Why voltage goes up?

[Luiso G]

Hi:
     
     I have two good questions. Look. I wanted to convert 125v AC to 12v DC. I looked on the internet for diagrams and founded one. When I finished building it... Happened that it wasn't 12v DC instead it was 18v DC. I didn't know what to do so I removed the capacitor an then I had 12v DC.

The questions are:

Is the capacitor doing something?
If yes. What is it doing?

I can't remember where the diagram was so I draw one. I tried to upload it, but there was an error saying that upload folder was full... Or something like that. Well anyway to see the diagram go here:
http://www.libertypr.net/jem_1910/diagram.html
Don't laugh at me page.

The materials are:
125v AC 15 amps to 12 or 25v DC 2 amps transformer. (Yes it has two outputs)
A rectifier diode bridge
An electrolytic capacitor 35v µf (micro farads)

Thank You

[cadstarsucks]

the voltmeter is averaging the wave form that the capacitor filtered.

the transformer puts out 12V RMS (the reading without the cap) but it is 18V peak (the reading with the cap)

[Luiso G]

Cool. I'm learning. I didn't really knew what was RMSV (Root Mean Squared Voltage). Just to be sure, it is correct to say that I should use a "true RMS" voltmeter? And it is correct to say the following?

12 * .707 = 8.48
12 + 8.48 = 20.48
that is the peak-peak voltage.

As you will see 18v is close to those 20.48v

[cadstarsucks]

Cool. I'm learning. I didn't really knew what was RMSV (Root Mean Squared Voltage). Just to be sure, it is correct to say that I should use a "true RMS" voltmeter? And it is correct to say the following?

12 * .707 = 8.48
12 + 8.48 = 20.48
that is the peak-peak voltage.

As you will see 18v is close to those 20.48v

No a bit backwards.

After the bridge with out the cap or load you will get RMS * SQRT 2 peak, but since the transformer is unloaded you get 20% more (transformer "regulation" of 20% means you get 20% more with no load than you do with full load)

When you add the capacitor it charges to the peak voltage.  When you add the load the capacitor charges to the peak and discharges to the load between peaks.

A true RMS voltmeter will pick up all the crap in the signal up to it's frequency response limit and give you the actual DC equivalent.  Normal meters respond to the average and are calibrated to read RMS assuming they are averaging a perfect sine wave.

RMS means Root Mean Square.  Take tiny little time segments, square the values, add them up, and take the square root.  In calculus it would be the square root of the integral of the square of the function.