Capacitors & Capacitance

[Roger Johansson]

Rikard Bosnjakovic <bos@REMOVETHIShack.org> wrote:

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

"Hotlinking not permitted".

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]

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.


--
Roger J.

 

[John Popelish]

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)

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.

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.

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

 

[Tom Biasi]

"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.

Hi Richard,
John has given you a technical and eloquent (as usual) explanation.
Your response indicated that you may not have comprehended the explanation.
You say "I know capacitors don't allow current to flow through them ".
Well if you look at DC you could be considered somewhat correct.
If you understand the capacitors mechanisms you may see that AC will 'Pass'.
You also say, "they can be charged and discharged and that the time taken to
charge and discharge can
be calculated. ".
After saying those things you still can not think of any uses for
capacitors?
Think some more,
Regards,
Tom

 

[John Popelish]

Richard Harris wrote:

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

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.

People who run motor windings with capacitors in series would be
amazed to hear that. While it is true that no particular electron
that goes in one side ever makes it out the other side, if you push an
electron in one side, a different one comes out the other side. And
that is still current.

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?

Charge flows into one sire and out of the other, even though no charge
makes it through the insulation between the plates. An electron
arriving onto the surface of one plate creates an electric field that
repels an electron on the other side of the insulation to leave the
other plate and leave the capacitor. Since there are electrons moving
through both leads, there is effectively current passing through the
capacitor. All that is required to make this current is to force a
change in the voltage applied to the capacitor. Once the voltage
stops changing, the current stops.

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

 

[Andrew Holme]

Richard Harris wrote:

"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.

Current *can* flow through a capacitor - but not forever, in the same
direction. As it flows, charge is taken from one plate and piled up on the
other. The greater the current, and the longer it flows, the more charge is
moved, and the greater the potential difference between the plates. If this
goes on for too long, the dialetric breaks down.

 

[Andrew Holme]

Andrew Holme wrote:

Richard Harris wrote:
"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.

Current *can* flow through a capacitor - but not forever, in the same
direction. As it flows, charge is taken from one plate and piled up
on the other. The greater the current, and the longer it flows, the
more charge is moved, and the greater the potential difference
between the plates. If this goes on for too long, the dialetric
breaks down.

That should be "dielectric" of course.

 

[John Popelish]

Richard Harris wrote:

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.

A bit overstated, I think. I would say that a capacitor is a device
that must pass current in order for the voltage across it to change.
There are other devices that oppose voltage change by other means than
capacitance.

Capacitors are often connected across DC supply rails for the effect
you describe. If only small and brief currents are involved, then
small capacitors may do (e.g. .1 uf across the power pins of a logic
chip). If larger and longer lasting currents are involved, then quite
large capacitors ar used (following rectifiers in power supplies, for
instance). In both cases, the voltage is certainly more steady with
the cap in place than it is without it. This is using caps as
something like small rechargeable batteries, except that chemical
batteries can supply current with almost no change in their output
voltage, while capacitors must always have some voltage change if they
are going to pass current.

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.

Again, a bit overstated, but not wrong. Change "the property" to "a
property".
There are other things that oppose current change that do not involve
inductance.

--
John Popelish

 

[Richard Harris]

Thanks guys

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

"Tom Biasi" <TomBiasi@optonline.net> wrote in message
news:W9eJd.1931$H43.998@fe11.lga...

"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.

Hi Richard,
John has given you a technical and eloquent (as usual) explanation.
Your response indicated that you may not have comprehended the
explanation.
You say "I know capacitors don't allow current to flow through them ".
Well if you look at DC you could be considered somewhat correct.
If you understand the capacitors mechanisms you may see that AC will
'Pass'.
You also say, "they can be charged and discharged and that the time taken
to
charge and discharge can
be calculated. ".
After saying those things you still can not think of any uses for
capacitors?
Think some more,
Regards,
Tom

 

[Gareth]

bxbxb3 wrote:

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


An inductor is a bit like a heavy object on wheels. The current in the

inductor is analogous to the speed of the object. The force applied to
the object is analogous to the voltage applied to the inductor.

