# Capacitor changes impedance?

Discussion in 'General Electronics Chat' started by DexterMccoy, Feb 19, 2014.

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1. ### GopherT AAC Fanatic!

Nov 23, 2012
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What a great moderator!

2. ### WBahn Moderator

Mar 31, 2012
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Yes, let's get the OP on the right path, which includes using clear and well-defined terminology (while acknowledge that there is, sadly, in practice some common deviations from it).

The reactance is the imaginary part of impedance. Thus is it a real number.

The impedance is a complex number. If you have a purely reactive component, then the impedance is an imaginary number.

Furthermore, reactance is a signed quantity.

$
Z \; = \; R \; + jX
$

where Z is the impedance, R is the resistance, and X is the reactance.

For an ideal capacitor:

$
Z_C \; = \; \frac{1}{j \omega C} \; = \; -j \frac{1}{\omega C}
\
\omega \; = \; (2 \pi radian/cycle) f
$

Thus the reactance of a capacitor is

$
X_C \; = \; - \frac{1}{\omega C}
$

3. ### WBahn Moderator

Mar 31, 2012
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Hardly. The ESR of a cap can become a significant part of the impedance without there being anything wrong with the cap at all (other than possibly it being a poor choice for a given application). Just as the ESL can (and will) make the part look like an inductor above a certain frequency without there being anything wrong with the cap at all.

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4. ### DexterMccoy Thread Starter New Member

Feb 19, 2014
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on the test fixtures at my work there is a switch that will ramp or sweep a DC voltage up or down. For series capacitors that are inbetween or coupled between stages of the circuit will PASS this ramping or sweep up DC voltage , but once the DC is stable , the series cap BLOCKS the DC voltage

My question is when the DC is ramping or sweeping up or down to a peak voltage. This is a rate of change of the DC voltage

The Capacitor will DC this DC voltage because its ramping or sweeping up to a Peak voltage , yes unipolar waveform.

The Capacitor's reactance and ESR will change in value when their is a ramp or sweep DC voltage apply to a cap?

Because a series capacitor will PASS a DC voltage that is ramping up to a peak voltage, which is a rate of change

But I don't know if the capacitors reactance and ESR changes when it's passing a DC varying voltage

I think a capacitors reactance and ESR only changes when their is a AC waveform or bipolar waveform right?

When a capacitor's reactance and ESR changes , then the in-circuit's impedance changes?

5. ### Georacer Moderator

Nov 25, 2009
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A lot of posts drawing conclusions and sentences upon the OP and a member have preceded this one.
These have been moderated.

It has been repeated in the past: New accounts are clean slates, here in AAC. We cannot be sure of what anyone's identity is, nor do we want to know.

The heads ups are useful for our moderating work and welcome, but don't jump the gun and start amassing a mob.

6. ### studiot AAC Fanatic!

Nov 9, 2007
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I know nothing about any of that.

But I did offer additional explanation in my post#5 and asked a particular question in my post#13 with further offer of help.

I have not had a response to that.

7. ### Georacer Moderator

Nov 25, 2009
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True, but this has nothing to do with branding the OP a troll and a disruptive member.

8. ### DerStrom8 Well-Known Member

Feb 20, 2011
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In that case I apologize, I felt it was only fair to notify the members here about the nature of who they were trying to help.

9. ### shortbus AAC Fanatic!

Sep 30, 2009
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I also apologize. But also reserve the right to say "I told you so".

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10. ### DexterMccoy Thread Starter New Member

Feb 19, 2014
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Stable DC never passes through a cap
But a ramp DC voltage to a peak voltage or a sweep DC voltage to a peak volt, is a change of voltage , even tho it's DC

Example:
DC voltage ramp or sweep

ramps or sweep a DC voltage from 0 to 10 volts
The capacitor will PASS the DC voltage when its going the the change of voltage from 0 to 10 volts DC

The Capacitors ESR changes when there is a change of voltage?
The Capacitors Reactance or impedance changes when there is a change of voltage?

It's a change of DC voltage

I know in electronic books they just focus on the change of AC voltage that passes through a capacitor

11. ### t_n_k AAC Fanatic!

Mar 6, 2009
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It would be rather unusual for the coupling capacitor voltage [say] in a typical BJT amplifier circuit to be a pure ramp, even when the input signal is a slowly increasing ramp. The capacitor voltage would be modified by the equivalent source & terminating resistances - such as the stage input resistance "seen" by an inter-stage coupling capacitor.

