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Author Topic: Silly question about capacitors  (Read 16882 times)

Offline dieter

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Re: Silly question about capacitors
« Reply #45 on: February 16, 2014, 10:49:03 AM »
This is pretty much the opposite of what I've learned yet. Resistance causes heat loss but has no "slow down effect", and I've never heared of cooling elements for caps, tho they obviously are common for transistors or ICs...

The cap in a simple ac/dc supply that smoothens the rectified current is in heavy duty to do exactly what we discuss here, but they are as uncritical as a cap can be.

It just doesn't make sense, sorry.

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Re: Silly question about capacitors
« Reply #45 on: February 16, 2014, 10:49:03 AM »

Offline MarkE

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Re: Silly question about capacitors
« Reply #46 on: February 16, 2014, 11:03:13 AM »
This is pretty much the opposite of what I've learned yet. Resistance causes heat loss but has no "slow down effect", and I've never heared of cooling elements for caps, tho they obviously are common for transistors or ICs...

The cap in a simple ac/dc supply that smoothens the rectified current is in heavy duty to do exactly what we discuss here, but they are as uncritical as a cap can be.

It just doesn't make sense, sorry.
Dieter, plug the circuit into any SPICE simulator as vasik041 did.  The amount of time that it takes for the second capacitor to come with 0.1% of 50% of the starting voltage is 6.9 RC time constants.  Increase C or R and the absolute time to stabilize to within any particular percentage of the 50% asymptote stretches out.

You can find capacitor with heat sinks easily.  Go take a look into your local power utility substation.

You are absolutely wrong about what you think about electrolytic capacitors.  Very often more capacitance is used than would be required to meet charge storage requirements in order to stay within ripple current requirements.  Ripple current requirements are set by internal temperature rise considerations.  There is lots of information on the web sites of the various electrolytic capacitor makers such as Nichicon and United Chemicon.  http://nichicon-us.com/english/products/alm_mini/pict_f.htm

Offline dieter

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Re: Silly question about capacitors
« Reply #47 on: February 16, 2014, 11:26:05 AM »
Ok, I may be wrong there and yes, some caps may not be charged and uncharged too fast, otherwise they may be destroyed.


But I came to an other explanation that works for me:


Since we connect the 2 caps in series, the total capacitance will suddently be only 50% anymore. This happens faster than SOL, but immediately. The charge in cap A is now too high for its capacitance and quantum mechanics, those impatient dudes, can't wait til those lazy electrons have moved trough the slow wire. So they simply kick 50% of the charge "out" somehow. The remaining half, that now still fills Cap 1, will then slowly flow to cap 2 until they re equalized. Now they are still 50% full each, but as soon as you seperate them from eachother, they will double their capicitance, resulting in 8 Volts= 25% of the energy in each one.


Now I just have to find out, where the 50% has gone. So you say that's where the heat is caused, by forcing electron charges into the dielectric surroundings?


EDIT: okokok, but what happens when we increase the capacitance of an empty cap? Could't that be at least a good way to collect charges, eg. from the surroundings?
Ok, a silly question again but phew, I can test that right now. ::)


EDIT 2: Wait a sec, who said the initial charge of cap 1 (16v) is the max capacitance? Maybe the reaction is the same no matter how full the cap is and immediately altering the capacitence cannot be compensated by charge flow in the cirquit? When this causes heat, does then heat cause a charge?


I actually just reallize that I don't know anything about caps.

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Re: Silly question about capacitors
« Reply #47 on: February 16, 2014, 11:26:05 AM »
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Offline MarkE

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Re: Silly question about capacitors
« Reply #48 on: February 16, 2014, 01:02:47 PM »
Dieter, yes you are wrong as the linked reference explains in Item 2.  Electrolytic capacitors must be applied such that they do not suffer excess ripple current which in turn would cause excess internal heating and premature failure.

That is certainly a creative interpretation you offer.  The accepted theory previously explained is backed by careful experiments.

There are machines that vary capacitance.  If a voltage is placed across them, they emit E/M radiation.  More commonly, these machines act as stress or fine position sensors.

I don't think anyone said that the capacitor was charged to its upper voltage limit, only that it was charged to a particular voltage.