If you apply a force to the heavy object on wheels it will slowly
accelerate. Similarly, if you apply a voltage to an inductor the
current in the inductor will ramp up smoothly.

If you stop pushing the heavy object it will slow down due to friction.
Similarly if you remove the voltage from an inductor current will
continue to flow through it but the current will decrease due to
resistance in the circuit (for a perfect frictionless object or a
perfect inductor with no resistance the motion or current will continue).

If you try to stop your heavy object quickly when it is moving fast the
inertia of the object resists this change. Similarly if you try to stop
the current in an inductor quickly the inductance opposes this change.
This is why you need to be careful when you switch inductive loads like
relays and motors.

The equations for a heavy object are:

F = ma

Where F = Force, m = mass, a = acceleration (rate of change of speed)

E = 1/2*m*(v^2)

Where E = Kinetic Energy, m = mass, v = velocity

The equations for an inductor are:

V = L*dI/dt

Where V = Voltage, L = inductance, dI/dt = rate of change of current

E = 1/2*L*(I^2)

Where E = Energy stored in the inductor, L = Inductance, I = current

If you think of inductance as mass, voltage as force and current as
velocity the equations are the same.

Gareth.

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

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

 

[John Popelish]

bxbxb3 wrote:

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

If you imagine that voltage is torque and rotational speed is current,
then inductance is something like the inertia of a flywheel. Apply
torque and the flywheel steadily increases its rate of rotation (apply
voltage across an inductance, and current ramps up). It takes a large
spike of torque the other way to bring the rotation to a halt (it
takes a large applied reverse voltage to bring an inductive current to
zero, quickly).

A nice thing about this analogy is that flywheels turn around an axis
while current goes around the magnetic field of an inductance.

--
John Popelish

 

[Roger Johansson]

"bxbxb3" <bxbxb3@yahoo.com> wrote:

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

An inductor is like a water pump without a motor, it is driven by the
water that is pushed through it.

It needs a pressure and a current to get going, and then it keeps on
running, pumping water, until the resistance slows down the current.

This is used in cars for creating a spark, or to create the starting
spark in a flouroscent tube.

Voltage is used to get the current running in a coil, and then suddenly
the connection is cut off, the current still pumps through the coil and
where is that current going to go? It has nowhere to go so the voltage
increases until a spark jumps over to ground, and that spark starts the
car or the fluoroscent tube.

Inductors have a resistance against quick changes in the current,
capacitors have a resistance against quick changes in the voltage.

The resistance changes with frequency, so inductors have little
resistance at low frequencies and high resistance at high frequencies.

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

This pdf is written in swedish but that doesn't matter because it is
only the diagram we need. In the index find the word " Induktanser" and
click on it, then scroll down one page, there is the diagram. Zoom in to
see the details. It is on page 33 in the pdf file.

You can see how the horizontal scale is the frequency scale, the vertical
scale is resistance, diagonally you see inductance and capacitance.

This diagram tells you what resistance a certain inductance or
capacitance has at a certain frequency.

For example, we want to know what inductor is needed for a loudspeaker
filter, it should have a resistance of 10 Ohms at 200Hz.

We go into the diagram from the horisontal 10 Ohm line, follow it to the
(vertical) 200Hz line, there is our working point. From there, follow the
diagonal line down left towards the border of the diagram and you see the
value 10uH.

So, we need a 10uH coil for this purpose.

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.


--
Roger J.

 

[PeteS]

Although I agree with that desccription, it might not go far enough, in
a way. (IMO)

Strictly speaking (and you have to, quite often), capacitance is
exhibited between any two points where there is a difference in
potential.
This leads to the interesting observation that a resistor with current
flowing through it is, in fact, a parallel RC network (the amount of
capacitance is roughly proportional to the resistance but
obviously depends on the physical dimensions of the resistor). This,
incidentally, is one of the reasons to avoid large resistor values in
the feedback loop of an opamp (it makes an integrator out of it).

It also means that there is capacitance between virtually any two
points on the
average PCB. That is a point of major importance to practising
designers. Thousands more examples may be found of where capacitance
exists (static discharge testing uses, amongst other things, the 'human
body model').

