Capacitors & Capacitance

[Richard Harris]

Hi,
Just been reading about capacitors and understand the property of
capacitance. The problem is I don't see how this property is of use, many
circuits use capacitors but I don't understand what there role is.

I know capacitors don't allow current to flow through them and that they can
be charged and discharged and that the time taken to charge and dischage can
be calculated. But how are these devices off any real use?

Thanks for ya time guys.

 

[Richard Harris]

"John Popelish" <jpopelish@rica.net> wrote in message
news:41F5586D.BC4DAC4C@rica.net...

Richard Harris wrote:

Hi,
Just been reading about capacitors and understand the property of
capacitance. The problem is I don't see how this property is of use,
many
circuits use capacitors but I don't understand what there role is.

I know capacitors don't allow current to flow through them and that they
can
be charged and discharged and that the time taken to charge and dischage
can
be calculated. But how are these devices off any real use?

Thanks for ya time guys.

Capacitors are a class of devices that have time dependent response.
The mathematical description of the relation between the "current
through a capacitor" and the voltage across it is
I=C*(dv/dt)

current through a capacitor never happens.

With I an amperes, C in farads and dv/dt, the time rate of change of
voltage, in volts per second.

Since pure DC has no rate of change, it produces no current through a
capacitor. but any other voltage waveform from varying DC to
sinusoidal AC or any other variation in voltage over time causes
current to pass through capacitors.

AC will charge and discharge a capacitor, reversr polarity of charge will
occur based on the frequency of the AC but no current will pass through a
capacitor.

There is also energy stored in any capacitor that does not have zero
volta across it. The energy is

E=(1/2)*C*V^2
With E being the energy in joules or watt seconds, C in farads and V
in volts.

The voltage cross a capacitor is also related to the total charge that
has passed through it since its had zero volts across it.
Q=C*V
Where Q is charge in coulombs, C is farads and V is volts across the
capacitor.

Agin if current can't flow through a capacitor how can voltage or a charge?

Capacitors are used for energy storage, filtering (frequency dependent
response) including resonance with inductors, DC blocking (while
passing AC riding on the DC bias), and timing circuits the measure the
time it takes for a specific voltage change caused by a charging
current. They can also be used to add up the total (integral) of a
signal over a period of time as a voltage change across the capacitor,
if the signal can be converted to a proportional current that charges
the capacitor.

--
John Popelish

Thanks for your time.

 

[Gareth]

Richard Harris wrote:

"John Popelish" <jpopelish@rica.net> wrote in message

Capacitors are a class of devices that have time dependent response.
The mathematical description of the relation between the "current
through a capacitor" and the voltage across it is
I=C*(dv/dt)


current through a capacitor never happens.

Well, that is true sort of true. I expect that is why John used the
quotes. If a capacitor is charging or discharging then it appears that
current is flowing though it.

You know that capacitors can be charged and discharged, well the charge,
Q, stored in a capacitor is:

Q = C*V [1]

Where V is voltage and C is capacitance. Now, if you change the voltage
you must also change the charge on the capacitor. This charge has to
come from somewhere in the circuit, so charge must be moving. Moving
charge is current, so we have some current, but only while the voltage
is changing. If the voltage is constant then no current flows.

Current is actually the rate of flow of charge. So if we increase the
charge on a capacitor by dQ (where dQ means a small amount of charge) in
a time dt (where dt means a short time) then the current which appears
to flow through the capacitor during the time, dt, is dQ/dt

so I = dQ/dt [2]

For example if the charge increased by 1 C (Coulomb) in 1 second, the
average current flowing though the capacitor in that time would be 1 Amp

According to [1] a change in charge would also mean a change in voltage,
dV. Using equations 1 & 2 we can eliminate Q from the equation. This
gives, as John said:

I = C*(dV/dt)

If you use the water flow analogy and think of wires as pipes full of
water and voltage as pressure, then a capacitor would be a stretchy
rubber sheet blocking a pipe.

Now, if you increase the pressure (voltage) the rubber sheet will
stretch and some water (current) will flow in the pipe. Note that as
the rubber sheet stretches the water on the other side of the sheet will
be pushed down the pipe. No water actually crosses the rubber sheet but
as you increase the pressure water flows into one end of the pipe and
water flows out the other end. It is different water, but we don't care
about that, as far as we are concerned we force water in one end, by
increasing the pressure, and water comes out the other end.

If the pressure is constant no water will flow. If you reduce the
pressure the rubber will contract and push water back down the pipe.

If you constantly increase and decrease the water pressure water will
flow up and down the pipe. No water will actually cross the rubber
sheet but water is flowing up and down the pipe.

A capacitor will do a similar thing.

