# Why is there a polarized capacitor symbol in an AC circuit?

I am confused. My question is very basic, someone please enlighten me.

If polarized capacitors should not be used in reverse polarity then why does every textbook (like the one I am using, Grob's Basic Electronics) show polarized capacitor symbols in AC circuit examples?

I couldn't find the answer by just googling.

• Please share the exact circuit you are talking about. Additionally, your deduction is not correct. You can use polarized capacitors in AC circuits, in situations where the polarity is not reversed. This can happen and does happen all the time. Commented Jun 13, 2023 at 4:37
• I am talking about simple AC circuit examples in the book where it shows charge discharge characteristic of a capacitor. it explicitly mention the reverse in current and voltage polarity through capacitor. I will try to upload the image
– Asim
Commented Jun 13, 2023 at 4:50
• Good job in updating your question with the relevant information! Commented Jun 13, 2023 at 7:55
• My opinion is that you are correct to bring this up. For me that symbol has always meant polarised capacitor, with or without the + sign, and its use for non-polarised devices grinds my gears. I hate it as much I hate the use of the acronym LDO instead of "regulator", but I suppose the lingo evolves, and I just have to suck it up. Commented Jun 13, 2023 at 8:14
• @SimonFitch yeah i can relate to it
– Asim
Commented Jun 13, 2023 at 9:34

As far as what I've seen used, that's the US symbol for a capacitor, the polarized kind have a little $$\+\$$ next to it. However, this is not fully correct (see edits/comments below) since the information I find is somewhat contradictory, depending on the source.

EDIT-1: As Justme pointed out, this is only partially correct since other sources refer to the capacitors in the bottom row as "Electrolytic Polarized (US), Fixed Non-Polar (UK), and Fixed Polarized (US)" (see 2nd comment).

EDIT-2: I'm basing my (edited) answer on DIN EN 60617-4 (1999) (unfortunately no access to newer versions since they switched to a database some time ago). It refers to the 2nd row of symbols as "symbols standardized in IEC 617-4 (1983) which are now deleted." (Annex A, page 17)

• And another diagram on the net says otherwise, the bottom three symbols being : Electrolytic Polarized (US), Fixed Non-Polar (UK), and Fixed Polarized (US). So curved without + means polarized and curved with + means electrolytic. This answer is marked accepted but it's not correct. Commented Jun 13, 2023 at 9:32
• Thanks. You are right. It made me think that these are polarized caps but actually the symbol is misleading.
– Asim
Commented Jun 13, 2023 at 9:32
• @Justme may be you are right, but the textbook i was referring to has the same definitions and symbols for the capacitors, so I thought it was right answer. but thanks for mentioning the other versions.
– Asim
Commented Jun 13, 2023 at 11:36
• @Justme: Thanks for pointing that out, I was not aware of that! I'm not sure if that makes my answer wrong, but it makes it at least incomplete. I edited the answer accordingly. I'm curious though, is there any sort of standardization for this or are we just trapped in this hellish limbo of never knowing what is actually meant? Commented Jun 13, 2023 at 13:23
• Thanks for specifying DIN EN 60617-4. Commented Jun 13, 2023 at 14:20

Referring to IEEE Std 315-1975 (Reaffirmed 1993), "Graphic Symbols for Electrical and Electronics Diagrams", section 2.2, the capacitor symbol with the curved plate is a Style 2 symbol.

2.2.1.1B — For style 2, if it is necessary to identify the capacitor electrodes, the curved element shall represent:
a) The outside electrode in fixed paper-dielectric and ceramic-dielectric capacitors;
b) The moving element in adjustable and variable capacitors;
c) The low-potential element in feed-through capacitors.

Section 2.2.2 covers the symbol for polarized capacitors which shows the Style 1 (plates represented by straight lines) & Style 2 capacitors with a plus symbol.

Hence, the symbol in Grob's book is fine. Personally, I've always considered the curved plate to represent the outside electrode in wound capacitors which were common in filter designs, especially using polystyrene capacitors, where the outside foil is connected to ground to reduce pickup.
Historically, the curved plate capacitor was commonly in use as a non-polarized capacitor in the pre-1940s through the 1960s and even in to the early 1970s as evidenced in some books and data books I have. Bernard Grob's (not Mitchel Schultz) book goes back to at least the mid 1970s which was used in our basic electronics class. Further research: the 1st edition was printed in 1959.

• Thanks for adding more info and reference. I didn't know the curved electrode of a capacitor symbol has historical reason.
– Asim
Commented Jun 13, 2023 at 17:10
• Also, in IC design, the curved electrode represents the one on the bottom, thus with substantial stray capacitance to substrate. Commented Jun 13, 2023 at 21:01

That capacitor symbol refers to a non-polarized capacitor. Yet its "plates" are distinctly different, one being curved.

Construction of metal-film capacitors put one of the metalized films facing out, which means that it can couple electric fields to other nearby components. When possible, this outside-facing plate should be grounded while the other (hidden) plate carries AC signals.

Variable capacitors also have preferred connections. One type's frame has electrical connection to moveable plates (rotor), and should be grounded if possible.

A trimmer capacitor might have one plate connected to the adjustment-tool frame. Best to ground this plate. All these non-polarized capacitors differentiate one plate from the other - the curved plate is the one most exposed.

The symbol is inappropriate in this context, treat it like a normal capacitor.

Such a component is strictly worse than an equivalent unpolarized capacitor, so it should be only used when other requirements make a polarized capacitor mandatory.

If part selection and layout is not up to the circuit designer, then this symbol can be used to indicate that voltage across this capacitor stays always unipolar, regardless of capacitor value/size constraints, but I am not a fan of this practice.