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Topic: Basic Capacitor Question (Read 6558 times) previous topic - next topic

card9inal

Hey all,

I want to make sure I have my head wrapped around the polarity of an electrolytic capacitor before I attempt a project in the next couple of days.

For an electrolytic capacitor, you connect the anode to the positive voltage, and the cathode to the negative voltage, so in reality, with caps that are marked with arrows pointing to the anode, current will then flow AGAINST the arrow, correct?

Like so:  +15V - ( <- <- <- EC) - Ground

Is that a fair judgment to make, that current flows AGAINST the arrow?

In another part of my circuit, I'll have two voltage sources coming into my circuit, and have been advised to place some EC's between them, like this:   +15V - ( <- <- <- EC) - Ground - ( <- <- <- EC) - -15V

Does that look right?

Thanks!

Grumpy_Mike

Quote
For an electrolytic capacitor, you connect the anode to the positive voltage, and the cathode to the negative voltage,

No an electrolytic capacitor does not have an anode or cathode. You connect the + end to the most positive voltage in your circuit and the - end to the most negative.
The marking on capacitors will vary, most likely one end is marked + so that tells you the other is -. The actual arrors don't mean anything as such.

majenko

Are you talking electron current, or conventional current?

JoeN

#3
Sep 04, 2012, 06:26 pm Last Edit: Sep 04, 2012, 08:21 pm by JoeN Reason: 1

Are you talking electron current, or conventional current?


Conventional.

So if you have the regular +5V/0V power supply connect the longer leg to the +5V side and the shorter leg which will be marked something like (-) to the +0V side.  Since these capacitors usually span between power and ground to provide noise surpression, it's generally easy to tell which is which.  If it spans a logic wire and ground connect the longer side to the logic wire, the shorter (-) side to the ground wire.  If it spans a logic wire and +5V connect the longer side to the +5V and the shorter (-) side to the logic wire.  That way it will never be reversed biased, only evenly biased.  But I don't think you see that setup very often at all.  An electrolytic cannot span two logic wires because the polarity would be incorrect at least part of the time which is a bad, disallowed, dangerous, and will kill the cap.

Electrolytics should look like this:

http://www.digikey.com/product-detail/en/UVZ1A102MPD/493-1270-ND/589011?WT.mc_id=PLA_589011

Longer is for the positive side, shorter with the grey bar and the negative sign is for the negative side.

Tantalums are polarized too, but the more postive leg is marked and there is no difference in the leg length.

http://www.faithfullink.com/images/C8.jpg
I will never ask you to do anything that I wouldn't do myself.

card9inal

Some (not all) of the electrolytic caps I've seen, have an arrow pointing to the positive terminal (which I guess I incorrectly correlated to the anode).  With these types of caps, conventional current would flow INTO the arrow, meaning into the positive side, correct?

Electron flow would obviously be the opposite and current would flow through the cap WITH the arrow.

Is that fair to say?

JoeN

#5
Sep 04, 2012, 06:37 pm Last Edit: Sep 04, 2012, 06:41 pm by JoeN Reason: 1

Some (not all) of the electrolytic caps I've seen, have an arrow pointing to the positive terminal (which I guess I incorrectly correlated to the anode).  With these types of caps, conventional current would flow INTO the arrow, meaning into the positive side, correct?

Electron flow would obviously be the opposite and current would flow through the cap WITH the arrow.

Is that fair to say?


Regardless of marking, if you have the cap installed correctly the unreal "current" is going to flow from the more positive side, however marked, to the more negative side.  In actuality, the electrons are going to flow from the more negative side, however marked, to the the more positive side.  This is hardly earth shattering knowledge.  It's is true of all parts of all circuits all of the time.  If the cap is installed incorrectly, it's still true, you are just going to kill the cap.  You just want to make sure to install the cap correctly.  Postive side to more positive, negative side to more negative.
I will never ask you to do anything that I wouldn't do myself.

majenko


Regardless of marking, if you have the cap installed correctly the unreal "current" is going to flow from the more positive side, however marked, to the more negative side.  In actuality, the electrons are going to flow from the more negative side, however marked, to the the more positive side.  This is hardly earth shattering knowledge.  It's is true of all parts of all circuits all of the time.  If the cap is installed incorrectly, it's still true, you are just going to kill the cap.  You just want to make sure to install the cap correctly.  Postive side to more positive, negative side to more negative.


That's the point I was trying to bring attention to with conventional vs electron current.

Conventional current flows from + to -, as it is the flow of "positive" current charge.

Electron current flows from - to +, as it is the flow of the actual electrons, which hold a negative charge.

JoeN



Regardless of marking, if you have the cap installed correctly the unreal "current" is going to flow from the more positive side, however marked, to the more negative side.  In actuality, the electrons are going to flow from the more negative side, however marked, to the the more positive side.  This is hardly earth shattering knowledge.  It's is true of all parts of all circuits all of the time.  If the cap is installed incorrectly, it's still true, you are just going to kill the cap.  You just want to make sure to install the cap correctly.  Postive side to more positive, negative side to more negative.


That's the point I was trying to bring attention to with conventional vs electron current.

Conventional current flows from + to -, as it is the flow of "positive" current charge.

