Forumla for rectified and filtered voltage

[defaced]

I picked up a transformer with a 12VAC (I'm assuming that's RMS voltage) secondary, rectified and filtered it and I get 18VDC.  Now I know why this happens, but I'd like to find the forumla that models this.  Any help?

The end result is that I need 12VDC, so a tranny with a secondary that when rectified and filtered can produce this.  My guess is I need a tranny with a 8VAC secondary. 

[ante]

Hi defaced,

This should be helpful I hope!

[defaced]

I want to make sure I'm reading this correctly.  For a few of the examples, there are two equations for E.  Looking at what reading I'm taking from my circuit and compairing it to those forumlas, E = .9 Erms is the voltage after rectification and E = 1.4 ERms is the voltage after filtering.  Is that correct?

[nickagian]

Hi defaced!

Well this is what I have learned in my university:

1)At a half-rectifier circuit (look at the first two circuits at the picture posted by Ante) the output voltage amplitude is given by the formula Vout(amp)=E(rms)xsqrt(2)-VD0

[sqrt->square root, E the voltage at the secondary of the transformer and VD0-> the forward voltage of the diode, about 0.7~0.8V]

Since sqrt(2) is approximately 1.4 you can say that Vout(amp)=1.4E(rms)-VD0
so roughly Vout(amp)=1.4E(rms).

This is one of the formulas that you can see in the picture.

Moreover the DC output voltage (practically the average voltage) can be calculated by the formula

VDC=1.4E(rms)/pi  -  VD0/2

and since 1.4/pi is approximately 0.45, you can say that VDC=0.45E(rms)

(That means probably the second formula at the picture)

However if you use a simple capacitor for filtering, the output voltage roughly equals the output voltage amplitude, thus 1.4E(rms).

2)For the full-rectifier circuit (both with the two diodes and the bridge-rectifier) the output voltage amplitude equals

Vout(amp)=2*1.4E(rms)-VD0 thus approximately 2.8E(rms)

and the DC level

VDC=2*1.4E(rms)/pi - VD0 so approximately 0.9E(rms)

That`s all!  8) ;)

[MP]

defaced,
Your unloaded output voltage measurement is pretty meaningless. You need to measure the voltage loaded to get an accurate measurement. Also, if you want a well regulated 12 VDC supply, you should use a regulator. Most regulators require an input voltage which is 2 to 3 volts (or more) above the output voltage. Just get a 7812 voltage regulator for this power supply and you will have exactly what you need. If you use a transformer with an 8 volt secondary, you will not have enough voltage after loading occurs on the output.

MP

[aartak]

MP is right
also check the resistance of secondary winding
if it is more than 3-4 ohm you may have problems at high loads
because 7812 needs minimum 13.5-14V to work properly
dont forget to put large cap for filtering