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Polarised capacitor has polarity (+ and -). Is it also called electrolytic capacitor? (Question 1)

Unpolarised capacitor does not have polarity (there is no + and -). Unpolarised capacitor can be connected in any direction, but this is not so with polarised capacitor.

Under what circumstances should one use polarised capacitor and similarly unpolarised capacitor? (Question 2)

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    \$\begingroup\$ This is not a duplicate question. There is one more question related to Unpol. Cap vs Pol. cap. ... but that is a different question. \$\endgroup\$ – NeedSomeLuck Sep 1 '20 at 23:45
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In any DC power supply or feed-lines on a circuit board, you will see many large value capacitors that MUST be polarized. That is the (+) lead must be more positive than the (-) lead. They can be electrolytic or the more expensive tantalum types. Hook these in reverse polarity and they might go BANG, depending on the current available.

They are used for bulk filtering of the power supply voltage. Smaller SMD ceramic or thru-hole ceramic are to filter high frequency noise, thus are very close to their point of use such as a CPU or MPU, or most any IC. Bulk capacitors that smooth low-medium frequency ripple can be inches away, and their high ESR is tolerated. Some are very large and need their own reserved space.

Tiny SMD capacitors (not polarized) have very low ESR so are used to filter out RF noise, and are often less than 1/4 inch from the device that needs them.

In DC power supply feeds both polarized and non-polarized are used and safe as long as polarized capacitors are inserted correctly and no capacitor is exposed to a voltage beyond its rating. In AC power feeds non-polarized capacitors are mandatory, and if connected to AC mains they need to be X or Y rated for safety reasons.

For AC / RF signals with no DC offset voltage it is best to use non-polarized capacitors. These circuits can have a high impedance which allows for tiny SMD packages to be use.

Some less common types include bipolar electrolytics for high-pass, band-pass and low-pass filters for speakers, often combined with inductors and low-ohm resistors.

The design engineer should plan out reserve space for all of these types of capacitors.

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    \$\begingroup\$ It depends on chemistry. Tantalum caps are somewhat small, low ESR and can be used for filtering. But they are polarized still, and turn into pyrotechnics when mounted backwards. \$\endgroup\$ – Lundin Sep 2 '20 at 6:51
  • \$\begingroup\$ @Ludin I like tantulums, better ESR than electrolytics but suffer from "trapped" electrons. Polypropylene is great for high current ultrasound. Too many cap types to explain without a book. \$\endgroup\$ – user105652 Sep 2 '20 at 7:31
  • \$\begingroup\$ The main issue with tantalum specifically is the "rare earth"/"conflict" metal crap, meaning that prices might got bananas at any time, as happened a couple of years back. Because of this they are too risky to use - not only might the cap itself explode when mounted backwards, the BOM cost might explode at any time too. \$\endgroup\$ – Lundin Sep 2 '20 at 9:33
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A polarised capacitor is also known as an electrolytic capacitor.

With its electrodes immersed in gel electrolyte, it is polarised by design, to form and maintain a thin oxide layer on the anode serving as the dielectric.

The resulting very low gap between the electrodes and the high surface area of the anode, obtained by etching, make for very high value capacitors having a smaller volume.

They are used for low-frequency coupling and decoupling, energy storage and filtering in power supplies.

An electrolytic capacitor is chosen mainly for its relatively small size and only for DC applications.

Reversal of polarity or application of AC would impair the dielectric layer and cause irreparable / catastrophic damage to the capacitor.

An electrolytic capacitor for a speaker crossover network would be like an identical pair of them connected in series opposition.

enter image description here

It would be specifically designed with two anodes in the same package serving as the non-polarised electrodes.

No damage would be caused by the audio (AC) coupling since the dielectric impairment and re-formation on the anodes would occur in alternate half-cycles.

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    \$\begingroup\$ @vuznan Not all electrolytics are polarized, as in those used for speakers, but they are clearly marked. \$\endgroup\$ – user105652 Sep 2 '20 at 7:46
  • \$\begingroup\$ Hi DAS, I have amended my answer, thanks to your comment. \$\endgroup\$ – vu2nan Sep 2 '20 at 11:22
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An answer to the second question: The main difference between a polarized and non-polarized capacitor is capacity, how much voltage it can store. Another difference is that non-polarized capacitors can run at much higher frequencies. Read more here.

An answer to the first question: As far as I know all electrolytic are polarized

Forgive me if my info is incorrect, I really am trying.

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  • \$\begingroup\$ Not really because they are polarized or unpolarized, per se. It's just that the capacitor technologies we have available that are polarized tend to offer higher capacities and those that are not polarized tend to offer better high frequency performance. \$\endgroup\$ – DKNguyen Sep 2 '20 at 0:28
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    \$\begingroup\$ OP's question is a bit like asking "when do we use white metal and when do we use orange metal"? That's the wrong question. It's not the colour that is the selection criteria. The proper question is "when do we use steel and when do we use brass?" \$\endgroup\$ – DKNguyen Sep 2 '20 at 0:30
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    \$\begingroup\$ @DKNguyen. By polarised Cap I mean those with + and - signs. There is something related to polarity with them. While an unpolarised capacitor does not have any + and - and so this can be connected in any way (direction doesn't matter). \$\endgroup\$ – NeedSomeLuck Sep 2 '20 at 0:33
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    \$\begingroup\$ @TheRealJoe That is an excellent link provided except it does not explain why a polarized capacitor must be under the specified polarity. The need to do so was implied with the image of a failed electrolytic capacitor, as in the cap could fail in a puff of smoke and bits of metal if under too much reverse voltage. Explaining some of the how and why would be helpful. Consider this is a critique of the link, not of the person providing the link. \$\endgroup\$ – MacGuffin Sep 2 '20 at 1:03
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    \$\begingroup\$ Bipolar electrolytics do exist and have been used in devices, so no, not all electrolytics are polarized. \$\endgroup\$ – Justme Sep 2 '20 at 4:38
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  • f -3dB BW =~0.25/(ESR*C=Tau @64%)

Because of the dielectric, electrode interface, non-electrolytic caps tend to have a much higher f-3dB yet much lower density than low ESR e-caps which are ~2us and 100x more bandwidth than general purpose e-caps ≈200us. Thus to span a wider f range, multiple caps are used.

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