# Questions about capacitor in bridger rectifiers

Discussion in 'Homework Help' started by Poussin De Calipoco, Nov 6, 2015.

1. ### Poussin De Calipoco Thread Starter New Member

Sep 26, 2015
6
0
Hi everyone, i have a question about capacitor in a full brigde rectifier.

I have two capacitors 10uF and 47uF

10uF gave me a mean value of the output (DC) of 7.75v and a ripple voltage of 1.42V
47uF gave me a mean value of the output(DC) of 8.02V and a ripple voltage of 372.6mV

The question is which capacitor give the smallest ripple. i said obviously the 47uF one but i cant explain why and we also have to tell which one gave the largest mean value in the output.
My question is how can i explain using theoricaly both answer

Thank you

2. ### dl324 Distinguished Member

Mar 30, 2015
3,244
622
$\small V_{ripple}=\frac{I}{2fC}$

Wendy likes this.

Sep 26, 2015
6
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wtf?

4. ### Papabravo Expert

Feb 24, 2006
10,144
1,791
The formula is actually for a full wave rectifier, not a bridge rectifier. It says:
"the ripple voltage is equal to the current that you draw divided by two times the frequency times the capacitance". All other things being equal, a larger capacitance leads to less ripple.
What's with the rude expletive. Are you in the habit of biting the hands that feed you?

5. ### Poussin De Calipoco Thread Starter New Member

Sep 26, 2015
6
0
im not looking for a formula, im looking for a theorical explanation whats wrong with you ?

6. ### #12 Expert

Nov 30, 2010
16,298
6,810
We don't do reports for your school assignments. We are problem solvers by nature, and that is what is wrong with us compared to you.

7. ### dl324 Distinguished Member

Mar 30, 2015
3,244
622
Sorry for trying to help; or, if this is homework, cheat.

8. ### MrAl Well-Known Member

Jun 17, 2014
2,433
490

Hi there,

Sometimes a formula is considered theory, but if the formula does not suite you then you might be looking for *more* theory, or what you might call deeper theory. You might find however that there is no formula that you are going to like for this 100 percent, because the parameters required to do an exact analysis are hard to figure out for more real life applications. We know capacitance, we know current draw, we know input voltage, but it's harder to estimate line resistance and inductance for example. Even knowing these things however you may not be satisfied with the results depending on how much theory you want to get into. I'll try to explain a little more here.

To start, for a more theoretical example, using one of the defining theories for the capacitor we have:
dv/dt=i/C
or:
dv=i*dt/C

and knowing the discharge time dt we can approximate the voltage change dv knowing the current draw i.

That's if we accept that level of theory. If we dont, then we have to go deeper, and the first attachment "Fullwave Feat" shows a solution with a more thorough application of theory for a full wave rectifier. What that solution excludes however is the capacitor ESR.
That led me to look into this further several months ago, and i found a general solution that includes ESR (shown in the second attachment) but you have to realize that the full expansion of those equations leads to an even MORE complicated solution than the first one!

So you have to figure out how deep you want to go...how theoretical you really want to get...and how much accuracy you really need.

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9. ### ScottWang Moderator

Aug 23, 2012
4,855
767
Any students post the questions in this homework forum, the members won't given the answer directly and only can be guiding how to solve the problem.

When you asked for help, please don't used the rude attitude to the members who tried to help you, otherwise this thread will be close soon, good luck ...

10. ### shteii01 AAC Fanatic!

Feb 19, 2010
3,392
497
The formula shows if you keep all other variables constant (I and f), then by increasing the capacitor you reduce the Vripple. All you have now is to find how that formula was derived, that is the theory you are looking for.

11. ### MrAl Well-Known Member

Jun 17, 2014
2,433
490
Hi,

What i was trying to point out was that if he was looking for an exact solution he's never going to find it. That's because the rectifier circuits are much, much more complicated than they look. This means we all have to accept some sub optimal solution so we can at least get an approximate result we can at least start from. Some of the "formulas" presented in this thread are the result of those approximations.