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Offline forest

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Re: Silly question about capacitors
« Reply #49 on: February 16, 2014, 01:12:44 PM »
sorry, I was not clear enough. Take two non-polar capacitors in paralell (maybe would be good to use two electrolytic to form non-polar to have bigger capacitance from start and lower leakage to air), charge them to the same voltage  (because of being parallel), then disconnect and connect them in series and measure. what is going on ? energy rise ????

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Re: Silly question about capacitors
« Reply #49 on: February 16, 2014, 01:12:44 PM »
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Offline dieter

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Re: Silly question about capacitors
« Reply #50 on: February 16, 2014, 01:54:33 PM »
Forest, that is most likely only the same like with batteries in series, double voltage, same current. As two caps in series are technically equal to doubling the gap between one cap's plates, it will reduce the Capacitance to 50%, but since the energy is the same, the voltage is forced to rise.
At least that's what I think, tho I might be rather unconcentrated and or confused ATM.


MarkE, they emit E/M Radiation ? How? RF? HV? But actually that's what I meant, losses trough radiation.


BTW. by "who said that" I meant "nobody said that", it's always tricky to translate sayings 1:1.

Offline MarkE

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Re: Silly question about capacitors
« Reply #51 on: February 16, 2014, 02:15:03 PM »
Forest, no there is no energy gain.  Two capacitors in series is effectively just the previous case but charging both capacitors first.  No charge transfers and no energy is lost:

Original energy = 0.5*2*C1*Vcharge^2.  = C1*Vcharge^2.
Qtotal = 2*C1*Vcharge

Vseries = Qtotal/Cseries
Cseries = 0.5*C1

Eseries = 0.5*C*V^2 = 0.5*(0.5*C1)*(2*Vcharge)^2 = C1*Vcharge^2 = Original energy

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Re: Silly question about capacitors
« Reply #51 on: February 16, 2014, 02:15:03 PM »
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Offline MarkE

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Re: Silly question about capacitors
« Reply #52 on: February 16, 2014, 02:17:06 PM »
Forest, that is most likely only the same like with batteries in series, double voltage, same current. As two caps in series are technically equal to doubling the gap between one cap's plates, it will reduce the Capacitance to 50%, but since the energy is the same, the voltage is forced to rise.
At least that's what I think, tho I might be rather unconcentrated and or confused ATM.


MarkE, they emit E/M Radiation ? How? RF? HV? But actually that's what I meant, losses trough radiation.


BTW. by "who said that" I meant "nobody said that", it's always tricky to translate sayings 1:1.
Dieter anytime charge accelerates there is E/M radiation.  Connecting a full capacitor to an empty capacitor accelerates charge impeded only by the resistance and inductance.

Offline poynt99

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Re: Silly question about capacitors
« Reply #53 on: February 16, 2014, 03:24:26 PM »
Dieter,

As Mark said, a tiny bit of energy is radiated at the moment you connect the two capacitors together. Turn on your AM/FM radio and you may in fact "hear" it.

However, most of the energy is dissipated (lost) in the combined resistance in the capacitors themselves, and the conductors between them.

If one could replace the connecting conductor with a pure inductance, there would be no energy loss at all during the transfer (assuming ideal capacitors).

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Re: Silly question about capacitors
« Reply #53 on: February 16, 2014, 03:24:26 PM »
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Offline dieter

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Re: Silly question about capacitors
« Reply #54 on: February 16, 2014, 03:27:06 PM »
Surrounded by scary formulas, I made more simple experiments. The two caps, empty, were connected in a loop, then gently heated to about 45 deg. C., then I doubled their capacitance by unconnecting them, still warm. Now one has 12 mvdc, the other -12mvdc charge.


Of course I don't think that the thousands of intelligent and educted scientists made a bad job so that I have to reveal the truth about caps after 10 Minutes of experiments  ;) , it's just that science is very if not only profit-oriented, so certain aspects are never really examined. Example given the back emf is usually seen as a nasty, dangerous thing that needs to be eliminated, eg. by heat dissipation, instead of  harvesting it to save energy. After all, they want us to consume electricity since one reactor gives about 1/3 billion $ in a year.

