# coupling capacitor

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#### DragonForce

##### New Member
Sorry, you need to explain that graphic to me - where was the measurment taken, after the capacitor? I've never been any good at reading graphs.

The blue trace shows that the voltage does not drop below zero, it does not change polarity - it is DC.

The capacitor will remove the DC offset and give an AC OUTPUT signal, sure - that was never the issue. The issue is, AC MUST change polarity in order to be AC - pulsating DC is still DC.

#### crutschow

##### Well-Known Member
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...............................

The capacitor will remove the DC offset and give an AC OUTPUT signal, sure - that was never the issue. The issue is, AC MUST change polarity in order to be AC - pulsating DC is still DC.
No. Your assertion that an AC voltage must reverse polarity is an unnecessary restriction on the definition of AC. AC is simply a time-varying signal. If you view the Fourier components of such a signal, it will have a DC component with AC components. As previously stated, a varying DC can be viewed as an AC signal riding on a DC offset. Passing the signal through a series capacitor to a resistor to ground, will remove the DC bias and give an AC signal going above and below ground. That's what the AC input on an oscilloscope does -- it allows you to view only the AC on an AC signal with a DC bias.

#### MikeMl

##### Well-Known Member
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Sorry, you need to explain that graphic to me - where was the measurment taken, after the capacitor? I've never been any good at reading graphs.

The blue trace shows that the voltage does not drop below zero, it does not change polarity - it is DC.

The capacitor will remove the DC offset and give an AC OUTPUT signal, sure - that was never the issue. The issue is, AC MUST change polarity in order to be AC - pulsating DC is still DC.

Let me correct your sentence: The issue is, AC Current MUST change polarity in order to be AC - a pulsating DC Voltage has a DC component.

The red trace in Eric's simulation is the current I(C1) which flows through all three components, the voltage source, the coupling capacitor, and the load resistor. Sure looks like only the AC current flows, and it is symmetrical +1A to -1A in the steady-state.

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#### DragonForce

##### New Member
I've had a word with an electronics engineer, who tells me that AC (alternating CURRENT) is not the same as alternating VOLTAGE - voltage can alternate without the current doing so. If the CURRENT does not alternate (change polarity) then it isn't AC - it MAY be that the voltage will alternate however - am I getting these two things confused? If CURRENT flows unidirectionally, it is DC - however, voltage does not equate to current.

I apologise to anybody who thinks me aggressive, please don't misunderstand my passion for this, my hobby of 40 years as me trying to upset or anger people. I understand you guys are only trying to help, and yeah - I see that I may have come over as being a little confrontational.

#### ericgibbs

##### Well-Known Member
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hi DF,

The definition of an Alternating Voltage is: Periodic voltage, the average value of which over a period is zero.

Eric

#### audioguru

##### Well-Known Member
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Hi
so hw does this coupling capacitor differ from an ordinary capacitor??Does'nt an ordinary cap too block an ac n send only the dc??
A coupling capacitor is not a special type of capacitor, it is an ordinary capacitor.
A coupling capacitor is in series and it blocks DC and passes AC. You wrote it backwards.
A filter capacitor is connected to ground and is fed from a series resistance. Then higher frequencies pass to ground and lower frequencies pass through the resistor to the output.

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