Electron current flows from - to +, as it is the flow of the actual electrons, which hold a negative charge.


Yep, totally true.  It's something everyone interested in electronics has to wrap their brain around and be prepared to think and talk about it either way.  Sometimes it takes a little time to get that straight in your head.  It's rare in science or technology to have two competing descriptions of one phenomenon and for the descriptions to be (at first glance) diametrically opposed. 
I will never ask you to do anything that I wouldn't do myself.

majenko




Regardless of marking, if you have the cap installed correctly the unreal "current" is going to flow from the more positive side, however marked, to the more negative side.  In actuality, the electrons are going to flow from the more negative side, however marked, to the the more positive side.  This is hardly earth shattering knowledge.  It's is true of all parts of all circuits all of the time.  If the cap is installed incorrectly, it's still true, you are just going to kill the cap.  You just want to make sure to install the cap correctly.  Postive side to more positive, negative side to more negative.


That's the point I was trying to bring attention to with conventional vs electron current.

Conventional current flows from + to -, as it is the flow of "positive" current charge.

Electron current flows from - to +, as it is the flow of the actual electrons, which hold a negative charge.


Yep, totally true.  It's something everyone interested in electronics has to wrap their brain around and be prepared to think and talk about it either way.  Sometimes it takes a little time to get that straight in your head.  It's rare in science or technology to have two competing descriptions of one phenomenon and for the descriptions to be (at first glance) diametrically opposed. 

And all because electricity is moved around by negative charge carriers ;)

Even harder is the concept in semiconductors of holes being positive charge carriers - there's nothing there ;)

retrolefty




Regardless of marking, if you have the cap installed correctly the unreal "current" is going to flow from the more positive side, however marked, to the more negative side.  In actuality, the electrons are going to flow from the more negative side, however marked, to the the more positive side.  This is hardly earth shattering knowledge.  It's is true of all parts of all circuits all of the time.  If the cap is installed incorrectly, it's still true, you are just going to kill the cap.  You just want to make sure to install the cap correctly.  Postive side to more positive, negative side to more negative.


That's the point I was trying to bring attention to with conventional vs electron current.

Conventional current flows from + to -, as it is the flow of "positive" current charge.

Electron current flows from - to +, as it is the flow of the actual electrons, which hold a negative charge.


Yep, totally true.  It's something everyone interested in electronics has to wrap their brain around and be prepared to think and talk about it either way.  Sometimes it takes a little time to get that straight in your head.  It's rare in science or technology to have two competing descriptions of one phenomenon and for the descriptions to be (at first glance) diametrically opposed. 


Well the story I like to hang my hat on is the when science was first coming to terms with the field of electronics in the 18th & 19th centuries they certainly could prove that current could flow is a circuit, but they couldn't actually tell in which direction, so they just kind of agreed by consensus that current flowed from + to -. The concept of atoms and electrons were not understood. This is what became to be called 'conventional' current flow and is still the 'current flow' used in EE studies. Later however when the atom was better understood and the electron was understood to be the current carrier it was clear that current flow was caused by electrons flowing from - to +, and that is how is was/is taught in military electronics fundamentals training courses.

Later the science when developing semiconductors, the concept of 'hole flow' where the current carriers are caused by the flowing absence of a electron ( a hole) in the valence electron layer of an atom, thus continuing/justifying the statement that current flows from + to -. A weak argument in opinion. So we are kind of stuck with dealing with both a current flow direction and an electron flow direction, which to me is an awkward thing to be stuck with. I'm sure others will have stronger and possible more valid reasons to justify the two different concepts for the what flowing electricity really is. Are the electrons moving, or are invisible holes moving, or perhaps they are both moving in opposite direction at the same time?

Lefty 

majenko


Are the electrons moving, or are invisible holes moving, or perhaps they are both moving in opposite direction at the same time?


Or maybe the cat escaped the box in search of some milk?

pluggy

Surely the direction of the current flow will depend on whether the capacitor is being charged or discharged at that moment ?. If its a static voltage there won't shouldn't be any current flowing at all. 

retrolefty

#12
Sep 04, 2012, 09:10 pm Last Edit: Sep 04, 2012, 09:12 pm by retrolefty Reason: 1

Surely the direction of the current flow will depend on whether the capacitor is being charged or discharged at that moment ?. If its a static voltage there won't shouldn't be any current flowing at all.  


Of course, but the question is which direction (- to + or + to -) is current flowing when charging or discharging the cap? And to just say it flows in opposite directions when charging Vs discharging is not an answer to that question. That current flows at times is not in question. Show your work!

winner10920

On electrolyic caps the negative terminal(refering to going to the more megative voltage) is marked usully by dashes(- - - -), ablackcolored terminal, nd sometimes the terminal being "different" in texture or just not plain flat
the ones I've seen atleast usually don't have a marking for the positive terminal
Either way once you feel confident id suggest trying it out in a safe enviroment(maybe with safety glasses) because electrolytic like to either smoke out alot, or sometimes just explode

MarkT

Anyone who has difficulty believing in hole-flow needs to learn about the Hall-effect - its possible to determine the sign of moving charge carriers directly.  The sign of the Hall coefficient for a semiconductor depends on whether its p-type or n-type.
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