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Sci.Electronics.Basics -> Can a capacitor let DC current through?
There are 54 messages in this thread.
You are currently looking at messages 40 to 54.
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Author: Anonymous.Date: 13:39 22-08-07
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"Bob Myers" <nospamplease@address.invalid> wrote in message
news:fahr76$r0e$1@usenet01.boi.hp.com...
> .....If we model AC this way, then you take
> one ball at the near end and alternately push it in and
> somehow suck it out AT THAT END. And at the far
> end, we have a ball which is behaving in exactly the same
> way, alternately popping out and being sucked back in.....
Which raises ethical questions about the behaviour of the
power companies! 50 times per second (60 in Yankland)
they give you some electrons with the right hand and then
take them back with the left hand.
As you ain't allowed to keep them, and they are only on loan
for such a brief period, then it seems morally wrong for them
to charge you for the privilege!
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Author: Bob MyersDate: 14:42 22-08-07
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"Anonymous." <me@privacy.net> wrote in message
news:fahsc5$21i$1@aioe.org...
> Which raises ethical questions about the behaviour of the
> power companies! 50 times per second (60 in Yankland)
> they give you some electrons with the right hand and then
> take them back with the left hand.
Precisely! Or, as a sign in the men's room at a local
drinking establishment succinctly puts it:
"We don't sell beer, we only rent it."
Bob M.
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Author: Rich GriseDate: 14:48 22-08-07
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On Wed, 22 Aug 2007 18:39:14 +0100, Anonymous. wrote:
> "Bob Myers" <nospamplease@address.invalid> wrote in message
> news:fahr76$r0e$1@usenet01.boi.hp.com...
>> .....If we model AC this way, then you take
>> one ball at the near end and alternately push it in and
>> somehow suck it out AT THAT END. And at the far
>> end, we have a ball which is behaving in exactly the same
>> way, alternately popping out and being sucked back in.....
>
> Which raises ethical questions about the behaviour of the
> power companies! 50 times per second (60 in Yankland)
> they give you some electrons with the right hand and then
> take them back with the left hand.
>
> As you ain't allowed to keep them, and they are only on loan
> for such a brief period, then it seems morally wrong for them
> to charge you for the privilege!
But on their trip to your house and back, they do work, like
lighting light bulbs, cooking, heating water, etc. - so what
you're paying for is the power you dissipate, or, over the
course of a month, the energy that the power company sends
to you, you use for awhile, and throw away. ;-)
Cheers!
Rich
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Author: Rich GriseDate: 14:49 22-08-07
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On Wed, 22 Aug 2007 12:42:45 -0600, Bob Myers wrote:
> "Anonymous." <me@privacy.net> wrote in message
news:fahsc5$21i$1@aioe.org...
>
>> Which raises ethical questions about the behaviour of the
>> power companies! 50 times per second (60 in Yankland)
>> they give you some electrons with the right hand and then
>> take them back with the left hand.
>
> Precisely! Or, as a sign in the men's room at a local
> drinking establishment succinctly puts it:
>
> "We don't sell beer, we only rent it."
Ick! Who wants to be the second cusomer to rent a particular pint?
Ewww!
Thanks,
Rich
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Author: Rich GriseDate: 14:50 22-08-07
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On Tue, 21 Aug 2007 23:36:31 -0400, Michael A. Terrell wrote:
> John Fields wrote:
>> On Tue, 21 Aug 2007 10:12:39 -0700, Don Bowey <dbowey@comcast.net>
>> >
>> >My thought about this is... No, capacitors do not let DC current through,
>> >nor AC currents either.
>> >
>> >Capacitors block DC voltage. AC signals appear to pass through capacitors,
>> >but they don't actually do that.
>> >
>> >Flame away.
>>
>> The flame of truth burns eternal. :-)
>
> Unfortunately, so do the sparks of stupidity. :(
We've noticed. Thanks for the reminder, Michael. :-)
Cheers!
Rich
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Author: Rich GriseDate: 14:57 22-08-07
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Author: ChuckDate: 19:39 22-08-07
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John Larkin wrote:
SNIP
>
> "AC" and "DC" are extremely vague terms, and a capacitor
doesn't care
> about terminology; most capacitors are made in foreign countries and
> don't even understand English. Whether a signal is one or the other
> depends on the time frame over which it's observed. A 1-cycle-per-year
> sine wave sure looks like DC if you observe it for an hour.
