# extra 10 volts why

#### jamroll

Oct 31, 2022
7
HI just started in this great hobby built my first mains project but i have a problem i have a mains transformer 230v into a full bridge rectifier i get 20v dc thats great now then i put a 2200uf 35v capacitor across the output to smooth it out like i have seen on youtube then the voltage goes to 30v ? like i said just started so not to technical please thank in anticpation
jamroll UK PS do not have any load connected .

#### bertus

Moderator
Nov 8, 2019
3,268
Hello,

Have a look at the attached PDF.
On page 22 you will see an overview of the relation between the voltages.
From page 34 you will see the effect of the capacitor.

Bertus

#### Attachments

• Unit12.pdf
788.3 KB · Views: 9

#### crutschow

May 7, 2021
834
i get 20v dc thats great now then i put a 2200uf 35v capacitor across the output to smooth it out like i have seen on youtube then the voltage goes to 30v ?
A typical voltmeter will give the average DC voltage of the rectified sinewave, which is 0.636 of the peak sinewave voltage.
When you add the capacitor, it charges up to the peak of the sinewave which is 1 / 0.636 = 1.57 of the average or 1.57 * 20V = 31V.

#### crutschow

May 7, 2021
834
The average value of the rectified sinewave should be 0.637 of the peak value, not 0.636.

#### kellys_eye

Jun 25, 2010
6,496
The average value of the rectified sinewave should be 0.637 of the peak value, not 0.636.
I'm sure the OP has the kit to measure the 0.001V difference.........

#### jamroll

Oct 31, 2022
7
Hello,

Have a look at the attached PDF.
On page 22 you will see an overview of the relation between the voltages.
From page 34 you will see the effect of the capacitor.

Bertus
thats very help full thank you i have lots to learn obviously.

#### jamroll

Oct 31, 2022
7
A typical voltmeter will give the average DC voltage of the rectified sinewave, which is 0.636 of the peak sinewave voltage.
When you add the capacitor, it charges up to the peak of the sinewave which is 1 / 0.636 = 1.57 of the average or 1.57 * 20V = 31V.
Very grateful thank you thats made me think a lot about capacitor effects

Feb 19, 2021
725

Regards, Dana.

#### crutschow

May 7, 2021
834
I'm sure the OP has the kit to measure the 0.001V difference.........
Well, you never know...............

#### crutschow

May 7, 2021
834
Below is an LTspice simulation of the two cases with a full-wave bridge:
Note that with the capacitor, the output goes to about 30Vdc (green trace) and without, the average voltage is about 20V (red trace and Waveform window).

#### juliasmith

Nov 13, 2022
11
The main thing for which it happens is the capacitor.

Oct 31, 2022
7
many thanks

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