when to use polarized caps

If that were true, then the apparent capacitance would be the same as either cap alone, not *half* that of one cap... which is what you get.

Best regards,

Bob Masta DAQARTA v4.51 Data AcQuisition And Real-Time Analysis

Scope, Spectrum, Spectrogram, Sound Level Meter FREE Signal Generator Science with your sound card!

--
Yes. 

Believe it or not, the capacitors themselves are diodes connected
series-opposing.

Not _great_ diodes, since they have some DC resistance in the forward
direction, but pretty good since they have quite a high DC resistance in
the reverse direction.

JF

--- Since the resistance of the cap will be somewhat higher than zero ohms, the power supply will still pump as much current as it can into that resistance, boiling the electrolyte and eventually causing the vent to open or the cap to explode.

The stiffer the supply, the shorter the time to cap failure.

JF

--- From:

"Special capacitors designed for AC operation are available, usually referred to as "non-polarized" or "NP" types. In these, full-thickness oxide layers are formed on both the aluminum foil strips prior to assembly. On the alternate halves of the AC cycles, one or the other of the foil strips acts as a blocking diode, preventing reverse current from damaging the electrolyte of the other one. Essentially, a 10 microfarad AC capacitor behaves like two 20 microfarad DC capacitors in inverse series."

JF

And the more violent, often with resulting flame that can cause an interplane short, going further down the slope to disaster.

At low reverse voltages, polarized elecs usually keep behaving like a cap. Numbers like 10% of rated voltage maybe.

At higher voltages, they start to leak, and the leakage current de-forms the AlO2 layer, which in turn increases leakage. The "short" is soft, time-dependent, and complex.

An NP electrolytic behaves strangely compared to a "real" NP cap. At small signal, no bias, it looks like C/2, two caps sections in series. Under various large-signal conditions, it can look more like C, but ugly/lossy/nonlinear and frequency dependent.

Don't use them in analog filters.

John

That all depends on how hefty the supply would be. It may turn into a nice show! :)

That's what I try to tell me wife! :)

The current, however, will be at maximum. DUH.

Power supplies are either constant voltage or constant current.

So, the max current a CC supply can give is defined by its shorted output figure.

Also, they are TWO caps in series (internally), so only one ever appears electrically as a short at any time, so the other is always operating as it should, yielding a cap that operates as a cap, regardless of the polarity with which it get charged at.

One way, two plates get charged across an electrolytic medium, and the other goes low R across its medium and passes all the current, the other way, they trade places.

Only one cap is ever active at any one time. The cap is physically (and plate area wise) twice as large as a similar polarized EL cap. That is because there are TWO caps inside that package.

Only one is ever active at any one time, and the other becomes a short.

So what is it you are missing, or what is it you are claiming happens?

Rather conflicts with the appearance that the powersupply was working properly. Granted, I didn't check for ripple at the time, but there can't be any ripple when the output is zero, and I rather think I'd have noticed sooner if there was no power going into the circuit I was developing.

Sylvia.

EL

where

Thge output is NOT zero. There is no such thing as a zero ohm short. So you would see a ripple on the output. It would be in the form of ripple on the current signature on a clamped output, however.

The "power" has to "go" somewhere, and if you had a cap that smoked, that's where it was going.

A power supply just does not simply fold up on itself to zero output.

Some have shutoff features, but most pump the load at the rated current into a short, and will pump the rated load at the rated voltage and current with the declared amount of ripple present. Furthewr loading usually will at some point kill the voltage regulation, and as that folds back, you should see the current rise to and remain at the maximum rating.

When working 'near' zero ohms, or zero volts or zero amps, you must remember that it does not automatically mean that all the other figures fall to zero. You need to understand that concept, and learn about the difference between the regulation methodologies employed in a constant current supply and a constant voltage supply.

There are not many things that are really at zero.

The original Sprague TE series caps were very good in reverse polarity; if i remember correctly they would withstand 10% of forward rating "forever" with little leakage, and up to about half rating for "short" periods of time with no damage. Most other electrolytics tend to act like diodes in the reverse direction (crummy ones tho) and so are usable back-to-back at voltages less than 1/2 voltage rating.

That depends if the cap "zapped" the supply before it (literally) blew its top.

These are NOT electrolytic caps. They are polypropylene caps. Two plates with a dielectric between. In this case two layers of foil with poly film in between. Such capacitors have no preferred charging direction. The shortcomings however is that they get VERY large when you need high values. But they are extremely linear and very low loss which makes them excellent for audio crossovers. They also can be used for power supply filtering and are excellent there too because of their low loss and high frequency handling ability. However the down- side is they are very large (even in low voltage types) and relatively expensive.

An electrolytic capacitor is basically two layers of foil (typically aluminum) with wet paper in between. The paper is soaked with an electrolyte. (hence the name...duh!) When a voltage is applied to the device a current flows and a layer of aluminum oxide is built up on one of the foils. One foil is the capacitor "plate", the oxide coating is the dielectric, and the electrolyte is the other "plate". The electrolyte is made contact with by the other foil. Since the oxide layer is so thin, the capacitance is very high for the size. However if you reverse the voltage on the capacitor the oxide breaks down and the capacitor tends to short or get very low in resistance. Hence the capacitor can only operate with a certain polarity and are so marked.

Now a non-polarized electrolytic capacitor is actually two electrolytic capacitors wired in series back to back. It uses the above characteristics so that either one or the other capacitor is holding off the voltage while the other one is more or less shorted. The whole arrangement is more or less a kluge and tends to not be very linear. This is why crossovers that use these in cheap speakers really suck. And not only that electrolytic capacitors can't really take a lot of current in the reverse direction without damage so this method does have a certain power limitation as well. Thus, using non- polarized electrolytics in power supply filtering is not a good idea since the reversed cap may be damaged (and explode) plus the capacitance is half for a given size, plus they cost more.

Now it IS common to use polarized electrolytics as power supply filters that are in parallel with ordinary dielectric capacitors as described above (not electrolytic types). The reason you do that is because conduction in liquids (electrolytes) tends to not respond well at high frequencies. Thus, the smaller standard cap handles the high frequency noise while the electrolytics handle the basic low frequency work. Tantalum electrolytic capacitors (same idea as aluminum only tantalum foil!) have better high frequency response and thus are often superior to the cheaper aluminum electrolytic capacitors for filtering.

Boy, you guys sure don't know much about electronic design! Ooops. Crossposted to sci.electronics.basics I see. Sorry noobs.

"Benj"

** You still spewing bullshit all over usenet ?

Seems so.

** That is UTTER BOLLOCKS !!

Where does this complete nonsense come from ?

Must be audiophool on web forums.

** Like a using a woofer and tweeter - right ??

ROTFLMAO !!!!

** Now THAT really is funny.

Ignore this usenet blow fly.

.... Phil

I did. No explanation given on the reverse polarity issue.

Yes, I know the good ones are not electrolytic non-polarized. I was pointing out how you are supposed to do it.

If the non-polarized cap depends on the reverse polarized cap to act like a diode, well, count me out. ;-)

Then you obviously do not know how to follow or read through a Usenet thread chain.

Two series caps in a single package. Tell us what your trouble with that configuration is again?