When designing filters or PCBs that are low frequency, you usually
don't have to worry about those effects but as soon as things get above
a few 10s of MHz or so, (and don't forget transients), they can become
of overriding importance. Note that in a switching power supply with an
oscillator of 100kHz (quite low by current standards), board layout
capacitive effects are of major significance (the transients are in the
order of 10s of MHz, typically).

Note my first statement - it is what led to the development of a slick
little arrangement called 'guarded leakage' used quite a bit in the
input to sample and hold circuits. You can find that in Horowitz & Hill
(don't ask for the page, it's been over 20 years since I read it)


As with many things electrical and electronic, the model one uses
depends on the circumstances.

So the answer to "what is capacitance" is long, and you have to decide
which parameters are of importance at a given time

PeteS

 

[Jamie]

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.


let me give you an example.

lets assume that you have the need for a negative pulse from a circuit
that only has a common and + supply of voltage.
using a 100Uf capacitor to test with, connect the - side to your +
input of your DM ( multimeter Voltage Scale). the - lead of your DM to
the common side of lets say a 9 volt battery which would be the -
terminal in this example.
using a simple thing like a double throw switch, connect the + side
of the capacitor to the center pole of the switch, one side of the switch
to common and the other side to the + terminal on the 9 v battery.
watch you DM as you switch from one side to the other.
you will noticed that you get a momentary pulse of + voltage when the
center pole of the switch is connected to the + terminal of the Battery.
now switching the center pole of the switch to the common side (which
would be the - side of your battery) yields a - voltage in your meter.
if you were to short the capacitor with a jumper and repeat the action
of connecting the center pole of the switch to + terminal of the battery
and then the common side (- terminal of the batter), you will see only a
+ reading on your meter and then only a 0 zero, you will not see a -
voltage develop.
just think of a battery's terminals being switched to give reverse
polarity.
the capacitor is in effect a cell (battery being 2 or more cells);
--
now lets apply this to a simple class A amplifier.
i will only talk about the basics here.
imagen using a resistor from the + terminal of your 9 volt battery to
supply current to the collector of a transistor.
now when the circuit is properly biasing the transistor (setting is
current state), you can measure the voltage at the collector and lets
assume that it is for now 50% of the supply voltage, in this case
using your 9 volt batter = 4.5 volts.
if you were to connect a speaker to this same point, the speak would
push to one side and stay there due to the DC , there for you would only
get a half cycle movement in the speaker. this is not good.
if you were to now decouple the connection with a capacitor of lets
say around 1000 uf or more (incase you want to experiment with this),
the variation of voltage at the collector and resistor will give you a
+ and - (true -) voltages. When the DC stabilizes, the cap will become
fully charged and thus no current will no longer flow leaving the
speaker cone at it's natural resting place.
So you can simply think of capacitors in circuits like alternate the
direction of flow to give you real AC voltage effects, and
also caps are used to suppress the variations voltages and for making
time constant effect circuits etc. ( the list goes on)
if you would look at a LM555 timer and some examples, you could a
good idea how they could be applied in timers, oscillators (tone
generators) etc.
you mite want to get your hands on one of those 101 kits. they are
very intuitive for introducing people to electronics.

I hope long winded message didn't bore you.

 

[Kitchen Man]

On Mon, 24 Jan 2005 19:53:47 +0000 (UTC), "Richard Harris"
<richard_harris_2@hotmail.com> 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.

You said it: "Time taken to charge and discharge can be calculated."
This allows us to use fancy math to make electrons dance any dance we
want, from pulling the buzz out of electricity that we want to be clean,
to putting the zing into electricity that we want to resonate - for
instance, creating a musical note in an electric piano. IOW, caps allow
us to remove and create frequencies accurately and reliably. This puts
music and speech on top of radio waves, alphabetic characters on a video
screen, and squeezes wall-shaking rumbles out of subwoofers.

You may have misread that documentation you have that says capacitors do
not pass AC. That's kind of generally what happens, but what your book
most likely means is that capacitors give us the means to control what
frequencies of AC are allowed, and which are not.

--
Al Brennan