# Capacitor Sizing - To Big/Small

Discussion in 'General Electronics Discussion' started by Sadlercomfort, Oct 28, 2015.

424
55
Feb 9, 2013
Hi Guys,

I need some help finding information about capacitors and sizing. I'm trying to learn about 'Failure Modes and Effects Analysis' reports, currently writing about capacitors and how under-sizing and over-sizing can effect the circuit.

I'm trying to find out how the value of C1 effects the circuit (See attached).

Thanks,
Ash

2. ### shrtrnd

3,824
519
Jan 15, 2010
Threw me for a minute there. I thought you were going to ask about physical size/dielectric composition of caps. (They've been getting substantially physically smaller over the years)
You just want to know about capacitor electrical characteristics.
I'll let others here give you electrical calculations. Did you check the 'Resources' heading at the top of this page?

424
55
Feb 9, 2013
There's a lot on the Internet about the dielectric failure modes, however this to be more specific to material sciences.

I just to know the basic effects of having a larger capacitance than required or a lower capacitance. As these are the two fault modes I'm looking into. Talking more about how this might effect the operation of the circuit or even damage it.

I'm not sure there's anything in the resource section

4. ### davennModerator

13,866
1,958
Sep 5, 2009
what specifically are the 2 fault modes you are referring to ?

424
55
Feb 9, 2013
One: A capacitor has a low capacitance value due to age or whatever

Two: A capacitor has a high capacitance due to the error of the designer

Each fault will effect the circuit differently, I need to identify how and why.

6. ### Torch

9
1
Jul 26, 2012

The circuit is a voltage regulator system and it designed to keep a constant reference output. Ideally we want the input to not change too rapidly as this would lead to poor regulation of the output. The capacitor is designed to filter out high frequency(rapidly changing) signals to allow proper functioning of the internal regulation circuit. The chosen capacitor value will allow for rapidly changing signals to be filtered out.

The first extreme would be choosing the value of capacitance too low, which would mean some rapidly changing signals will still pass through to the regulator circuit and lead to an unstable output. The second extreme is seen from this graph:

capacitors are not ideal and actually start to behave as inductors for very high frequency signals, this depends on the value as well as type of the capacitor. Choosing a to high value will actually cause the opposite effect of what the capacitor is chosen to do. Leading to again an unstable system. Hope I was clear enough

duke37 likes this.
7. ### duke37

5,364
772
Jan 9, 2011
The graph shows that the larger capacitors are only effective at lower frequencies. The dip depends on the capacitor proper and the length of the leads. The leads should be as short as possible.

In your diagram C3 is placed further away from the regulator than C2. Shift it as close as possible to reduce lead length.