General question regarding capacitors

[stallik]

Stupid question from numpty.com
Will someone please explain how putting 2 polarised caps in series (cathodes connected together) can create a bipolar cap? My niave belief was that current flows only one way through a polarized cap rather like a diode which is why orientation is important. Is that wrong or is it something else that's polarised rather than current?
Just want to get my head round it

[Govmnt_Lacky]

Gilmour??

This form of "series capacitors" has been used in many pedals. The Echoflanger comes to mind. Basically, by connecting the (+) sides together it puts the caps in series and effectively halves the capacitance AND make it a non-polarized capacitance. So, if you tied the (+) sides of two 33uF caps together and solder in the (-) sides into the circuit, you essentially have a 16.5uF non-polar cap 

[GGBB]

Gilmour??

This form of "series capacitors" has been used in many pedals. The Echoflanger comes to mind. Basically, by connecting the (+) sides together it puts the caps in series and effectively halves the capacitance AND make it a non-polarized capacitance. So, if you tied the (+) sides of two 33uF caps together and solder in the (-) sides into the circuit, you essentially have a 16.5uF non-polar cap  

I thought you were supposed to tie the cathodes together.  Maybe it works both ways?

[stallik]

Yes guys, it is for the Gilmour, I was just wanting to understand why it works.

[pappasmurfsharem]

Yes guys, it is for the Gilmour, I was just wanting to understand why it works.

I'm curious about this as well.

[GGBB]

Yes guys, it is for the Gilmour, I was just wanting to understand why it works.

I'm curious about this as well.

Make that 3 of us.  Where are the EEs?   

[Mark Hammer]

I'm sure they'll be along momentarily.

I'll stick my name to this thread to lure them out to correct my mistakes.   

[waltk]

Will someone please explain how putting 2 polarised caps in series (cathodes connected together) can create a bipolar cap? My niave belief was that current flows only one way through a polarized cap rather like a diode which is why orientation is important. Is that wrong or is it something else that's polarised rather than current?

Before the EEs show up, here's my lame non-EE explanation  . 

First, electrons don't flow through a capacitor.  The dielectric between the + and - side of a cap is an insulator, so DC current (electrons) can't flow through it.  If you apply a voltage to one side of a cap, charge builds up on it, and causes an opposite charge on the other side.  You can only apply so much charge to one side of cap (for a particular voltage) then no more electrons will flow into it.

If you alternately charge and discharge one side of the cap, the charge on the other side reflects that - so alternating current (AC) appears to flow through the cap.

For electrolytic caps, it's the cap that's polarized - not the current you put into it.  Polarized aluminum electrolytic caps have a thin layer of aluminum oxide on the anode (+ side) as the dielectric.  As long as the anode has higher voltage than the cathode (- side) the thin aluminum oxide layer is maintained.  If you reverse-bias the cap - with a higher voltage on the negative side, the dielectric will be destroyed (the cap will be shorted, current will flow, the electrolyte will heat up, and the cap will pop).

When you put 2 polaraized caps back-to-back (in serial) positive-to-positive or negative-to-negative, they are each protected from being reversed-biased, and won't blow up.

...anyway, that's my story, and I'm sticking to it until someone shows up to correct me.

[pappasmurfsharem]

I'm sure they'll be along momentarily.

I'll stick my name to this thread to lure them out to correct my mistakes.   

Mark doesn't know? I kind of feel like the apocalypse is coming now.

When you put 2 polaraized caps back-to-back (in serial) positive-to-positive or negative-to-negative, they are each protected from being reversed-biased, and won't blow up.

But why does that work

[stallik]

Thanks Walt, I hope nobody corrects this as it's a very clear explanation and one I can understand.

[PRR]

What Walt said.

AC current always flows through a capacitor.

Any "good" capacitor, DC current will not flow. Either direction.

E-caps are "imperfect" capacitors. One way, DC current will not flow. The other way, it does flow.

So why use a crappy cap? Because they are much cheaper and much smaller than "good" caps.

AND in many electronic situations we "know" the DC will only be the right way (at least never the wrong way). Prime example is power filters. We got 9V DC, it wobbles, a cap absorbs the wobble. We "know" which side is + and -. We put the cap that way, it never sees voltage the "wrong" way, it serves us well. And cheap. And small.

In an electrolytic cap, the outside can is the cathode. When you MAKE bipolar electrolytics, common-cathode with two foils is more logical than two cans. When you assemble a bipolar from two polars, either way works, and is equivalent.


One detail. Reverse voltage does not always "destroy" the e-cap. Depends how much current flows. If you put a 10uFd 16V backward across a car battery, about 1,000 Amperes will flow through the teeny part, it will burst, spew gunk in your face. (So don't try this.) If you have 10uFd output coupling cap from a 10K resistor to +9V, and get the "+9V" backward, less than 1mA will flow. The cap happily passes this much current. Its life may be shortened from decades to days, and it won't "block DC" until you put the DC right, but it isn't sudden death.

BTW, you might think that an e-cap can be used as a Rectifier. It can. Before Silicon, Germanium, Selenium, e-cap-like devices were used as Electrolytic Rectifiers. They are not particularly efficient, or cheap. Also early ones were crude, and tended to eat themselves up. Better chemistry (industrial purity) helps, but the same improvements in materials also made the other rectifier schemes more practical than electrolytic rectifiers.

[merlinb]

Thanks Walt, I hope nobody corrects this as it's a very clear explanation and one I can understand.

With back-to-back caps (either orientation will work) one of the caps may indeed be reverse biased. But because the other cap sucessfully blocks the DC, the reverse current in the unhappy cap is limited to a tiny value (i.e. the leakage current of the happy cap), which is not enough to heat up the insides of the unhappy cap and destroy it.