when to use polarized caps

I was just wondering... when should I use polarized caps? What's the benifit here over a regular one? I'm setting up a voltage regulator... a LM317.... and I'm putting a cap at the input voltage and a cap across the output voltage... can I use non-polarized caps here? Does it matter?

Much thanks!

Reply to
panfilero
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The polarity is necessary in the capacitor design. All but small value capacitors will be polarized, its part of the process. If the capacitor you are using is not polarized you need not worry about it usually. A filter capacitor in your output will be polarized. You can obtain non-polarized capacitors in larger values but these are special use and are not necessary in your case.

Tom

Reply to
Tom Biasi

The question is rather "when must I use non-polarized capacitors?".

Polarized capacitors aren't specified because of some need, they are specified because the value of the capacitor is relatively high, and the only way to get that capacitance in a decently small package is to go to scheme that forces the capacitor to be polarized.

If you look in the catalogs, you'll see that there are no small value electrolytic capacitors (and hence polarized capacitors). The smallest I've ever seen are something like 0.47uF, and I can't figure out why those exist since it's a value readily available in other forms. At about the 1uF level, polarized capacitors start to take over, and anything much above that is going to be polarized.

You can get non-polarized capacitors in the 1uF to maybe 30uF range, but they are bigger and more expensive and hard to come by. You don't use them unless you have a specific need for non-polarized capacitors.

There are few instances where you'd use such large value non-polarized capacitors. Crossovers in speakers come to mind, they handle AC. Some filter circuits maybe, but usually one can get away from large value capacitors (use a larger inductor, or a more modern filter that allows for smaller value capacitors), but even then, that would be for very low frequency filters. AC motor starting capacitors are a specialized breed. I'm sure there are some other exceptions, but that's what they are, rare instances when you actually really need a non-polarized capacitor.

Otherwise, you can live with polarized capacitors when you need high value capacitors. Most of the time, they are power supply filtering or bypass capacitors, where they are dealing with a polarized voltage. Even coupling capacitors in active circuits generally have one side that is more positive than the other.

Michael

Reply to
Michael Black

it

Thanks for the reply, so... basically it's something more to do with the way the capacitor is designed than it is something that's useful in any particular kind of circuit? I can see that in AC circuits it could be bad to have a polarized cap... but in DC circuits, does it not matter whether a cap is polarized or not... they can be used interchangeably... it just happens that maybe certain values of caps have a smaller footprint if they're polarized...

Reply to
panfilero

Polarized capacitors are Aluminum or Tantalum Electrolytics - they typically have much greater capacitance than a non-electrolytic type of a similar size, so are used where large capacitance is required.

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Peter Bennett, VE7CEI  
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Reply to
Peter Bennett

e

t it

Ohhh... "A filter capacitor in your output will be polarized." why can't it be non-polarized?

Reply to
panfilero

Some regulators need their output caps to have some minimum ESR, as polarized caps usually do. Many regs are unstable with low ESR caps, like ceramics or film types. Read the datasheet carefully.

You usually get a lot more C per dollar from a polarized cap.

Older data sheets, like the 317's, tend to be fuzzy in this regard. Look for tiny footnotes near the application examples.

John

Reply to
John Larkin

Ohhh... "A filter capacitor in your output will be polarized." why can't it be non-polarized? It can be, it's just not likely that you will come by one, or want to spend the money when it's not necessary.

Tom

Reply to
Tom Biasi

panfilero mused

Some regulators, like the LM317 and the 78xx family, have been around for 20-30 years. When their datasheets were written, you couldn't get cheap large value ceramic capacitors. It was assumed that for 1uF or more, you would use a polarised capacitor.

These days you can get 10uF ceramics which fit in 0805 packages. If you use them, you need to watch the voltage rating (small components often have low voltage ratings), and check that the dielectric (X5R, Y5V etc) has a low enough temperature coefficient that the capacitor will not drop below the minimum level of required capacitance at any expected operating temperature. Having said this, I have used ceramic capacitors with voltage regulators successfully.

I hadn't thought of the ESR but if John says it is a potential problem then it is a potential problem 8) I looked into using some tantalum (polarised) capacitors the other day, and found several types that had surprisingly high ESR's.

--
Nemo
Reply to
Nemo

Polarized caps have some other shortcomings as well. Non linearity comes to mind. No big deal in a power supply, but not good in audio circuits, for example.

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Paul Hovnanian     mailto:Paul@Hovnanian.com
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Reply to
Paul Hovnanian P.E.

Of course, those datasheets often specified type of capacitor, the capacitance value varying with the type.

Those capacitors are there to ensure the 3 terminal regulator doesn't oscillate, so the value specified was a reflection of the capacitance and the inductance.

Michael

Reply to
Michael Black

So what of nonpolarized electrolytics?

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Dirk

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Reply to
Dirk Bruere at NeoPax

An NP EL cap is really two caps in one. That is why you will always see the physical size being twice that of a similarly valued polarized EL cap.

They are two caps in series (by way of the manufacturing process) where one is polarity reversed with respect to the other.

Reply to
Mycelium

Wouldn't you say a non polarized cap is always more reliable than a polarized cap? If you can get by with say a 1uF ceramic, you will be better off than just brick-walling with a 4.7uF electrolytic, especially since the electrolytic has a big tempco. I guess my point is sometimes less is more.

Reply to
miso

Thanks - didn't know that.

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Dirk

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Reply to
Dirk Bruere at NeoPax

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re

I don't see how two caps in series prevents reverse biasing the caps. Are there diodes in the circuit?

Reply to
miso

Such bipolar capacitors are often used in loudspeaker crossover networks. When two capacitors are connected in series in this way, the total capacitance is half of the individual capacitor.

This is used in some cases, here the capacitance is the same as the individual capacitor, however, using diodes in this way might not be a good idea in audio circuits :-).

Paul

Reply to
Paul Keinanen

When you reverse polarity in a polarized EL, you get a short.

In an NP El, you get one shorting through and the other working, regardless of which way it gets hooked up or excited, so the end result always merely appears as a properly working, properly attached cap.

Reply to
Mycelium

where

This still doesn't answer the question regarding how the reverse bias is prevented if you don't use diodes.

I have a few crossover caps that are non-polarized. I think they are just freakin' huge to get the capacitance. I don't believe they internal contain two caps, though I have no way to prove this without cutting them open. They don't have the "crimp" in them like an electrolytic.

Here we go:

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Reply to
miso

It was explained. Read the thread.

Reply to
Mycelium

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