Capacitor Basics

[elixier]

I understand the role capacitors play in electronics and even understand the formulas involved. However, I have never understood when to choose a certain type of capacitor, and in some cases, the rating for the capacitor.

Case in point. I know that most ICs require, or its recommended that they have, bypass capacitors between the GND and VCC pins. On very rare occasion, the IC manufacturer will state the size of capacitor. More often than not, they don't. I'm not exactly sure how to derive the size of the capacitor from the data in the data sheet. I'm still learning to read data sheets and am having to teach myself, so that may be part of the problem. Something tells me trial and error play a part here, but I want to be sure. How can I tell if I should use a .1uf or 47uf or whatever capacitor. I know that I should know this, but alas, I don't.

Also, most engineers use polarized electrolytic capacitors for bypass applications. Why not use tantalum or an unpolarized disk capacitor? In any case, why use one type over the other? I have scoured introductory electronics texts for this information and it seems that without fail, they show the various formulas, explain what a farad is and describe the differences in capacitor types, but never explain when to use a particular type or why. This has got to be something that is taught. Components are just to expensive to play guessing games with.

Also, In the course of doing research for the project I'm building, I ran across some rather old information regarding solderless breadboards. The author of the text mentioned that breadboards should not be used in applications where frequencies approach or exceed 8MHz due to ringing. That's a problem for me because the project I'm doing, a MC68000-based robot controller, is going to be running at or above 8MHz. If I cannot use a solderless breadboard, what can I use or is this information outdated?

[Steve Joblin]

Here is a good overview of the different types and how to select the right one for your application: http://www.uoguelph.ca/~antoon/gadgets/caps/caps.html

Selecting one type over another usually involves making decisions on cost, operating temperature, frequency, etc..  For example, polarized electrolytic capacitors for bypass applications are used because they are cheap, but some also add tantalum to deal with filtering higher frequencies.  If your project is sensitive to high frequencies, then you might want to add a tantalum... if not, you may be able to eliminate it... it all depends on what you are trying to accomplish.

[Soeren]

Hi,

Something tells me trial and error play a part here, but I want to be sure. How can I tell if I should use a .1uf or 47uf or whatever capacitor. I know that I should know this, but alas, I don't.

Most digital IC's will be quite happy with 100nF (i.e. 0.1uF), for TTL and CMOS40k, most of the time 1nF is OK, but if you make it a habit to allways use 100nF, then you are on the safe side.

Also, most engineers use polarized electrolytic capacitors for bypass applications. Why not use tantalum or an unpolarized disk capacitor? In any case, why use one type over the other?

That's something that you'll have to learn over the years, but each type has certain advantages and drawbacks - dry tantalum's e.g. doesn't like to be used for power bypass, aluminum electrolytics are bad at higher frequencies and you want low ESR caps if you need to filter fast transients like in digital circuits and SMPSU's - but it's a huge topic, not something to even scratch in a forum post.

Also, In the course of doing research for the project I'm building, I ran across some rather old information regarding solderless breadboards. The author of the text mentioned that breadboards should not be used in applications where frequencies approach or exceed 8MHz due to ringing. That's a problem for me because the project I'm doing, a MC68000-based robot controller, is going to be running at or above 8MHz. If I cannot use a solderless breadboard, what can I use or is this information outdated?

Solderless breadboard can be a pain, especially as the frequency goes up, but 8MHz should not be a problem, provided that the breadboard is fairly new (spring contacts still good).
That said, I almost never use solderless breadboards, as they can give so many unexpected errors. I can't even count the number of times where I have helped someone having problems with one and recommended a solder job which has solved the problem.

Personally, all my solderless boards collect dust, bar one which is less than an inch square, good for only very small circuits. For the rest, I use Manhattan Style prototyping or go directly to a PCB, depending on the circuit and how much I expect having to prod, probe and change.