In any event, a pure constant-rate-of-change voltage ramp across an ideal capacitor would produce a fixed current whilst ever the ramp persists - per the usual relationship ....

i(t)=C(∂V/∂t)

Would the notion of capacitive reactance or impedance have any meaning in such circumstances?

Suppose one considers the instantaneous "impedance" Rinst=V(t)/i(t). This would be a constantly increasing value governed by the rate of rise of the ramp voltage.

Suppose one alternately considers the "dynamic" or incremental impedance r=∂V/∂i. This would be infinite since ∂i/∂t is zero if the current is constant.

Neither perspective is of much use and any consideration thereof is pointless.

Last edited: Feb 22, 2014
12. ### DexterMccoy Thread Starter New Member

Feb 19, 2014
429
2
Yes the input signal to the capacitor is a DC ramp from 0 to 10 volts dc voltage

Does the capacitance reactance stay the same? or does it change?
Does the capacitance ESR stays the same? or does it change?

The Rate/speed of rise ramp voltage as the input signal to the capacitor will change the reactance and ESR right?

13. ### WBahn Moderator

Mar 31, 2012
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You are trying to make fine distinctions where the course distinctions themselves are not strongly defined.

What is DC? What does it mean to have a direct "current" voltage?

You need to be talking about the spectra of the waveform and how different parts of the spectra are affected by the capacitor.

14. ### studiot AAC Fanatic!

Nov 9, 2007
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So I was wasting my time.

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15. ### t_n_k AAC Fanatic!

Mar 6, 2009
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I sense you are not alone in that respect. There seems to be a predilection for obtuse behavior on the part of some.

shortbus likes this.
16. ### ErnieM AAC Fanatic!

Apr 24, 2011
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Correct. Since:

I = C * {rate of voltage change}

when the voltage does not change (=0) then I must also be zero.

Exactly. Say your voltage changes 1 volt per second across a capacitor of 1 uF. Then the current will be:

I = 1 uF * 1 volt/sec
= 10^-6 * 1 * Farad * volt / sec
= 10^-6 Amp
= 1 uAmp

I do not follow.
If the voltage across a capacitor changes, a current thru it will result.
Conversely, if a current is passed thru a cap then the voltage will change.

"Reactance" is a term used then doing "AC analysis." When discussing a changing DC wave you are doing DC analysis so it is not useful to discuss reactance. A reactance is only defined for a single frequency sine wave, and if you are not using such a wave the analysis technique is not very useful.

True.

The reactance is undefined unless you have a sine wave.
Reactance is a useless method to analyze a capacitor's reaction to DC.

True, except remove "bipolar." One can have say 1V of AC riding on 100V of DC (not a bipolar wave) and send it thru a cap. The cap can be analyses as if there was just 1VAC across it. (Disclaimer: add "in the steady state" if you want to ask about the turn on transients.)

Yes. Double the frequency of the driving sine wave and you half the reactance.

I've ignored ESR because you should too until you completely understand the greatly predominant reaction of the capacitance inside a capacitor.

17. ### DexterMccoy Thread Starter New Member

Feb 19, 2014
429
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A DC voltage that ramps up or a sweeps up is a Unipolar waveform? or what else can it be called?

Any kind of rate of change voltage will be passed by a capacitor , a capacitor will only block a steady constant voltage

What is it called when doing DC analysis since it's not called Reactance?

The Reactance and ESR goes lower when the frequency is higher? why is that

18. ### profbuxton Active Member

Feb 21, 2014
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I do not think the statement "that current passes through a capacitor" is correct. Remember the dielectric!. A capacitor charges and discharges a quantity of charge on its plates in response to the voltage applied across it.
Apply a DC voltage to a discharged capacitor and a charge will flow till a "charged state" is reached and the capacitor voltage is effectively equal to the applied DC.
Reduce, reverse or short the capacitor and the stored charge will flow out of the plates till the charge state is equal to the applied volts.
If reduced then new charge state will equal reduced applied volts, if reversed then new charge state will equal applied volts but opposite polarity, if shorted then charge state will be effectively reduced to zero.
If an AC voltage is applied to the capacitor then the effect is the same as if reversing the voltage at a rate determined by the AC frequency. Charge will flow into and out of the capacitor at the AC rate.
Placing a resistor in series with the AC voltage source and capacitor will allow measurement of a voltage indicating a charge flow into and out of a capacitor at the AC frequency. This does not mean the capacitor is allowing current flow through it. It indicates the charge discharge cycle current flowing into and out of the capacitor.

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