How capacitor blocks dc current?

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SUMMARY

Capacitors block direct current (DC) while allowing alternating current (AC) to pass due to their ability to charge and discharge. The fundamental equation governing this behavior is I(t) = C * (dV(t)/dt), indicating that current through a capacitor is proportional to the rate of change of voltage. When connected in series, capacitors effectively prevent DC from flowing, while in parallel configurations, they still block DC but can allow AC to pass through multiple paths. The discussion also highlights the importance of capacitor value in smoothing out voltage fluctuations in power supplies.

PREREQUISITES
  • Understanding of basic electrical concepts such as voltage, current, and resistance.
  • Familiarity with capacitor operation and terminology, including capacitance and charge storage.
  • Knowledge of AC and DC current characteristics.
  • Basic calculus for interpreting the derivative in the equation I(t) = C * (dV(t)/dt).
NEXT STEPS
  • Study the role of capacitors in power supply circuits, focusing on smoothing capacitors in rectifiers.
  • Learn about the differences between half-wave and full-wave rectifiers and their impact on voltage output.
  • Explore the concept of ripple voltage and its significance in power supply design.
  • Investigate the effects of capacitor value on circuit performance, particularly in filtering applications.
USEFUL FOR

Electronics students, electrical engineers, and hobbyists interested in understanding capacitor functionality in circuits, particularly in relation to AC and DC current behavior.

  • #31
Yeah, it does for sure..the rate of change in voltage show the 90 degree phase shift for a capacitor, how it will react etc.

It's pretty cool..
 
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  • #32
90 degrees for AC and an exponential change for an applied step function ('turning on the DC').
 
  • #33
FOIWATER said:
a capacitor doesn't block DC until it is charged, in which case there is no difference in potential for current to flow

I would be careful to say that there are cases that a capacitor doesn't block DC. An ideal capacitor ALWAYS blocks DC. The technicality is that when you charge a capacitor with a DC source and there is a transient charging, the signal applied is not DC but rather a step function.
 
  • #34
DragonPetter said:
I would be careful to say that there are cases that a capacitor doesn't block DC. An ideal capacitor ALWAYS blocks DC. The technicality is that when you charge a capacitor with a DC source and there is a transient charging, the signal applied is not DC but rather a step function.

There are also two kinds of DC...flat line DC from batteries (ω=0)...and non flat lined DC produced by rectifiers (ω≠0). The cap will block true flat lined DC...but will fluctuate with rectified DC to the the dv/dt effect stated earlier in thread.

One more way of saying what everyone else has been saying...

The impedance of a capacitor is 1/(Jωc)

When ω=0 like in a true flat lined battery...you can clearly see that you have infinite resistance...or the cap will not let current flow.

When ω equals anything but zero...you will have current flow.

Anyone disagree with this? I've been wrong many times before...and many times to come!
 
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  • #35
psparky said:
There are also two kinds of DC...flat line DC from batteries (ω=0)...and non flat lined DC produced by rectifiers (ω≠0). The cap will block true flat lined DC...but will fluctuate with rectified DC to the the dv/dt effect stated earlier in thread.

Ummmmm, be careful here. The ripple portion of rectified DC is not DC. It is the AC component.
 
  • #36
Averagesupernova said:
Ummmmm, be careful here. The ripple portion of rectified DC is not DC. It is the AC component.

Ahhhh yes...thank you.

But the rippled portion isn't actually alternating...it's magnitude is just changing over time??

Does anyone have a graph of the current thru cap superimposed over rectified voltage across a capacitor. I've tried to graph it...It seems like the current in the cap will not be able to reverse direction because of the diodes of the rectifer...confused once again!
 
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  • #37
psparky said:
Ahhhh yes...thank you.

But the rippled portion isn't actually alternating...it's magnitude is just changing over time??

Does anyone have a graph of the current thru cap superimposed over rectified voltage across a capacitor. I've tried to graph it...It seems like the current in the cap will not be able to reverse direction because of the diodes of the rectifer...confused once again!

