Capacitor Types for Applications

[mityeltu]

I had an interview yesterday for a position in a testing lab. One of the questions asked was really intriguing to me. "What types of capacitors are you familiar with, what applications are they used in and why?" Now I can rattle off all kinds of caps and a bunch of different applications, but I could not think of any good reason to use one cap over another in any particular application.

For instance (I did some research last night and this morning), I understand that a film type cap is not good for high-frequency decoupling (as opposed to a ceramic cap), but I don't know why. Is there a resource available (preferably a book) that spells out why one cap is better suited to a certain application as opposed to another? I understand there are various physical and electrical characteristics (dielectric strength of the insulator, voltage/frequency/temperature responses etc.), but why does ceramic make a better HF decoupler? Why is aluminum electrolytic better for motor start while a non-pol film cap is better for motor running? Why are film caps better for snubbers?

Do you see what I'm asking? Is there something out there that would spell this out? This was a really thought provoking question to me and whether I get the position or not, I really want the answer to this.

 

[steve66]

I would think you could find all you would ever want to know just by searching the internet for "types of capacitors".

I found this as one of the first hits:

http://www.radio-electronics.com/info/data/capacitor/capacitor_types.php

 

[junkhound]

Also, nearly all the cap manufacturers have application notes on their web sites with the type information you are looking for.

Search for 'app notes' on the various mfg sites, such as AVX, CDE, ECI, Presidio, etc.

 

[gar]

140503-2346 EDT

mityeltu:

A few broad concepts.

Ceramic as a broad class has high dielectric constant but quite temperature and voltage sensitive. Dielectric strength is good. Good for high frequency bypass because of high capacitance in small package and low internal series inductance. Bypass does not require constant capacitance.

Some ceramic capacitors, by material adjustment, have good temperature stability, but are not a high dielectric constant.

Mica capacitors are good for high frequencies, have stable characteristics, and high voltage capability.

Air capacitors have good stability, are easily variable, but low capacitance. Can be made for high breakdown voltage.

Foil wound capacitors generally have higher internal series inductance making them less useful at high frequencies. Different dielectric materials produce some different characteristics. Polystyrene is very low loss and absorption, but is temperature limited (melts at a low temperature).

Electrolytic capacitors are generally unipolar, high loss, high capacitance, and low voltage (up to hundreds of volts for some, but not thousands).

Oil, paper, Mylar, and other plastic materials are generally wound with foil or metalized.

Supercapacitors are more like an electrochemical battery.

Vacuum capacitors are low capacitance and useful for high voltages.

Any capacitor will have a series resonant frequency. Above its resonant frequency it is basically an inductor.

A semiconductor diode with reverse polarity is a useful voltage variable capacitor.

Some ceramic capacitors make a useful variable capacitor. I built a FM VFO using such a capacitor in the mid 50s. Tom Butler and I wrote an article on this modulator for QST in the late 50s. I still have a transmitter built using the ferroelectric modulating capacitor. I have not tried it in many years. Following is a not very easily read report relating to this area. Some of the techniques discussed have shown up in modern equipment.

Unrelated to capacitors. In the QST issue for November 1946 on page 63 there is reference to a 175 mile contact I had over Lake Erie on September 19th. This was a very foggy night that produced temperature inversion. My power was about 25 W from an 829 to a 16 element beam in the 2 meter band. The receiver was a super-regenerative type.

http://deepblue.lib.umich.edu/bitstream/handle/2027.42/3988/bab9578.0001.001.txt?sequence=4

.

 

[gar]

140529-2138 EDT

Some more information for mityeltu.

Ceramic capacitors are made of a thin layer of ceramic material (thickness is a function of voltage rating, true of any capacitor ) with a separate metalized layer on each of two opposite sides. Very low series inductance. Some ceramic materials have a high dielectric constant, and therefore high capacitance in a small space. Today there are multilayer ceramic capacitors and these can still have low series inductance.

An electrolytic capacitor is typically made of aluminum foil. Aluminum is a good conductor of electricity. A side note, electrical conductivity and thermal conductivity are generally similar in their relationship. Copper is a good electrical and thermal conductor, iron less so, and stainless steel poorer. Aluminum oxide (anodized aluminum) is an insulator. For more details see http://www.engineersedge.com/instrumentation/electrolytic_capacitor_review_9881.htm .

Capacitors formed as a roll of conductive and insulating material have a higher internal series inductance than do flat capacitors.

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