Gareth

--
-----------------------------------------------------------------------
To reply to me directly:

Replace privacy.net with: totalise DOT co DOT uk and replace me with
gareth.harris

 

[John Popelish]

Richard Harris wrote:

Thanks guys

My documentation must be inaccurate as it states that AC can not pass.

If it does then it is.

--
John Popelish

 

[Richard Harris]

I have two statements I would like to know if they are correct and that my
interpretation of them is good.

1.) Capacitance is the property that opposes changes in voltage in a
circuit.
There fore a capacitor can be used to steady voltage and keep it constant.

2.)Inductance is the property that opposes changes in current in a circuit.
There fore a coil can be used to steady current and keep it constant.

Thanks

"Richard Harris" <richard_harris_2@hotmail.com> wrote in message
news:ct3job$imc$1@sparta.btinternet.com...

Hi,
Just been reading about capacitors and understand the property of
capacitance. The problem is I don't see how this property is of use, many
circuits use capacitors but I don't understand what there role is.

I know capacitors don't allow current to flow through them and that they
can
be charged and discharged and that the time taken to charge and dischage
can
be calculated. But how are these devices off any real use?

Thanks for ya time guys.

 

[bxbxb3]

Hi,
Its been a bit late, but I hope someone will read this. Could anyone tell
me, if a capacitor is compared to a rubber sheet connected to a water
pump, what could be the possible analogy for an inductor. That example was
pretty good to compare and imagine. Thanks

 

[Rodney Kelp]

It's used in filtering power to smooth it out.

"Richard Harris" <richard_harris_2@hotmail.com> wrote in message
news:ct3job$imc$1@sparta.btinternet.com...

Hi,
Just been reading about capacitors and understand the property of
capacitance. The problem is I don't see how this property is of use, many
circuits use capacitors but I don't understand what there role is.

I know capacitors don't allow current to flow through them and that they
can
be charged and discharged and that the time taken to charge and dischage
can
be calculated. But how are these devices off any real use?

Thanks for ya time guys.

 

[Rodney Kelp]

You should read up on ELI the ICE man.

"Richard Harris" <richard_harris_2@hotmail.com> wrote in message
news:ct3job$imc$1@sparta.btinternet.com...

Hi,
Just been reading about capacitors and understand the property of
capacitance. The problem is I don't see how this property is of use, many
circuits use capacitors but I don't understand what there role is.

I know capacitors don't allow current to flow through them and that they
can
be charged and discharged and that the time taken to charge and dischage
can
be calculated. But how are these devices off any real use?

Thanks for ya time guys.

 

[Richard Harris]

What is ELI and ICE ?

"Rodney Kelp" <Rodneykelp605@hotmail.com> wrote in message
news:aa6dndQMz-7l6WvcRVn-ow@adelphia.com...

You should read up on ELI the ICE man.

"Richard Harris" <richard_harris_2@hotmail.com> wrote in message
news:ct3job$imc$1@sparta.btinternet.com...
Hi,
Just been reading about capacitors and understand the property of
capacitance. The problem is I don't see how this property is of use,
many
circuits use capacitors but I don't understand what there role is.

I know capacitors don't allow current to flow through them and that they
can
be charged and discharged and that the time taken to charge and dischage
can
be calculated. But how are these devices off any real use?

Thanks for ya time guys.

 

[Ross Mac]

"John Popelish" <jpopelish@rica.net> wrote in message
news:41F5586D.BC4DAC4C@rica.net...

Richard Harris wrote:


Since pure DC has no rate of change, it produces no current through a
capacitor. but any other voltage waveform from varying DC to
sinusoidal AC or any other variation in voltage over time causes
current to pass through capacitors.

And so explaining the old term "Blocking Condensor"....

Great explanation John, as usual!.........Ross

 

[Mike Webster]

Roger Johansson <no-email@home.se> wrote in
news:Xns95E9963BA400386336@130.133.1.4:

There is a diagram over these factors which I think is very useful
but I can only find it in a pdf file from a swedish company.
http://www.elfa.se/se/fakta.pdf

I wish there was a better way to find such a diagram, better than
to have to download a pdf file in swedish and find the diagram. If
anybody knows about such diagrams in other places on the web, tell
us about it.

These diagrams are useful because you only need your eyes to focus
and follow lines, there is no need to do calculations or touch
anything, I have this diagram in front of me all the time at the
work bench, and use it very often.

Is it anything like the Smith chart listed here: ?
http://dbserv.maxim-ic.com/appnotes.cfm/appnote_number/742

About halfway down, the blown up version, unfortunately, has writing on
it.

Ah, sweet Google to the rescue, check here:
http://www.sss-mag.com/smith.html

BTW, This group has instantly restored my faith in Usenet after
languishing for years in the alt. domains ...

Mike Webster
Sophomore, ECE, NCSU

 

[Roger Johansson]

Mike Webster <mdw*nospam*@mikeandjudith.net> wrote:

These diagrams are useful because you only need your eyes to focus
and follow lines, there is no need to do calculations or touch
anything, I have this diagram in front of me all the time at the work
bench, and use it very often.