I guess the term DC may depend on the context and how the person has DC defined in their head.

To me, DC is the component of any signal that has a frequency of 0.

The rippled portion can be decomposed into its Fourier components, which are AC signals of varying magnitude/frequency/phase.
 
  • #38
DragonPetter said:
I guess the term DC may depend on the context and how the person has DC defined in their head.

To me, DC is the component of any signal that has a frequency of 0.

The rippled portion can be decomposed into its Fourier components, which are AC signals of varying magnitude/frequency/phase.

So in your mind and hopefully others...half and full wave rectifers do NOT produce DC voltage? Interesting...because alternators in your car go thru full wave rectifers to charge your battery. So technically it is DC...just the voltage is bouncing up and down in one direction over time.

What is the true definition of Direct current? Does it mean that it flows only in one direction...or does it mean it flows only in one direction and is a flat line with zero frequency?

And more importantly, how do you get that cool signature after your replies on here like Sophie and some of the others have?
 
  • #39
psparky said:
So in your mind and hopefully others...half and full wave rectifers do NOT produce DC voltage? Interesting...because alternators in your car go thru full wave rectifers to charge your battery. So technically it is DC...just the voltage is bouncing up and down in one direction over time.

They produce a voltage with a definite DC bias (average voltage), but AC components are created too because of the diodes turning on and off. That's what the capacitors are there for, to filter out the higher frequencies and only leave the DC bias (in other words, average).

psparky said:
What is the true definition of Direct current? Does it mean that it flows only in one direction...or does it mean it flows only in one direction and is a flat line with zero frequency?

It sounds like you are confused about AC signals on a DC bias. Just because a signal is always positive does not mean it can't have AC components, or that all of its components only have current direction that is positive. The net voltage and current may always be positive, but some frequencies that are biased on the DC bias will have negative voltage. This is rather easy to show mathematically if you add a sine wave to a constant that is greater than the sine amplitude, you see that the sine wave has periods of negative voltage/current, but when it is biased, the two superimpose and the voltage is always positive, even though the AC signal is still there.

psparky said:
And more importantly, how do you get that cool signature after your replies on here like Sophie and some of the others have?
No idea, I think you edit your profile. You might have to be a PF contributor.
 
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  • #40
A 'Unidirectional' signal would always be positive (or negative) DC signal would have a constant value (flat line). The mean value of a unidirectional signal (or any time varying signal) would be the DC component. If you pass a time varying signal through a series capacitor then, after a while, the resulting signal will have a mean value of zero. That's what we call AC coupling. It's the same effect that you'd get if you passed the signal through a 1:1 transformer - the DC component is 'blocked'.
 
  • #41
sophiecentaur said:
A 'Unidirectional' signal would always be positive (or negative) DC signal would have a constant value (flat line). The mean value of a unidirectional signal (or any time varying signal) would be the DC component. If you pass a time varying signal through a series capacitor then, after a while, the resulting signal will have a mean value of zero. That's what we call AC coupling. It's the same effect that you'd get if you passed the signal through a 1:1 transformer - the DC component is 'blocked'.

Almost there...explain this more please.
 
  • #42
yeah, I can see from the formula that when you have no frequency you have an infinite capacative reactance.

So an ideal capacitor blocks DC you say, what about the charge time? an ideal cap will block ALL current you are saying?

I don't know I am kind of thinking, if there is a difference in voltage, there will be current flowing to it. Even in the case of an ideal capacitor?

Can you set me right here
 
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  • #43
psparky said:
Almost there...explain this more please.

If you measure the current over a long period of time and then find the mean value, that is the 'DC' value. The capacitor has zero net difference in charge from start to end of the operation. It has discharged by the same amount as it has charged over that period of time as the AC components rippled through it.
 
  • #44
Borek said:
ac-dc.png


DC is stopped, AC passes through.

Let's face it...there is no better explanation than this...simple and to the point :-D
 
  • #45
FOIWATER said:
So an ideal capacitor blocks DC you say, what about the charge time? an ideal cap will block ALL current you are saying?

