# Big Fat capacitor and dc blocking

#### subatomic particle

Joined May 8, 2018
76
Hello everyone,
I am wondering why do we use capacitors that are so big to block the DC part of a signal. As far as i know the voltage drop across a capacitor will decrease if the capacitor gets bigger, which means less votlage will drop on the capacitor and the rest of the voltage will drop across the input. So how does this work? Isn't it better to use a small one?

#### ericgibbs

Joined Jan 29, 2010
17,972
hi sp,
The capacitive Reactance of a capacitor is Zc = 1/( 2pi *f *C).
Consider the attenuation effect at lower frequencies.

E

#### subatomic particle

Joined May 8, 2018
76
hi sp,
The capacitive Reactance of a capacitor is Zc = 1/( 2pi *f *C).
Consider the attenuation effect at lower frequencies.

E
I understand that part but we also want to block the DC voltage and using a big capacitor will not block it completely Right? I mean consider you want to block the DC part at the the gate of a MOSFETs. You will put before the gate a big fat capacitor and the gate source capacitor is probably smaller than the big fat capacitor, so the voltage drop will higher between Gate and sources which means we didn't really block the dc voltage from entering the gate

#### ericgibbs

Joined Jan 29, 2010
17,972
hi,
Post a simple drawing or example of the type and location of the capacitor that you are querying.
E

#### subatomic particle

Joined May 8, 2018
76
Ok here is what i mean

#### crutschow

Joined Mar 14, 2008
32,840
we also want to block the DC voltage and using a big capacitor will not block it completely Right?
Wrong.
No matter how bit (fat) the capacitor, it always blocks DC.
It it didn't block DC it wouldn't be a capacitor.

The size of the capacitor is determined by how low an AC frequency you want to pass, and what the rest of the circuit impedance is.

#### nsaspook

Joined Aug 27, 2009
11,738
I understand that part but we also want to block the DC voltage and using a big capacitor will not block it completely Right? I mean consider you want to block the DC part at the the gate of a MOSFETs. You will put before the gate a big fat capacitor and the gate source capacitor is probably smaller than the big fat capacitor, so the voltage drop will higher between Gate and sources which means we didn't really block the dc voltage from entering the gate
What? The capacitor passes changes in electric field energy between the plates.

https://www.tdk.com/en/tech-mag/cap...on as the power,a capacitor blocks DC current.

#### subatomic particle

Joined May 8, 2018
76
Wrong.
No matter how bit (fat) the capacitor, it always blocks DC.
It it didn't block DC it wouldn't be a capacitor.

The size of the capacitor is determined by how low an AC frequency you want to pass, and what the rest of the circuit impedance is.
I meant only in case we have other capacitor (which is here the Input Capacitor of the gate

#### dl324

Joined Mar 30, 2015
15,810
I understand that part but we also want to block the DC voltage and using a big capacitor will not block it completely Right?
By definition, capacitors will block DC. The size of the capacitor used depends on the application (frequency). Capacitors have impedance so, at a given frequency, a high value capacitor will have a lower impedance than one with a lower value. But there are parasitics that could come in to play which make the components perform in a less than ideal manner.
Ok here is what i mean
In most cases, this would be an inappropriate usage. You're using some sort of MOSFET, but it looks like the wrong polarity to me, and you've drawn the symbol for a P channel depletion mode MOSFET which would rarely be used in the way you've shown.

#### subatomic particle

Joined May 8, 2018
76
By definition, capacitors will block DC. The size of the capacitor used depends on the application (frequency). Capacitors have impedance so, at a given frequency, a high value capacitor will have a lower impedance than one with a lower value. But there are parasitics that could come in to play which make the components perform in a less than ideal manner.
In most cases, this would be an inappropriate usage. You're using some sort of MOSFET, but it looks like the wrong polarity to me, and you've drawn the symbol for a P channel depletion MOSFET which would rarely be used in the way you've shown.
Hallo,
in my literature we use this Symbol for N- Channel

#### dl324

Joined Mar 30, 2015
15,810
in my literature we use this Symbol for N- Channel
Your literature is wrong. The arrow on MOSFETs and JFETS JFETs points in for N channel and out for P. We sometimes are careless about indicating depletion mode from enhancement mode when hand drawing schematics because it takes too much effort to dash the vertical line that indicates the channel. If they used that symbol in a book, it's wrong in multiple regards.

This is how N and P channel JFETs and enhancement mode MOSFETs would usually be drawn (there are some confusing variants, but they all agree on arrow direction):

The dashed vertical line on the enhancement mode MOSFETs would be solid for depletion mode devices. I don't know of any part numbers off the top of my head.

EDIT: corrected typo on JFETs.

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#### Audioguru again

Joined Oct 21, 2019
6,151
I agree that the symbol in the book is wrong:
1) The arrow points in the direction of a P-channel Mosfet or an N-channel Jfet.
2) The bar is solid for a depletion type but nearly all Mosfets are enhancement types with a dashed bar.
3) The gate lead does not bend down to the source lead.
4) The diode is missing.