Offline MileHigh

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Re: Silly question about capacitors
« Reply #55 on: February 16, 2014, 03:41:28 PM »
Dieter:

If it made economic and/or technical sense to harvest back-EMF it would be done.  But in general it doesn't make sense so it is not done.  To say that something like back-EMF is "never really examined" is not true at all.  There is a huge amount of material to learn if you want to learn about electronics and a wise person would not make judgements or pronouncements without first understanding all of the issues.  I am not directing this comment specifically at you, it is a comment that applies to everyone.

MileHigh

P.S.:  And I will defer to comments by MarkE below about how back-EMF is being harvested and recycled in a lot of applications nowadays.  I am also on a learning curve that I will never get off of.

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Re: Silly question about capacitors
« Reply #55 on: February 16, 2014, 03:41:28 PM »
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Offline MarkE

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Re: Silly question about capacitors
« Reply #56 on: February 16, 2014, 03:41:43 PM »
Surrounded by scary formulas. I made more simple experiments. The two caps, empty, where connected in a loop, then gently heated to about 45 deg. C., then I doubled their capacitance by unconnwcting them, still warm. Now one has 12 mvdc, the other -12mvdc charge.


Of course I don't think that the thousands of intelligent and educted scientists made a baf job so I have to reveal the truth about caps after 10 Minutes of experiments  ;) , it's just that science is very if not only profit-oriented, so certain aspects are never really examined. Example given the back emf is usually seen as a nasty, dangerous thing that needs to be eliminated, eg. by heat dissipation, instead of  harvesting it to save energy. After all, they want us to consume electricity since one reactor gives about 1/3 billion $ in a year.
BEMF is recycled in virtually all high efficiency power converters and motor drives.  It is a necessity of smaller packaging where we can't afford the space needed to dissipate the wasted heat of dissipative BEMF discharge circuits.  40 years ago people threw away a lot of power in order to save component cost.  That all changed about 20 years ago.  Only in the lowest end consumer products do we see low efficiency power converters and motor drives.

Offline MarkE

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Re: Silly question about capacitors
« Reply #57 on: February 16, 2014, 03:44:38 PM »
Dieter:

If it made economical and/or technical sense to harvest back-EMF it would be done.  But in general it doesn't make sense so it is not done.  To say that something like back-EMF is "never really examined" is not true at all.  There is a huge amount of material to learn if you want to learn about electronics and a wise person would not make judgements or pronouncements without first understanding all of the issues.  I am not directing this comment specifically at you, it is a comment that applies to everyone.

MileHigh
It really is done everywhere now.  Virtually all stepper motor drives that are in everything from 3D printers to robotics are full bridge drivers that recycle BEMF.  All high efficiency motorized appliances use one form of bridge driver or another that recycles BEMF.  Virtually all switching DC-DC, and DC-AC converters to recycle BEMF.

Offline dieter

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Re: Silly question about capacitors
« Reply #58 on: February 16, 2014, 04:06:01 PM »
Today probably, yes, but for some 30 years it wasn't an issue. Would it be wise to say, Science is not financed by economy? Conclusion?


Anyway, what I have said about the -12 and +12 Mvdc charge may be caused by my body, acting as an antenna. Since I had them in my pocket while I was sitting in front of a heater fan.


Basicly I was referring to a document, I think it was "the secret life of capacitors" from the download section, that discovers a relation between self-charging and wheather changes, in pressure or temperature.
BTW Milehigh, I didn't say "something like a back-emf is never really examined", as you impressed. The idea of harvesting it wasn't really examined, at least for a long time. Thats a fact, just watch some older schematics, how they used to deal with bemfs between some cmos chips.

Offline Vladokv

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Re: Silly question about capacitors
« Reply #59 on: February 16, 2014, 09:01:30 PM »
Energy is lost as heat. Condesators heat up dramatically at high current. Look at induction heating - condesator  in LC tank heats up dramaticaly. Polyester condesators with no electrolyte heats up! Currents in that very moment are enormous and so energy losses are. Your setup looks like Colpitt oscillator with very low inductance(and straight wire have inductance). Current jumps back and forth maybe and million times(some miliseconds) before come to stop.

 

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