I don't know I am kind of thinking, if there is a difference in voltage, there will be current flowing to it. Even in the case of an ideal capacitor?

Can you set me right here

You are right that a voltage difference will cause current to flow and that there will be a charge time in a non-ideal capacitor with ESR, or in a capacitor with an impedance before it. In an ideal capacitor, the current will be proportional to dV/dt and so charging a capacitor directly from a step function should give an instantaneous surge in current to a new voltage on the capacitor in ideal circuit theory.

If it is charging, that means the voltage has changed from its initial condition to a new voltage level at some time that is usually considered at t= 0. DC voltage does not change, but the voltage at the capacitor must if it has an initial voltage different from the applied DC voltage, and so that applied signal is actually not just DC; the charging condition would involve other frequencies.

See the Fourier transform of the step function (boxcar, not heaviside) below to see the spectrum:
http://www.aanda.org/index.php?opti...l=/articles/aa/full/2001/30/aa10486/img32.gif
 
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  • #46
This thread could be ended if people only accepted what the term DC really means.
"DC, but changing a bit" isn't DC.
 
  • #47
sophiecentaur said:
This thread could be ended if people only accepted what the term DC really means.
"DC, but changing a bit" isn't DC.

It depends on the definition.

Wikipedia defines DC as a unidirectional flow of electrical charge, not a constant flow.

250px-Current_rectification_diagram.svg.png

"types of direct current"

An alternative, usually used in electronics, is that DC is an offset from zero of the average value of a waveform.
So, a 20 volt peak to peak sinewave, which goes from minus 5 volts to plus 15 volts, has a "DC component" of plus 5 volts.
A series capacitor will remove this offset "DC" voltage and (depending on its size), will pass all or most of the sinewave.

It is a bit like the heated arguments you can start if you even hint that Ohm's Law is very useful in examining the behaviour of diodes, capacitors and inductors. It all depends on the definitions.
 
  • #48
This is yet another of "those PF threads" about what words to use, I'm afraid.

Come on chaps, if a raw, rectified AC waveform can be called "DC" then what is the "DC component" of that waveform?
The blanket use of the term DC for any unidirectional waveform is sloppy and misleading. In fact it is meaningless. Imagine that the raw output from a rectifier circuit (termed "DC") had just a few millivolts of negative excursion for a brief time during each cycle, would that suddenly make it "AC"?

If people have to misuse the term 'DC' then they must specify that it is 'Raw'. Scientific language surely attempts to be specific and to avoid confusion. Use the appropriate term at the appropriate time unless you have already specified your alternative use of the term.
 
  • #49
It is an archaic term anyway. What does "direct current voltage" really mean anyway?

This is just a hangover from the 1920s when rotating machines produced either AC or DC and there was not much in between.

Now we can produce just about any kind of pulse that might be unidirectional or not, the old concept of "DC" is pretty meaningless unless we are still talking about mains power supplies.
 
  • #50
hi samieee, i try to explain what i know.
in my guess i find two answers.
1.(i) in basically capacitor have a structure of two separated plates are placed in some minimum distance. so its a basically open circuit.
(ii) we well know dc supply does n't pass through the open circuit. ( ac may connect two separated point. due to magnetic induction)
2.(i) current flow was performed by movement of electron in both conductor and semiconductor.if dc was supply was given to the open circuit electrons are settled in anyone corner(unidirectional) of the open loop. so there is no possible way to electron movement.
(ii) if ac supply was given to the conductor(/ semiconductor) electrons are not settled in any corner, electron moves only front and back manner. so induced voltage was continually flow in conductor...!

thanQ
 
  • #51
samieee,

Capacitors are widely used in electronic circuits to block the flow of direct current while allowing alternating current to pass,how it does the job?

