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Question about capacitor size vs capacitance

I have been getting myself into electronics for some time as a hobby. And often run into questions I am unable to answer with just search engines. This is one of them.

I am getting my head wrapped around the storage capacity of capacitors in relation to volts and amps. But one thing I cannot figure out is capacity VS size.

I have a few large capacitors, one from a microwave, a bunch I believe from stereo amplifiers, another is a motor start capacitor... all currently completely discharged.... of course.

The have the following capacitance and approx dimensions. uFarads, Voltes, (Inches) Microwave: 0.8 uF 1200v 5'x2'x3' Oval-ish shape Stereo: 12000 uF 71V 2.5'x1.5' Cylinder Motor Start: 40 uF 340V 7'x5'x4' (approx) Box shape (its huge) And there are super caps which are fairly small about 1v to 5.5v and can store a full Farad

What interests me the most that I have had trouble organizing is the differences in these caps any why.

Ex: The Stereo Amp capacitor is tiny compared to the motor start capacitor but is 300 times the rated farads and the large one is only about 4.8 times more voltage ability.

The microwave I can see being large basing its construction so that It does not break down at high voltage. But the difference between the motor start being VERY large, but only having very tiny capacity in comparison.

What is the reason for this ? Can it output its energy faster? Does it actually store more ? in some round about kind of way and my understanding that the Farad capacity of a capacitor is not really how "MUCH" energy a cap can actually store ? Or is it so the capacitor can handle a large amount of amps passing THROUGH its structure ? Where the stereo cap would be relatively low amps ? And then there are car audio capacitors which advertise anything from one F to 12F and higher, I don't have any of these but I have learned their advertised capacities are often quite wrong. And these are also quite large. How do these compare to the very large motor start capacitor ? Both are large, one has 40uF other has 1F.

- E Any reading material that would help with this or explanations would be great.

Reply to
Eric Bauld
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For one thing, the type of capacitor and how it is manufactured is important.

The capacitor from the stereo is an electrolytic capacitor. It packs a lot of capacitance in a relatively small volume. The size goes up with both capacitance and voltage rating. Most electrolytic capacitors fall into the range of 3 to 6 volts minimum to 500 volts maximum. They are polarized, except the seldom used non-polar electrolytics which internally are two back-to-back electrolytics. Electrolytic capacitors can be made of aluminum or tantalum with the tantalum capacitors being physically smaller (and more expensive).

The microwave oven capacitor is most likely a film dielectric capacitor. Again the size goes up with both capacitance and voltage rating. Film capacitors generally start at 50 volts and go up from there. Voltage ratings of over 1000 volts tend to be uncommon expensive. They are non-polar.

The motor start capacitor is most likely a film capacitor, although nonpolar electrolytics are sometimes used with a low duty cycle. They are rated for AC voltage, but can be used at significantly higher DC voltages.

The supercaps currently pack the most capacitance per volume. They are all low voltage units, generally 5 volts or less. In some senses they are more like batteries than capacitors.

For any given capacitor technology (i.e. aluminum electrolytic, polypropylene film, mica, etc.) the volume is proportional to the charge the capacitor can store which is the product of the capacitance and the working voltage (Q = C*V).

I have written many posts on electrolytic capacitors in the past. If you are interested in reading them, put electrolytic capacitor and my surname into a Google search to find them.

Dr. Barry L. Ornitz WA4VZQ my_ham_call snipped-for-privacy@live.com

Reply to
Barry

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snipped-for-privacy@live.com

Thanks, explains a few items. One thing that stood out. Why would the non polarized AC capacitor be usable at a "significantly" higher DC voltage ? I have searched for you posts on caps, taking a read now.

- Eric

Reply to
Eric Bauld

The peak to peak voltage of an AC waveform is 2*SQRT(2) = 2.83 times the RMS voltage. Also there are spikes on the typical AC line voltage. To survive these, the capacitor must have a large safety margin. Capacitors used for across-the-line or antenna coupling service have special requirements under Underwriters Laboratories' rules that require testing with high voltage impulses. You can read more about these in

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The selection of the right capacitor for any given application can sometimes be rather complex. "Real" capacitors always have series inductance and resistance as well as shunt conductance. ESR, or equivalent series resistance, is important in electrolytic capacitors, especially when used in modern switching power supplies. Low series inductance is important when the capacitor is used in high (radio) frequency circuits. Audio circuits often require low dielectric absorption capacitors. As you read more about electronic components and become more familiar with the terms, this will make more sense to you. ALWAYS consider safety when selecting electronic components. If you are unsure, feel free to ask in the various electronic newsgroups. This knowledge does not come overnight. :-)

Dr. Barry L. Ornitz WA4VZQ ham_call snipped-for-privacy@live.com

I know I am showing just how ancient I am, but I can remember when spam did not exist and you did not have to hide your email address.

Reply to
Barry

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