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Capacitance rules...

TenderTendon

Dec 20, 2014
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The rules involving capacitors in series and in parallel have always confused me. Even today, when I see a little subject involving a simple DC power supply filter, I still scratch my head. The confusion comes from the fact that when putting 2 capacitors in parallel, the capacitance is doubled, but the voltage rating stays the same. One attribute changes, the other stays the same. Now, if you put them in series, the voltage capacity is doubled, but the capacitance does not stay the same. It is halved. BOTH attributes change. What phenomenon causes this? The only thing I can think of to compare it to would be batteries. If I put 2 batteries in parallel, the capacity is doubled, but the voltage stays the same. If I put them in series, the voltage is doubled, but the capacity stays the same. At least that is my understanding. I would think that capacitors follow the same rules as batteries, but they don't seem to. I can easily find 20 websites that teach the parallel/series capacitor rule, but none explain what causes the behavior. I guess I need to read up on this more.
 
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crutschow

May 7, 2021
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It should be rather obvious that if you put capacitors in parallel, the capacitance would double, and the voltage rating stays the same, since they both experience the full voltage.

If two identical capacitors are in series, then the applied voltage is divided between the two, with the voltage across each thus being 1/2 the applied.
Since the charge into or out of a capacitor is proportional to the voltage (Q = CV), then the charge into each capacitor for a given applied voltage is 1/2 of a single capacitor of the same capacitance rating.
Thus the apparent capacitance of the two in series is 1/2 that of a single capacitor.

If you look at the charge that flows into and out of a capacitor due to a change in the applied voltage across the capacitor, then it should be clearer as to what is happening in each case.
 

Bluejets

Oct 5, 2014
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I
The rules involving capacitors in series and in parallel have always confused me. Even today, when I see a little subject involving a simple DC power supply filter, I still scratch my head. The confusion comes from the fact that when putting 2 capacitors in parallel, the capacitance is doubled, but the voltage rating stays the same. One attribute changes, the other stays the same. Now, if you put them in series, the voltage capacity is doubled, but the capacitance does not stay the same. It is halved. BOTH attributes change. What phenomenon causes this? The only thing I can think of to compare it to would be batteries. If I put 2 batteries in parallel, the capacity is doubled, but the voltage stays the same. If I put them in series, the voltage is doubled, but the capacity stays the same. At least that is my understanding. I would think that capacitors follow the same rules as batteries, but they don't seem to. I can easily find 20 websites that teach the parallel/series capacitor rule, but none explain what causes the behavior. I guess I need to read up on this more.
I'm guessing you probably have trouble with water pipe flow capacity as well.
 

TenderTendon

Dec 20, 2014
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It should be rather obvious that if you put capacitors in parallel, the capacitance would double, and the voltage rating stays the same, since they both experience the full voltage.

If two identical capacitors are in series, then the applied voltage is divided between the two, with the voltage across each thus being 1/2 the applied.
Since the charge into or out of a capacitor is proportional to the voltage (Q = CV), then the charge into each capacitor for a given applied voltage is 1/2 of a single capacitor of the same capacitance rating.
Thus the apparent capacitance of the two in series is 1/2 that of a single capacitor.

If you look at the charge that flows into and out of a capacitor due to a change in the applied voltage across the capacitor, then it should be clearer as to what is happening in each case.
I'm trying to wrap my head around this, but if I read this right, it seems that you are saying that the capacity of a capacitor changes depending on how much voltage is applied to it. Is that the case, or is it simply that the amount of energy stored is halved, because the applied voltage is split between the two?
 

crutschow

May 7, 2021
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if I read this right, it seems that you are saying that the capacity of a capacitor changes depending on how much voltage is applied to it
The capacitance of a capacitor is the amount of charge it stores per unit of voltage.
The capacitance does not change with voltage but the amount of stored charge does.
You do not see to have a proper understanding of what a capacitor does.
is it simply that the amount of energy stored is halved, because the applied voltage is split between the two?
That's basically true.
 

danadak

Feb 19, 2021
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Regards, Dana.
 

