Hi there,

I am new to the field of electronics and I am trying now to learn how to build a PIC programmer from on of the sites :

http://www.mcuhobby.com/articles.php?article_id=7&rowstart=3

My question is there are 2 types of capacitors used to build this programmer, and as I understood they are metalized film and ceramic capacitors. What is the difference between them and why the auther decided to use one type at some locations and the other one on other locations.

Regards

 

A good start is the table you find in this link:
http://en.wikipedia.org/wiki/Types_of_capacitor

Ceramic caps have better high frequency characteristics.
Also, here they are used for the oscillator input, so they have only a few pico farad (22pF to 33pF typically). I don't think the other cap type he used you could find them with such a low capacitance.

 

Actually, there are three different types used; ceramic, poly metal film, and aluminum electrolytic.
Ceramic caps are generally available in very small values, from fractions of 1pF up to around 0.1uF for large ones. But, ceramic caps can be quite physically large for the higher uF values.

Poly metal film caps are available in more medium ranges, from around 1nF (1,000pF) to several uF. They're usually used for 0.1uF/100nF bypass caps on IC's. You can use ceramics too, if you don't mind the extra real estate used by them.

Aluminum electrolytic caps are used for larger values of capacitance; around 1uF and higher. Aluminum electrolytic caps are small for their value of capacitance, and relatively inexpensive to manufacture. However, they have more "parasitic" properties than the two other cap types mentioned; ie: more inductance and resistance due to the physical size of the device. This is why you often see a small 0.1uF/100nF cap in parallel with a much larger polarized capacitor. The big capacitor takes care of the low frequency transients, and the small cap takes care of the high frequency transients.

The length of the leads on capacitors is important, as even straight wire has inductance. The less inductance, the better - so mount those small caps with the leads as short as possible.

 

first, thank you all for the quick reply and a special thank to SgtWooki for the detailed description. and since I understand now what are the types used there -thanks again- I have a question for SgtWooki :

in specific for that circuit you are right , he used ceramic for lower capacitance and poly metal film for higher capacitance. but if I use ceramic capacitors for all locations , what would be the effects on the circuit rather than taking care of the extra size of the ceramic capacitors as you mentioned.
And if it is possible to replace them (poly metal film) by ceramic capacitors why the auther preferred to use poly metal film capacitors at those locations in specific.

regards

 

in specific for that circuit you are right , he used ceramic for lower capacitance and poly metal film for higher capacitance. but if I use ceramic capacitors for all locations , what would be the effects on the circuit rather than taking care of the extra size of the ceramic capacitors as you mentioned.

If the value of capacitance is approximately the same, you won't notice any difference. About the only capacitors that are of a somewhat critical value in that circuit is the 22pF caps around the xtal; and the actual capacitance required may vary depending on the xtal used.

And if it is possible to replace them (poly metal film) by ceramic capacitors why the auther preferred to use poly metal film capacitors at those locations in specific.

Physical size is the most likely reason, but poly metal film caps also have a somewhat "self-healing" characteristic.

If you start building a number projects like this, you'll likely buy a variety of capacitors and other components to have them on hand. 0.1uF/100nF caps are required for virtually every IC, and most typical linear voltage regulators (like the LM317, LM337, 78xx series, etc.) so it makes sense to have them handy.

 

thank you again SgtWookie , with only your 2 posts I learned alot. I was googling info since yesterday without results. these info are very helpful.

Regards

 

There are many reasons for choosing a particular type of capacitor, among them are size, cost, availability, voltage rating, tolerance, inductance and application,

and there are so many types to choose from, ceramic, polystyrene, mica, aluminum, tantalum.

We can classify capacitors into two broad ranges, below 1uF and above 1uF and three areas of applications (that I can think of) AC coupling, timing and power filtering.

For power filtering, most applications will require large value polarized electrolytics. You would choose aluminum or tantalum electrolytics. Measured values tend to be about 20% higher that the stated value. Here, tolerance and accuracy is not a concern. You will tend to place a 0.01 to 0.1uF ceramic in parallel to suppress high frequencies.

For AC coupling, ceramic is pretty much the standard except when larger values >1uF are required. If you have to substitute a polarized electrolytic make sure you install it with the correct orientation with respect to polarity. Again, tolerance is not important and your application may not notice a 20% or more deviation from design requirements.

Where tolerance and accuracy is important, such as precise timing or filter applications, polystyrene or mica is preferred. This will give better stability and accuracy over temperature and time. Note that when using electrolytics for a timing circuit such as a 555 timer, accuracy and stability will suffer.

Large value non-polarized electrolytics are required in speaker cross-over circuits and CRT horizontal drive circuits, for examples.

 

These info are also very helpul, so as a result ( correct me if I am wrong):
**Use poly metal film capacitor over ceramic when physical size and accuracy are important factors. is that correct?


Regards

 

Hello,

A lot of information on capacitors can be found at the cap site:
http://my.execpc.com/~endlr/index.html

This is a link from the EDUCYPEDIA page on Capacitors

Bertus

 

Thank you all guys this thread was very helpful to me

Regards