridge Rectifier that has no center tap and 4 diodes.

    Figure 1.1.3 certainly does have a center tap on it's secondary and is using two diodes.
     
  8. Mustwin351

    Mustwin351

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    Apr 10, 2013

    So figures:
    1.1.4
    1.1.5
    1.1.6 should all have center taps on the transformer secondaries connected between the two diodes correct?

    I guess without a center-tapped transformer the best you can achieve is half wave rectification?
     
  9. Gryd3

    Gryd3

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    Jun 25, 2014
    Nope.
    1.1.4
    1.1.5
    1.1.6
    Are all using a Bridge Rectifier which does tot require a center-tapped transformer secondary.

    1.1.3
    Is using 2 diodes, and requires a center-tapped transformer secondary to rectify a full wave.

    This was a misunderstanding. The page correctly refers to figure 1.1.3 which had 2 diodes and a center-tapped transformer, but you had accidentally looked at 1.1.4 instead which is the first figure to use a bridge rectifier.
     

    Attached Files:

  10. Mustwin351

    Mustwin351

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    Apr 10, 2013
    I gotcha now. Sorry a little further reading would have helped. The center tapped transformer with a rectifier is just more efficient than one without a center tap.

    Thank you very much!
     
  11. davenn

    davenn Moderator

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    Sep 5, 2009
    you still might be misunderstanding ... the way you just wrote that is misleading and not what the www page said ;)

    they said ....
    they are telling you its more efficient that a single winding transformer and only ONE rectifier diode as referenced to in Fig 1.1.2

    A single winding secondary and a bridge rectifier is still efficient. and has the advantage of full wave rectification without the need for a centre tapped secondary :)

    Dave
     
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