Why are you distinguishing between direct current and alternating current? A good capacitor blocks any and all charge flow from passing through the insulating dielectric. Charge can accumulate on one plate and deplete on the opposite plate, but that is only transitory. One direction of the alternating current can exist during this transitory period, but the charge flow never passes through the capacitor. When the AC changes direction, the same thing happens.

well that good but suppose you connect a dc battery,a bulb and a capacitor in a circuit..this time dc current is flowing through the circuit isn't it?(when capacitor become charged if we disconnect the battery then capacitor itself will lighten the bulb

The current is existing in the circuit, but it is not passing through the capacitor. The charge is accumulating and depleting on the capacitor plates, and the capacitor is becoming energized. When the battery is removed, the energized capacitor has a voltage, which can be used to light the bulb until the charge between the place becomes equalized, thereby de-energizing the capacitor.

Ratch
 
  • #52
Capacitors of course allow DC current to pass through. Only if you impose zero voltage change condition, then the stored charge has to be removed by current of opposite polarity. In reality, you could charge a cap with 1 Amper for an hour and then discharge it with 1 Amper for an hour. Theoretically, the current profile has zero DC component.
 
  • #53
SunnyBoyNY,

Capacitors of course allow DC current to pass through.

I am going to enjoy seeing you prove that!

Only if you impose zero voltage change condition, then the stored charge has to be removed by current of opposite polarity.

How can there be a charge imbalance between the two plates if current is allowed to pass through the dielectric insulator? How can anyone impose a "zero voltage change" when there is a charge imbalance between the two plates?

A capacitor is never "charged". The excess of charge on one plate is balanced by a depletion of charge on the opposite plate for a net charge change of zero. This imbalance of charge causes a voltage to appear across the plates, and causes an electrostatic field to form. This electrostatic field contains energy, thereby causing the capacitor to be energized. So a capacitor energized to 100 volts has the same net charge as a capacitor with zero volts across it.

In reality, you could charge a cap with 1 Amper for an hour and then discharge it with 1 Amper for an hour.

What is an "Amper", and why is it capitalized? Have you figured out how many coulombs one "Amper" for one hour represents and what voltage you would need to imbalance a capacitor by that many coulombs? Hint: voltage = coulombs/farads.

Theoretically, the current profile has zero DC component.

What do you mean by that statement?

Ratch
 
  • #54
Ok, here we go:

DC Power supply in current mode provides DC current regardless of output voltage. Connect a cap across the power supply. Will it charge? Can you pass DC current through the device? Of course you can.

What don't you understand about 1 Amper current? It's a DC current with magnitude of 1 Amper.

Btw. you don't have to lecture me about electronics. I work with capacitors every single day (power electronics engineer) and just this morning I charged a cap bank with 100 A for about ten minutes. That is well in excess of 1 A of 3600 s. Ouch ooo... Yes, I was talking about ultracapacitors. Still caps.

Zero DC component: net zero current over the whole charge/discharge period (http://en.wikipedia.org/wiki/Fourier_transform)

Sure, electrons in current with DC frequency cannot move through the cap directly. Instead they are stored on one electrode while the other electrode supplies those electrons. However, from the circuit perspective the cap can conduct DC current. Voltage charge/discharge is the "side effect" for some.
 
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  • #55
Ratch said:
A capacitor is never "charged".
Ratch

Yep, and there is no voltage across no capacitor since capacitors cannot be charged. Just do not touch the two terminals at the same time.

You are talking semantics here. I have never heard "energized capacitor", it's "charged capacitor".
 
  • #56
SunnyBoyNY,

DC Power supply in current mode provides DC current regardless of output voltage

Only up to its upper voltage limit. When that limit is reached, it no longer acts as a current source.

Connect a cap across the power supply. Will it charge?

No, the capacitor will energize. It will contain electrical energy in the form of an electric field. It will have the same net charge as it did when the voltage was zero.

Can you pass DC current through the device? Of course you can.

Of course you can't. If you could you would have a leaky capacitor which should be discarded. The dielectric insulator is just that, an insulator. You are beguiled into thinking that current exists through a capacitor, because a charge flows onto one plate and flows away from the opposite plate. But never does any charge pass through the capacitor.

What don't you understand about 1 Amper current? It's a DC current with magnitude of 1 Amper.