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#### MisterBill2

Joined Jan 23, 2018
15,579
The capacitor will stop the DC current from flowing AFTER it is charged. Until the capacitor is charged, current will flow in to charge the capacitor. That is what causes the thump in the speakers when you switch on an amplifier that does not have the timed relay to keep the turn on capacitor charging thump from damaging the speakers.
Capacitors hold "charge", they seldom arrive charged, and so charging current flows into a capacitor until it is charged. THEN a capacitor couples the AC signal while blocking the DC portion, because it s already charged.

#### k1ng 1337

Joined Sep 11, 2020
701
Wrong.
No matter how bit (fat) the capacitor, it always blocks DC.
It it didn't block DC it wouldn't be a capacitor.

The size of the capacitor is determined by how low an AC frequency you want to pass, and what the rest of the circuit impedance is.
Would it not be more realistic to say a capacitor allows DC to pass in a decaying fashion to where it becomes blocked only when completely full. According to the math this never happens as an ideal capacitor response is asymptotic.

Even with a 99% charge at 5RC, the idea that capacitors block DC doesn't make much sense to me. Another one of those cases in electronics where terms appear to be bastardized which leads to confusion.

#### MisterBill2

Joined Jan 23, 2018
15,579
Back in 1959 I learned that a capacitor between the plate of one amplifier tube and the grid of the next amplifier tube would prevent the 150 volts DC from being coupled to the grid of the following stage. It was easy to think that it was blocking the DC while passing the audio signal. And the 0.47 Mfd capacitor across the cathode resistorallowed the audio current to flow while not letting the DC voltage flow. Coupling and blocking are simple concepts for a kid to grasp,even if the actual process is more complex, which it really is. But it worked well for me until I got to engineering school many years later. So we can say that FUNCTIONALLY it is blocking the DC and be correct.

#### nsaspook

Joined Aug 27, 2009
11,738
Would it not be more realistic to say a capacitor allows DC to pass in a decaying fashion to where it becomes blocked only when completely full. According to the math this never happens as an ideal capacitor is asymptotic.

Even with a 99% charge at 5RC, the idea that capacitors block DC doesn't make much sense to me. Another one of those cases in electronics where terms appear to be bastardized which leads to confusion.
What does 'full' mean? A capacitor doesn't store electrical charge, it stores electrical energy from a potential difference between plates caused by charge separation.

Analogy time.
A low reactance (at the AC signal energy) electronics coupling capacitor is a lot more like a special type of good electrical conductor (low value resistor) than a energy storage device or a physics capacitor. The AC electrical energy surrounding the wire couples across the capacitor dielectric insulator from plate to plate as an electric field due to close proximity (much like how a transformer couples AC magnetic energy across separate coils) while the dielectric charge carrier insulator separation stops DC currents from a static electric field.

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#### k1ng 1337

Joined Sep 11, 2020
701
What does 'full' mean? A capacitor doesn't store electrical charge, it stores electrical energy from a potential difference between plates caused by charge separation.

Analogy time.
A low reactance (at the AC signal energy) electronics coupling capacitor is a lot more like a special type of good electrical conductor than a energy storage device or a physics capacitor. The AC electrical energy surrounding the wire couples across the capacitor dielectric insulator from plate to plate as an electric field due to close proximity (much like how a transformer couples AC magnetic energy across separate coils) while the dielectric charge carrier insulator separation stops DC currents from a static electric field.
Well you got me with charge vs energy which is exactly my point. People use analogies (words) instead of numbers which is the problem in my opinion.

Then of course people will argue back and forth using words and not numbers (the proof) which makes matters worse.

To make matters even worse, I am still learning an incredibly complicated subject!

#### nsaspook

Joined Aug 27, 2009
11,738
Well you got me with charge vs energy which is exactly my point. People use analogies (words) instead of numbers which is the problem in my opinion.

Then of course people will argue back and forth using words and not numbers (the proof) which makes matters worse.

To make matters even worse, I am still learning an incredibly complicated subject!
Everyone says charge and it's understood in that context but we need to be careful what that means with capacitors (Capacitive coupling) in our mental picture.
https://en.wikipedia.org/wiki/Capacitive_coupling

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#### MisterBill2

Joined Jan 23, 2018
15,579
For a good capacitor, "The Jolt of the Volt is Proportional to the Large of the Charge."
Likewise with the "Arc of the Spark".

#### k1ng 1337

Joined Sep 11, 2020
701
Everyone says charge and it's understood in that context but we need to be careful what that means with capacitors (Capacitive coupling) in our mental picture.
https://en.wikipedia.org/wiki/Capacitive_coupling
I often make mistakes but that won't stop me from pursuing the truth.

Frankly, I am learning it's best to just stay out of conversations like this until I gather 100 years worth of experience. It is clear my opinions are rudimentary compared to life long engineers.

Folks have made me sorry more than once for my input which is unfortunate as I enjoy discussing things that I do and do not know.

Carry on without me!

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