Question about capacitor as a DC block

Discussion in 'General Electronics Chat' started by yuvi1, Jul 16, 2013.

  1. yuvi1

    Thread Starter Member

    Feb 1, 2013
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    Hi everyone

    it is well known that capacitors block DC and allowing AC and also used to filter signals , so my question is : If capacitors used to block DC , why do we see them in all kinds of DC circuits ?

    thanks !
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Blocking DC is only one of the functions of a capacitor.

    The inverse is that a capacitor passes AC. Hence it is used to pass AC signal to ground. Hence it is used to remove noise.

    A capacitor also stores charge. Hence it is used to smooth out the voltage in a DC power supply.

    In essence, the capacitor's behavior to DC and AC signals are the same. The application of this behavior is what differs.

    There is a misunderstanding of what we refer to as AC. AC is not only alternating current. A 100V signal with a 1V ripple is considered to be a signal with both DC and AC. The AC signal does not have to reverse direction for it be designated the term AC.
     
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  3. yuvi1

    Thread Starter Member

    Feb 1, 2013
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    thanks MrChips !

    when you say the the capacitor is used to smooth , do you mean that it used as a stabilizer or as a regulator?

    and is it correct to say that a capacitor pass DC until it is fully charged ?
     
  4. BobTPH

    Active Member

    Jun 5, 2013
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    When we say that a capacitor blocks DC, we mean that no current will flow into or out of the capacitor as long as it's two terminals are at the same voltage. Any change in voltage will cause the capacitor to either charge to higher voltage or discharge to the lower voltage with current flowing in or out.

    Bob
     
  5. PackratKing

    Well-Known Member

    Jul 13, 2008
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  6. MrChips

    Moderator

    Oct 2, 2009
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    Is a smoothing capacitor a stabilizer or is it a regulator?

    Both yes and no.

    I take blood pressure medication to stabilize and regulate my blood pressure.
    I would not get too hung up on the usage of the two words as applied to electronics.
    When we use the word "regulate" we then to refer to an active feedback control system that attempts to bring a given parameter to a preset value.

    A smoothing capacitor is more like a reservoir or storage tank. If it is large enough it can hold a large amount of charge so that when you take a small amount of charge the voltage is only slightly disturbed.

    From a circuit analysis perspective, the fluctuating demand for charge represents a fluctuating signal (which we call the AC component) superimposed on the average voltage (which we call the DC signal). Because the capacitor is a low impedance to AC, it conducts the AC to ground resulting in a smoother voltage (reduced AC component).
     
    Last edited: Jul 16, 2013
  7. wayneh

    Expert

    Sep 9, 2010
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    Are you sure of that? In common usage, does the "alternating" term just mean fluctuating in amplitude instead of direction? I'm not being snarky - I'm really wondering about the definitions.

    I think that would be a failure of the vocabulary. Current that truly alternates directions is a different thing than fluctuating DC. It should have a distinct term.
     
  8. WBahn

    Moderator

    Mar 31, 2012
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    Like many terms in technical parlance, the term "alternating current" is widely used to mean a number of things that are, in some cases, only superficially related. The specific meaning usually has to be determined by context. This is not only a source of potential miscommunication because of the general lack of a precise definition, but also risks hard to spot miscommunication between two people that each believe the know the "one true definition" of the term but their definitions are different but close enough so as not to be obviously so.
     
  9. MrChips

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    Oct 2, 2009
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    An obvious miscommunication I experience with students all the time is the labeling on the input selector switch of an oscilloscope AC-GND-DC.

    Here is an example where AC in this case does not mean Alternating Current.
    AC in this case refers to the signal component with the DC removed.
    We call this the AC component.
     
  10. BobTPH

    Active Member

    Jun 5, 2013
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    Since all voltages are relative, any signal that is changing periodically will be alternating in sign with respect to some voltage (like the RMS voltage for instance).

    Bob
     
  11. wayneh

    Expert

    Sep 9, 2010
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    That doesn't apply to current flow though. Current is not arbitrary, like voltage. And the term is alternating current, not alternating voltage, which might be more accurate.
     
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