I think you mean amperes with no capitalization. Yes, amperes or amps are one coulomb of moving charge per second. AC or DC, it makes no difference.

Btw. you don't have to lecture me about electronics. I work with capacitors every single day (power electronics engineer) and just this morning I charged a cap bank with 100 A for about ten minutes. That is well in excess of 1 A of 3600 s. Ouch ooo... Yes, I was talking about ultracapacitors. Still caps

Yes, 60,000 coulombs is far more than 3,600 coulombs. You energized those capacitors. They still had the same net charge afterwards as they did at the beginning, specifically zero coulombs.

Zero DC component: net zero current over the whole charge/discharge period (http://en.wikipedia.org/wiki/Fourier_transform)

Do you mean average current? What has the Fourier transform got to do with it? It probably does not matter anyway.

Sure, electrons in current with DC frequency cannot move through the cap directly. Instead they are stored on one electrode while the other electrode supplies those electrons. However, from the circuit perspective the cap can conduct DC current. Voltage charge/discharge is the "side effect" for some.

They don't pass through the capacitor with AC, either. If you knew that a capacitor was an energy storage element, then why did you say "Capacitors of course allow DC current to pass through."? A capacitor being able to store a charge imbalance is not the same as acting like a resistor. If it did conduct current, then it would be a wire. A voltage is always present when the capacitor charge is imbalanced.

Yep, and there is no voltage across no capacitor since capacitors cannot be charged. Just do not touch the two terminals at the same time.

They cannot have a net charge other than zero, but they can be energized by applying a voltage across them.

You are talking semantics here. I have never heard "energized capacitor", it's "charged capacitor".

I am talking about real happenings. Capacitors are never charged with coulombs, they are charged with energy, in which case you might as well say energized.

Ratch
 
  • #57
Ratch,

1) You should take it easy, please. When I say that DC power supply behaves as a current source, you retort that only up to a certain limit. I know that (not to mention that I have built a number of 10kW+ power supplies from the scratch).

2) Cap is energized, charged, that's the same thing. The argument is just repeated over and over. Do that at your work -- correct people like that. You will get laughed at hard (do not mean it badly).

3) Of course, no real DC or AC current flows through the cap. That's the physics behind the insulator/separator. But from the circuit perspective you can pass DC current through a cap while charging it; electrons are stored on one side and leave on the other. Thus, from the circuit perspective, you have current flow. While I understand your point, AC and DC currents can be passed through a capacitor from engineering perspective. In other words, cap is a mere black box. Current flows in, current flows out. Again, it's just semantics.

4) Amper or amper, the main point has gotten across. Anyway, I have learned something new.

5) Charged caps have non-zero charge -- Q = V * C. 1 F, 1 V, 1 C. We are talking ideal capacitors here. Real-capacitors can be modeled as a ladder network of caps, series/parallel resistors, and a series inductor. I am considering all parasitic components to be zero.

6) ...

7) ...

8) ...

Ok, Mr. Ratch, I do not wish to fight no more. Each of us is right. I will play with capacitors and charge them with DC current and claim that they are charged and have fun otherwise while you can be correcting other people and play smart.
 
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  • #58
Why don't you guys just look at the formula for the reactance of a capacitor?

Xc = 1/jωC

If ω is zero, then X is infinite, whatever value of C you use. That formula is easy to derive and the derivation can be found all over the Net.
What is there to argue about if you acknowledge that 'DC' means ω=0?
 
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  • #59
sophiecentaur,

Why don't you guys just look at the formula for the reactance of a capacitor?

Xc = 1/jωC

I think I can also speak for SunnyBoyNY when I aver that we both are cognizant of that relationship.

If ω is zero, then X is infinite, whatever value of C you use. That formula is easy to derive and the derivation can be found all over the Net.
What is there to argue about if you acknowledge that 'DC' means ω=0?

We argued about several things, but that was not one of them.

Ratch
 
  • #60
Xc = -j/(wC)

Xc = reactance offered by Capacitor & w = 2*pi*f
since f=0 , for dc . Xc = infinite
 

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