# Capacitor calculation for DC power supply

Discussion in 'General Electronics Chat' started by KanchanB, Jul 31, 2014.

1. ### KanchanB Thread Starter New Member

Jul 19, 2014
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Hi, am trying to calculate capacitor value for full wave bridge rectifier with reference to below formula

C = Laod Current × Capacitor discharge time ÷ acceptable ripple amplitude
I.e.
C = (I × T÷2) ÷ (ΔV)

Capacitor discharge time is approximated to T/2 for full wave bridge rectifier.
For, I = 2.2 A, ΔV = 2V and,
(T÷2) = 1 ÷ (2 × F) =1 ÷ (2 × 50Hz) = 10ms

Thus substituting these values in above equation, i get,

C= 2.2A × 10ms ÷ 2V = 11000 μF

Do I need such a big Capacitor??
Am i going wrong anywhere..?

2. ### Jony130 AAC Fanatic!

Feb 17, 2009
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You can use C in range between 6800uF and 10000uF.
Because in real life discharge time is less then 10ms.

3. ### MikeML AAC Fanatic!

Oct 2, 2009
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That formula is sound, but slightly too simplistic and pessimistic:

Here is a simulation that shows the amount of ripple at three different values of C1, namely 6,000uF (green trace), 8,000uF (red trace), and 10,000uF (blue trace).

I had to guess at the magnitude of the filtered voltage, and the series resisance of the voltage source (transformer secondary), both of which somewhat effect the magnitude of the ripple across the filter capacitor.

Usually, the purpose of this exercise is to see if there is sufficient capacitance to hold the filtered voltage high enough so that it a downstream voltage regulator would not drop out of regulation...

If you have any details about the transformer, the rectifiers, the Vdo of your regulator, and the final desired voltage, write back and I will modify the simulation...

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4. ### ErnieM AAC Fanatic!

Apr 24, 2011
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I agree with your calculation. It's slightly simplistic but well a founded approximation, and the same one I use.

Every part has a tollerance. Electrolytic caps such as tyou may use tend to be spec'ed as -10/+20 percent or larger on the + end, meaning you may get a little less, but probably get more.

I plan for the cases where I get a little less so I'd bump to the next larger size.

Just remember: "you can't put too much capacitance in a power supply filter cap."

5. ### MikeML AAC Fanatic!

Oct 2, 2009
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Oh yes you can. If you make the filter capacitor too big, the reduced ripple raises the average voltage. If you are going into a linear regulator, the power wasted (and heating) in the regulator is increased. You should select the capacitor to leave just a bit of headroom above the regulator's Dropout voltage.

6. ### alfacliff Well-Known Member

Dec 13, 2013
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also, larger caps means higher startup current when turning it on. did you include the 120 hz frequency of the bridge rectifier? full wave bridge has double line frequency, and less ripple in the first place than half wave. a three phase bridge has almost no ripple, and is easy to filter.

7. ### MrChips Moderator

Oct 2, 2009
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Also larger caps increase the peak diode current (already discussed this in a previous thread).

8. ### ErnieM AAC Fanatic!

Apr 24, 2011
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WHAT?

I just said "you can't put too much capacitance in a power supply filter cap." Now you accurse me of putting in too much cap where I literally say you can't do that?

Actually that phrase is a take off of a Saturday Night Live sketch where the guy who knows leaves on vacation telling the staff "you can't put too much water in a nuclear reactor." They argue the point back and fourth, then decide the safest course is to drain all the water out right before they leave for the night.

My sentence is (intentionally) semantically null, and should have been marked with a smiley face.

Besides, the value of the cap may influence the duration of the inrush current, but never the peak value. The peak is limited by the resistance of the circuit, or inductance from an EMI filter, or both.

9. ### MrChips Moderator

Oct 2, 2009
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Language, language, language!

The expression "you can't put too much capacitance in a power supply filter cap." translates to mean there is no maximum limit to how much capacitance you can put in a power supply cap.

The qualifying word is too.

As an example:

"You can't put too much emphasis on the importance of acquiring a strong background in mathematics if you want to become a competent engineer".

Last edited: Jul 31, 2014
10. ### MikeML AAC Fanatic!

Oct 2, 2009
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Well, I fell for it. You see, English is NOT my native language.

11. ### ErnieM AAC Fanatic!

Apr 24, 2011
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Exactly... though my intent here was hoping to provoke a discussion of inrush current rather then language.

The phrase "you can't put too much X into Y" in entirely dependent on context. If one stated "you can't put too much load on that beam when looking at a creaky old building the interpretation would be of a strict maximum rather than an unlimited maximum.

Besides, if I was to put too much emphasis on importance of acquiring a strong background in mathematics I might scare some people away from this field.

12. ### MrChips Moderator

Oct 2, 2009
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Let us consider the statement:

"you can not put too much load on that beam”

where I have placed the emphasis on two words.

Now remove the not:

"you can put too much load on that beam”

This translates to mean that it is possible to exceed a certain safe limit.

Now put the not back into the sentence:

It translates to: there is no max limit.

13. ### THE_RB AAC Fanatic!

Feb 11, 2008
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Too much time on your hands today? Things slow at work?

Come to AAC and discuss the meanings of words.

(not directed at any one person)

14. ### ErnieM AAC Fanatic!

Apr 24, 2011
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Let us consider the statement:

"you can not put too much load on that beam”

where I have placed the emphasis on four words.

Now don't randomly remove anything:

"you can not put too much load on that beam”

This states there is a uncertain safe limit one cannot exceed.

No further translation is required.

Now see above for a complete contrary explanation.

Hence the statement is null and meaningless.

15. ### MrChips Moderator

Oct 2, 2009
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Take the statement:

"you can not put too much load on that beam”

Now replace the word can with shall or must

and you get a totally different result.

16. ### ErnieM AAC Fanatic!

Apr 24, 2011
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I get the same result with shall or must, so let's not change the sentence.

Try it this way:

"you can not ....... put too much load on that beam

"you can not put ....... too much load on that beam