# What is the rule of choosing capacitor value?

Discussion in 'The Projects Forum' started by Project Boy, Sep 7, 2013.

1. ### Project Boy Thread Starter New Member

Sep 7, 2013
2
0
Hello, I am new in All About Circuits.I am an engineering student.During doing project, I become confused about choosing the capacitor.What valued capacitor should I choose? What is the calculation of calculating the capacitance? Please can anyone please describe this briefly and easily?

2. ### wayneh Expert

Sep 9, 2010
12,380
3,236
The choice depends on the function you want it to perform, so there is no answer without more detail regarding the context.

That said, the job of any capacitor is to store charge, usually measured in coulombs. 1 Coulomb = 1 ampsec. As you add charge to a capacitor, the voltage increases. The amount it increases depends on its capacitance. A 1 farad capacitor will increase 1V for each coulomb of charge added to it. Of course there is a limit on the voltage the capacitor can survive, so there is a limit of how much charge it can hold.

3. ### Project Boy Thread Starter New Member

Sep 7, 2013
2
0
Actually i am working with a project of "How to make a 0-24v dc powers supplier". I know that this project requires capacitors. But i don't know how many capacitor should i use?? Also wish to know about the value of capacitor. I don't know how i calculate the exact capacitance that i need to perform this project. I wish to know about working with capacitor in all other sectors of project. If you can help me any way, please...

Jul 18, 2013
10,839
2,513
The capacitor sizing after a bridge depends on the continuous current and the % of ripple desired, little known is if increasing the value more than necessary will increase the required VA value of any transformer, however.
Max.

Last edited: Sep 7, 2013

Aug 17, 2013
858
39
You mainly need to look at 2 values: the voltage and the capacity -both are written on most capacitors-. For example, if you are going to charge a capacitor with 24V, you need to make sure your capacitor will support that voltage; so you'll need a capacitor for at least 25V (plus error margin). The capacity will depend on how much you need in each instance, how much time it will take the capacitor to charge/discharge, etc...

Aug 17, 2013
858
39
If you're asking which ones you should buy to get started for your projects... well, I had that same doubt a few weeks ago, so I got 3 of each in multiples of 10; and so far I had no need to buy any more.

You need to get:
3 x 100uF
3 x 10uF
3 x 1uF
3 x 100nF
3 x 10nF
3 x 1nF
When you need different values you can combine 2 of those. For example: 2 100uF in series make a 50uF capacitor, and 2 100uF in parallel make 200uF. This way you can make any value you need.

Jul 18, 2013
10,839
2,513
Go with the smallest value that will give you satisfactory results for your application.
Max.

8. ### t06afre AAC Fanatic!

May 11, 2009
5,939
1,222
Some use 1000-2000 uF approx pr max ampere as a rule of thumb. But it is better to do some proper calculations. In a power supply it is many design considerations to take. A thing many overlook is the correct spec regarding the transformer VA rating then designing a linear power supply

9. ### wayneh Expert

Sep 9, 2010
12,380
3,236

Until you have some experience, you should follow schematics that have been designed by others, and reviewed by expert eyes here. (There is a lot of junk out there, but trained eyes can spot it for you.)

When the time comes that you need to make a change or customize a design, propose your ideas here and you'll get plenty of feedback.

ErnieM likes this.
10. ### MrChips Moderator

Oct 2, 2009
12,625
3,451
It is as the same question as "I don't know what value resistor to use. What value resistor should I choose?"

A capacitor, like a resistor, is a component in a circuit.

You have to present the circuit and analyze the the function of each and every component.
There is no single answer on how one goes about choosing the value of a specific component.

Knowledge and experience will lead you to the appropriate component values.

11. ### shortbus AAC Fanatic!

Sep 30, 2009
4,095
1,680
Every thing I've ever seen about this, voltage for caps in a circuit, said 50% over the expected voltage of the circuit. Higher voltage rating doesn't hurt, a voltage rating too close could. Due to tolerances in manufacturing.

12. ### wayneh Expert

Sep 9, 2010
12,380
3,236
I think you'll hear a rebuttal to that. It makes sense to me but I'm pretty sure there is a reason (beyond cost) to not grossly over rate.

I think the 50% "rule" comes in part from the typical wall wart experience where an AC voltage (rms) rectifies to a 41% higher DC voltage (ignoring diode drop) if it's unloaded.

Aug 17, 2013
858
39
I couldn't remember what was the exact convention -reason why I just said "plus error margin"-, so thank you for posting that detail.

I personally bought them all way over what I use in my experiments, which is normally between 3V and 5V (12V max), so all of them are over 25V.

Last edited: Sep 7, 2013
14. ### tubeguy Well-Known Member

Nov 3, 2012
1,157
197
My 2 cents.

Lately I've seen 'newer' equipment (90's or newer) with blown electrolytics which had a 10-20% voltage margin. Whereas, much older equipment will be fine with those same margins. We know electrolytics deteriorate with age.
But we also know that electrolytics physical size has decreased over the years.

I'm thinking tighter specs and tolerances also mean that only a slight defect in manufacturing can cause early failure.

My Point? Years ago I would have said a 20% margin is fine. Now, my opinion is 50% safety margin is not a bad idea for the voltages up to maybe 100 volts. For voltages in the 100's of volts, this is not always economically feasible.

shortbus likes this.