Maker Pro
Maker Pro

Choosing between capacitor types

idris

Jun 19, 2011
33
Joined
Jun 19, 2011
Messages
33
I know this is really really basic, but I want to be able to design and build small circuits (mostly 555 and/or logic based). But the one thing I've never really got my head round is when to use which type of capacitor. I've read loads of pages online and even got out a couple of old text books, but none have explained it in a way that helps. This is probably my failing. :rolleyes:

I've got a "beginners components kit" from eBay and it contains the following caps:
10 x 10nF Ceramic disc capacitors.
10 x 100nF Polyester box capacitors.
10 x 1uF Electrolytic capacitors.
10 x 10uF Electrolytic capacitors.
10 x 100uF Electrolytis capacitors.
3 x 220uF Electrolytic capacitors.
3 x 470uF Electrolytic capacitors.

Other than based on value, how do I know when to use which type.
 

davenn

Moderator
Sep 5, 2009
14,254
Joined
Sep 5, 2009
Messages
14,254
Hi,
welcome to the forums

depends on the specific circuit, thats a good starting range of values and would be good for things like timing circuits. Smoothing caps on input and outputs of voltage regulators. coupling between stages of an audio preamplifier and also in a main amplifier. many many uses.

As I said it depends on the specific cct.

here's a section of a citcuit that uses some of those values, see if you can identify them :)

attachment.php


cheers
Dave

PS I didnt draw this cct, so before anyone condems my spelling, I do see Audio is misspelt ;)
 

Attachments

  • 23IFSND.gif
    23IFSND.gif
    6.6 KB · Views: 1,904
Last edited:

idris

Jun 19, 2011
33
Joined
Jun 19, 2011
Messages
33
Thanks Dave.
I can identify them easily. :D That's not the problem. :( I just don't know how to pick which type. :confused:
Let me put it another way ...
Hypothetically, if the only variable between an electrolytic cap and a ceramic cap was the fact that one is polarised, why would you choose one over the other? What is the specific benefit of polarised / non polarised?

(And thanks for the welcome.)
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Joined
Jan 21, 2010
Messages
25,510
The problem is, you don't have 2 different types of capacitor that are alike in all other respects.

Consider the size of a 0.1uF (100nF) polyester capacitor. Now imagine that you wanted something with 1000 times the capacitance. In general, volume is proportional to capacity and voltage rating for a given composition. Now look at the 100uF electrolytic. Is it anywhere near 1000 times larger?

OK, so why not make everything electrolytic? Well, electrolytic capacitors have numerous deficiencies. Firstly they are polarised. I can't think of a reason you'd prefer a polarised capacitor over a non polarised capacitor if all other things were equal. Secondly they have comparatively high leakage. Thirdly, they tend to be quite inductive. Fourthly, they deteriorate badly over time (some have a lifetime of only 500 hours when operated near their rated limits).

So there's 2 reasons. firstly the need to keep size within reasonable limits, and to have desirable characteristics. There is no perfect capacitor, so there are many different types of capacitor.
 

idris

Jun 19, 2011
33
Joined
Jun 19, 2011
Messages
33
Thanks Steve. I think I'm starting to understand.
With the caps I've got, and the circuits I'm interested in, do I need to concern myself with what type I'm using? (Obviously I don't have any values in both polarised AND non-polarised.)

And a supplemental question ...
In the circuit Davenn posted, all the polarised caps except one have the +ve tied to the positive rail or -ve tied to gnd. The exception being the output of the second op amp, where the -ve pin of the cap is connected to the output. (I suspect I should be embarrassed to bs asking this, but...) in the latter situation, how do you know which way round the cap should be?
 

davenn

Moderator
Sep 5, 2009
14,254
Joined
Sep 5, 2009
Messages
14,254
snip

And a supplemental question ...
In the circuit Davenn posted, all the polarised caps except one have the +ve tied to the positive rail or -ve tied to gnd. The exception being the output of the second op amp, where the -ve pin of the cap is connected to the output. (I suspect I should be embarrassed to bs asking this, but...) in the latter situation, how do you know which way round the cap should be?

basically, the positive of the polarised cap is going to go towards the "most" positive (higher potential) part of the circuit. So on this case the point between the 100k and the 4.7k resistor is at a more positive potential than the output pin of the op-amp.
NOTE that the other side of the 4.7k resistor isnt going to the negative rail. Its held above negative by the 100pF capacitor.

cheers
Dave
 
Top