The diagram for calculating relations between voltage, resistance,
current and power (wattage) in a linear (ohmic) resistance:

http://humanist.250free.com/VAOW2.jpg

If you know the voltage and the resistance of a resistor you follow the
line from the voltage scale and the line from the resistance scale to the
point where they meet. From that point go to the other scales and you see
the current and the wattage (power) on the other scales.

Use a pen to point at the meeting point in the beginning, later it is
easy to just focus your eyes on a certain point. Very handy when your
hands are occupied with a soldering iron and components.

I have added some colors to the diagonal resistance scale which are the
third ring color on resistors, and I have extended the diagram upwards to
calculate higher currents than the original diagram.

Sorry about the messy look but I have used this for many years and added
lines as I needed it. I used a scanner to get it off the front cover of
my homemade workbench handbook. I would love to see a better version of
the diagram.

.......

The diagram for calculating inductance, capacitance, impedance and
frequency:

http://humanist.250free.com/LCFO2.jpg

This is also very useful for calculation of resonance frequency in LC
circuits, see where the L-value and the C-value meet, and see what
frequency that gives.

Is it anything like the Smith chart listed here: ?
http://www.sss-mag.com/smith.html

It works in the same way, it gives a simple visual graphic solution
instead of using equations and a calculator.

BTW, This group has instantly restored my faith in Usenet after
languishing for years in the alt. domains ...

There are more groups which are very helpful and relatively nice, try
alt.comp.freeware for discussions and announcements of free software for
all your needs. The participators in a.c.f vote for the best freeware in
all categories every year, so you can easily find the best free software
for any purpose.

http://pricelesswarehome.org/

This year's list, divided into categories:
http://pricelesswarehome.org/2005/about2005PL.php


--
Roger J.

 

[John Fields]

On 31 Mar 2005 05:07:53 +0200, Roger Johansson <no-email@no.invalid>
wrote:

Roger Johansson <no-email@no.invalid> wrote:

The diagram for calculating relations between voltage, resistance,
current and power (wattage) in a linear (ohmic) resistance:
http://humanist.250free.com/VAOW2.jpg
The diagram for calculating inductance, capacitance, impedance and
frequency:
http://humanist.250free.com/LCFO2.jpg

In the angloamerican world it is seen as some kind of cheating to use
diagrams, they want people to memorize a lot of formulas instead.

In Europe (Germany) it is much more accepted to use diagrams. The first
diagram above is actually copied from a german book originally.
They use diagrams for many types of calculations.

I read an article in Wireless World many years ago about english and
american engineers visiting an electronics congress in Germany, and the
article made fun of the way german engineers always had a pocket book
(Schlau Buch = Sly Book = Handbook) with them and how they looked up
things instead of having everything memorized in their brains as the
angloamericans tried to do.

It has been an ideological difference in the way we teach and learn
things.

The germans think more like this; If we simplify everything as much as
possible we can free more energy and time to learn even more instead of
wasting our time with remembering equations, and making calculations with
pen and paper.

The british always want to do things "the right way", learning the proper
equations and doing the math. And american like to memorize things, even
when it fills no sensible practical purpose, like learning strange
sentences to remember the colors in the resistor color code.


I have tried to create better looking versions of these diagrams, using a
graph paper design program which could create logaritmic scales
vertically and horisontally. Then I took a copy of that diagram into a
paint program, rotated it 45 degrees and tried to superimpose it on the
original diagram, to create logaritmic scales diagonally as well as
vertically and horisontally.

But it didn't work so well, the program didn't produce diagrams which
were suitable for this. So I'll wait for better graph paper design
programs, or hope somebody else will create such diagrams.

You are free to copy and distribute these diagrams as you like, I cannot
keep them on a free web site indefinitely. Such web sites are often
closed down unexpectedly.

---
And handbooks can be lost, or stolen, or destroyed, but having once
heard "Bad Boys Rape Our Young Girls But Violet Gives Willingly", it's
with you for life.

--
John Fields
Professional Circuit Designer

 

[Roger Johansson]

John Fields <jfields@austininstruments.com> wrote:

And handbooks can be lost, or stolen, or destroyed, but having once
heard "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx foul language", it's
with you for life.

As others have already pointed out this introduces an unnecessary step in
the thinking process. It is a lot easier to learn to associate the color
with a number directly.

The best way to learn the colors is to sort a few hundred mixed resistors
using a color code table.

After that you need only look at that table occasionally to reinforce the
color you are not sure about.

How do you learn the multiplication table in your schools?

Do you have some foul language mnemonic sentences for that too?

And what happens if you see a few movies from Hollywood filled with more
extreme foul language wordings which I will not exemplify here.