SNIP
Part of the problem is that AC and DC
are, as John said, extremely vague
terms. They have no fundamental basis in
theory other than as a shorthand in
those cases where the context makes
their meaning clear.
AC and DC are descriptive terms that can
cause confusion whenever applied to
anything other than a pure, time-varying
sinusoidal voltage or a time-invariant
voltage, respectively. That these are
quasi-bogus terms becomes clear when we
recall that they have no units! They
don't appear in formulas. We can't
measure the AC-ness or DC-ness of a
circuit or device. Would electronics
theory even miss AC and DC?
I wonder whether there are other
descriptive terms used in electronics
the way AC and DC are used. Offhand, I
can't think of any at all!
As someone else pointed out, the
confusion disappears when we consider
the basic definitions of capacitors and
inductors (which do not include AC or
DC) rather than "capacitors pass AC but
not DC".
Chuck
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Author: vorangeDate: 21:58 22-08-07
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Hi,
could you take a look at the first animation on this page (below). Is
that what happens when (steady state) DC encounters a capacitor for
the first time. Does the capacitor pass a small "blip" of a charge
INITIALLY on startup and then block all further DC current.
http://www.talkingelectronics.com/html/Page03.html
If so then my mental model is right. I liken it to blowing air out of
my lungs (in one breath) onto a paper fan. The fan turns at first (a
blip) however I eventually run out of air in my lungs to blow out
(just like the opposite plate of the cap runs out of free electrons to
push away). And that's why the load (the paper fan) stops turning.
But there is that initial blip is there not? If so one should be
careful when using caps.... what if the input pin of a microcontroller
registers that 'blip' as a HIGH input signal and does something in
response to it!? Would it be wise to place a resistor ahead of the
cap if its going into some low impedance input pin of some digital/
logic chip?
Thanks to all for your help.
> On Aug 22, 7:22 am, John Popelish <jpopel...@rica.net> wrote:
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Author: John PopelishDate: 22:26 22-08-07
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vorange wrote:
> Hi,
>
> could you take a look at the first animation on this page (below). Is
> that what happens when (steady state) DC encounters a capacitor for
> the first time. Does the capacitor pass a small "blip" of a charge
> INITIALLY on startup and then block all further DC current.
Yep. Once the voltage stops changing, the current fades
toward zero.
> http://www.talkingelectronics.com/html/Page03.html
>
> If so then my mental model is right. I liken it to blowing air out of
> my lungs (in one breath) onto a paper fan. The fan turns at first (a
> blip) however I eventually run out of air in my lungs to blow out
> (just like the opposite plate of the cap runs out of free electrons to
> push away). And that's why the load (the paper fan) stops turning.
>
> But there is that initial blip is there not?
Yes, there is. And if you look further down, it shows that
you can extract the reverse of that blip by connecting a
load across the capacitor.
> If so one should be
> careful when using caps.... what if the input pin of a microcontroller
> registers that 'blip' as a HIGH input signal and does something in
> response to it!? Would it be wise to place a resistor ahead of the
> cap if its going into some low impedance input pin of some digital/
> logic chip?
The main capacitor you have to worry about is your body. If
you walk across a rug, you may mechanically charge up your
body capacitance and when you touch the processor, you can
dump that charge into a pin. It can be a large enough pulse
to damage the device.
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Author: Anonymous.Date: 04:38 23-08-07
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"Rich Grise" <rich@example.net> wrote in message
news:pan.2007.08.22.18.51.00.762838@example.net...
> On Wed, 22 Aug 2007 12:42:45 -0600, Bob Myers wrote:
>> "Anonymous." <me@privacy.net> wrote in message
>> news:fahsc5$21i$1@aioe.org...
>>> Which raises ethical questions about the behaviour of the
>>> power companies! 50 times per second (60 in Yankland)
>>> they give you some electrons with the right hand and then
>>> take them back with the left hand.
>> Precisely! Or, as a sign in the men's room at a local
>> drinking establishment succinctly puts it:
>> "We don't sell beer, we only rent it."
> Ick! Who wants to be the second cusomer to rent a particular pint?