TenderTendon

Dec 20, 2014
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The capacitance of a capacitor is the amount of charge it stores per unit of voltage.
The capacitance does not change with voltage but the amount of stored charge does.
You do not see to have a proper understanding of what a capacitor does.

Your first 2 sentences here really got me thinking and I think I've finally got it. I had a pretty good understanding of what a capacitor does, but my confusion was with the way capacitors are rated. For example: If I saw a capacitor rated at 20uF @ 500v, I viewed this as a 20uF capacitor with a 500v maximum rating (two separate, unrelated ratings). What I never took the time to grasp was that the 500v rating is not just a limit, but a requirement to achieve the rated 20uF of storage and if 250v were applied to this same capacitor, only 10uF would be stored (all theorhetical). Is this the correct way to view this?
 

crutschow

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For example: If I saw a capacitor rated at 20uF @ 500v, I viewed this as a 20uF capacitor with a 500v maximum rating (two separate, unrelated ratings).
That is correct.
the 500v rating is not just a limit, but a requirement to achieve the rated 20uF of storage and if 250v were applied to this same capacitor, only 10uF would be stored (all theorhetical). Is this the correct way to view this?
Again no, that is not what I said.
Do not confuse the capacitance value with the charge stored on a capacitor, which are two related but independent values. as you initially stated.
The term "rated 20uF of storage" is not a proper statement.
It's 20uF of capacitance, but the charge "stored" is a function of both the capacitance and the voltage.
There is no relation between the capacitance value, which does not change with voltage, and the capacitor voltage rating.
Whether the capacitor is rated a 10V or 500V, 20uf is 20uf, and the circuit cannot tell the difference between the two (unless the capacitor voltage is exceeded and it blows up).

What does change is the amount of charge stored on a capacitor with the applied voltage according to the formula Q = C*V where Q is the charge, C is the capacitance, and V is the voltage.
V is unrelated to the capacitors voltage rating (other than it it limited to that voltage)
Once you understand that, you will understand capacitors.
 
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bertus

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Nov 8, 2019
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Hello,

Have a look at the following page:

Bertus
 

TenderTendon

Dec 20, 2014
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Do not confuse the capacitance value with the charge stored on a capacitor, which are two related but independent values. as you initially stated.
The term "rated 20uF of storage" is not a proper statement.
It's 20uF of capacitance, but the charge "stored" is a function of both the capacitance and the voltage.

Wow, that finally did it. I WAS confusing capacitance with stored charge. Given this information, it's clear to see that a capacitor rated 20uF @ 500v can store much more energy than one rated 20uF @ 50v, even though they have the same capacitance.
 

crutschow

May 7, 2021
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it's clear to see that a capacitor rated 20uF @ 500v can store much more energy than one rated 20uF @ 50v, even though they have the same capacitance.
Especially since the stored energy is proportional to the square of the applied capacitor voltage.
 

TenderTendon

Dec 20, 2014
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Do not confuse the capacitance value with the charge stored on a capacitor, which are two related but independent values. as you initially stated.
The term "rated 20uF of storage" is not a proper statement.
It's 20uF of capacitance, but the charge "stored" is a function of both the capacitance and the voltage.
There is no relation between the capacitance value, which does not change with voltage, and the capacitor voltage rating.
Whether the capacitor is rated a 10V or 500V, 20uf is 20uf, and the circuit cannot tell the difference between the two (unless the capacitor voltage is exceeded and it blows up).

What does change is the amount of charge stored on a capacitor with the applied voltage according to the formula Q = C*V where Q is the charge, C is the capacitance, and V is the voltage.
V is unrelated to the capacitors voltage rating (other than it it limited to that voltage)

Thank you for taking the time to explain this. I've read many articles and watched many videos. None ever explained it as clearly as you did.
 

crutschow

May 7, 2021
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I'm guessing you probably feel like a big man when you ridicule people on a public forum, using your supreme intelligence.
Unfortunately, for whatever reason (bad day at home?), there are a few on these forums who sometimes act like they are still in high school, and demean and bully neophytes.
Perhaps they think they are funny.
Just ignore them.
 
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