Do you get your colors codes and multiplication tables all mixed up then?

I am pretty sure you don't even use such mnemonics yourself when you look
at a resistor. You associate the color with a number directly, don't you?


--
Roger J.

 

[John Fields]

On 31 Mar 2005 15:50:25 +0200, Roger Johansson <no-email@no.invalid>
wrote:

John Fields <jfields@austininstruments.com> wrote:

And handbooks can be lost, or stolen, or destroyed, but having once
heard "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx foul language", it's
with you for life.

As others have already pointed out this introduces an unnecessary step in
the thinking process. It is a lot easier to learn to associate the color
with a number directly.

---
Bullshit. It's an index which co-relates a number and a color with a
"naughty" phrase.
---

The best way to learn the colors is to sort a few hundred mixed resistors
using a color code table.

---
For you, maybe. For someone else, maybe not.
---

After that you need only look at that table occasionally to reinforce the
color you are not sure about.

---
Duh...
---

How do you learn the multiplication table in your schools?

---
I learned them by memorizing them, and I suspect that's how it's still
done, although maybe we've gone the German route and all the little
kids carry handbooks around so they don't have to clutter up their
minds with all that memorized garbage.
---

Do you have some foul language mnemonic sentences for that too?

And what happens if you see a few movies from Hollywood filled with more
extreme foul language wordings which I will not exemplify here.

Do you get your colors codes and multiplication tables all mixed up then?

---
Oh, my! How clever...
---

I am pretty sure you don't even use such mnemonics yourself when you look
at a resistor.

---
No more than you have to look at your color table, but if you got
confused and couldn't find your table what would you do? Probably the
same thing I would, and that would be to remember: "Bad Boys Rape Our
Young Girls But Violet Gives Willingly".
---

You associate the color with a number directly, don't you?

---
Of course, but I didn't always.

--
John Fields
Professional Circuit Designer

 

[Rikard Bosnjakovic]

Roger Johansson wrote:

http://humanist.250free.com/VAOW2.jpg

"Hotlinking not permitted".


--
Rikard Bosnjakovic http://bos.hack.org/cv/

Anyone sending unwanted advertising e-mail to my address will be
charged $250 for network traffic and computing time. By extracting
address from this message or its header, you agree to these terms.

 

[c_t]

I personally have grown weary of how electronics is taught in America. I
wanted to see your diagrams, but it said that there is no Hotlinking,
(whatever that is), and to visit the site directly.

Can you help me out?

Thanks,



"Roger Johansson" <no-email@no.invalid> wrote in message
news:Xns962AA0766BB5386336@81.174.12.30...

John Fields <jfields@austininstruments.com> wrote:

And handbooks can be lost, or stolen, or destroyed, but having once
heard "xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx foul language", it's
with you for life.

As others have already pointed out this introduces an unnecessary step in
the thinking process. It is a lot easier to learn to associate the color
with a number directly.

The best way to learn the colors is to sort a few hundred mixed resistors
using a color code table.

After that you need only look at that table occasionally to reinforce the
color you are not sure about.

How do you learn the multiplication table in your schools?

Do you have some foul language mnemonic sentences for that too?

And what happens if you see a few movies from Hollywood filled with more
extreme foul language wordings which I will not exemplify here.

Do you get your colors codes and multiplication tables all mixed up then?

I am pretty sure you don't even use such mnemonics yourself when you look
at a resistor. You associate the color with a number directly, don't you?


--
Roger J.

 

[Rich Grise]

On Thu, 31 Mar 2005 09:59:12 -0600, John Fields wrote:

I learned them by memorizing them, and I suspect that's how it's still
done, although maybe we've gone the German route and all the little
kids carry handbooks around so they don't have to clutter up their
minds with all that memorized garbage.

Nah. Pocket calculators. And then when they go to work, they only
have to press the buttons with the pictures of the burgers and fries,
and the POS terminal does all of the math for them.

Thanks,
Rich

 

[Roger Johansson]

"c_t" <willtop29@hotmail.com> wrote:

wanted to see your diagrams, but it said that there is no Hotlinking,
(whatever that is), and to visit the site directly.

Can you help me out?

Ooops, sorry.

I had no problems clicking on the links I gave you, so I thought it would
work for others too, but now I realize they have put a cookie on my
computer so they know I am the owner, so I am allowed to go to the
picture directly.

For them to be available for other people I had to re-edit my index.html,
and I have done that now. I hope it will work now.

Try this link http://humanist.250free.com/

Then click on the two .jpg files at the bottom of the list.

Please tell me if it doesn't work, 'cause I want them to be available to
you and others.


--
Roger J.

 

[Roger Johansson]

John Fields <jfields@austininstruments.com> wrote:

And handbooks can be lost, or stolen, or destroyed..

Cool!

--
Roger J.