A pseudo-science TV programme over here in Britland suggested
that statistically in every glass of water that you drink is at least
one molecule that was urinated by Isaac Newton!
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Author: ChuckDate: 08:35 23-08-07
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John Popelish wrote:
SNIP
>
> The main capacitor you have to worry about is your body. If you walk
> across a rug, you may mechanically charge up your body capacitance and
> when you touch the processor, you can dump that charge into a pin. It
> can be a large enough pulse to damage the device.
Well, the body is a capacitor in the
sense that distributed somewhere in the
universe is the "other plate" containing
the opposite charge. The resulting
electric field created by that capacitor
is usually not strong enough to overcome
the repulsive effects of the charges on
the body when the body comes into
contact with another object.
The problem with describing the body as
a capacitor is that one might then
imagine that touching one plate of a
"conventional capacitor" will discharge it.
Pedagogically, it is probably better to
describe the body as a charged object,
which is not the way we normally
describe a capacitor.
Chuck
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Author: Rich GriseDate: 13:57 23-08-07
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On Wed, 22 Aug 2007 18:58:59 -0700, vorange wrote:
> Hi,
>
> could you take a look at the first animation on this page (below). Is
> that what happens when (steady state) DC encounters a capacitor for
> the first time. Does the capacitor pass a small "blip" of a charge
> INITIALLY on startup and then block all further DC current.
>
> http://www.talkingelectronics.com/html/Page03.html
>
> If so then my mental model is right. I liken it to blowing air out of
> my lungs (in one breath) onto a paper fan.
Think of it more like a chamber with a rubber diaphragm stretched
across the middle:
----------------------
| / |
| \ |
| / |
------- \ -------
------- / -------
| \ |
| / |
| \ |
----------------------
When you blow in the left, at first the diaphragm moves to the right,
and displaces the air on its right. Eventually, the diaphragm "bottoms
out", and the flow stops (or, you're blowing as hard as you can, even
though the diaphragm is only part-way, but the back-pressure stops you).
When you release the pressure, it blows the air back in your face. That's
like the cap discharging, except that for the charge to flow like the air
does here, you need wires and a complete circuit. :-)
That's the flaw I see in the fluid models of electricity - the fluid
needs to be in a pipe. If you break the pipe, all the fluid spills out.
If you break a wire, the flow stops. So they're kind of opposites in
that respect, but otherwise it seems to work well.
Hope This Helps!
Rich
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Author: Anonymous.Date: 14:48 23-08-07
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"Rich Grise" <rich@example.net> wrote in message
news:pan.2007.08.23.17.59.18.83551@example.net...
> That's the flaw I see in the fluid models of electricity - the fluid
> needs to be in a pipe. If you break the pipe, all the fluid spills out.
> If you break a wire, the flow stops. So they're kind of opposites in
> that respect, but otherwise it seems to work well.
Actually it's not so much a fluid, more a case of blue smoke.
When the circuit goes wrong, something breaks and you see
the blue smoke all over the place.
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Author: zDate: 11:44 24-08-07
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On Aug 19, 10:54 pm, vorange <orange...@yahoo.com> wrote:
> 1st question : Till now, I believed that capacitors only let AC
> signals through while blocking DC. But then, I saw a schematic whee
> they put a capacitor on the output line of an opamp. The signal into
> the opamp was a square wave signal (which I imagine is DC and not
> AC). How then does the output of the opamp (presumably DC as well)
> pass through the capacitor? This has confused the hell out of me.
> Is there something I'm missing here?
>
> 2nd question : Is it fair to say that if a signal goes from say +5 to
> -5 volts and then back to +5...etc that is is an AC signal because its
> reversing its direction. But if it goes from +10 to 0 volts and then
> back to +10 that it is a DC signal because its not reversing
> direction?
>
> I'm confused :(
It's actually a little more complicated. it helps if you understand a
little calculus. The capacitor passes the derivative of the signal,
i.e. the rate of change. So, with DC, there is no change, there is no
current throughput. With your square wave, the signal changes where it
rises and falls, so at those points you will get spikes through, on
the flat parts, nothing, i.e. a spike waveform plus and minus around a
baseline of zero. The reason "capacitors pass AC" is that AC as people
generally think of it is made of sine waves, and the derivative of a
sine wave turns out to be the same sine wave, just shifted a bit in
time.
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