Please educate me on ac to dc conversion

[whomeno]

OK I built a ac to dc. I used a 20 volt Transformer giving 22 volts. after running it to dc I get 30v dc. I should get less dc right? what am I doing wrong?
Here is what I used.

[GibsonGM]

AC is 'different'...the value used as 'AC voltage' is an average.   This is done because AC is always changing, and they needed a way to compare the work that AC could to with DC.   They can do equal work when AC is multiplied times .707, and this is the 20VAC value you see on the transformer.  There are many tutorials on AC theory on the net, so I won't go too far into this  

Basically, your DC output should roughly equal your AC PEAK value, not the average value we're always seeing.

Your measured voltage peaks at 1.414 * 22VAC value, so 31.1V.   If we subtract the drops of the diodes we are very close to what you are getting!  Some more reading on full wave rectification might help, I still have to brush up on that stuff sometimes too since I rarely design power supplies...  HTH!

[pringe]

As GibsonGM says I think you might be confusing RMS/peak and the effect of smoothing capacitors on the ‘average’ DC voltage from an AC signal - this site has some useful info and may help clear it up for you:

http://www.skillbank.co.uk/psu/trfrec.htm

[antonis]

And substitute GND for DC-, 'cause it could be confusing enough when dealing with dual supply circut..

P.S.
Needless to say, BEWARE of the "other" side of transformer..!!
Although there are properly insulated PCB mounted transformers, the risk of primary wiring & tracking is always there..
(a light smear of solder or an inperfect cut completion on the low voltage side could lead to circuit troubleshooting  but on the HIGH voltage side could lead to much more severe situatiuon..)

[merlinb]

The reservoir capacitor 'fills in the gaps' up to the peak voltage, not the RMS voltage.

[antonis]

So, next step should be to calculate/estimate diode conduction phase angle..

P.S.
I presume, it should be better for OP to proceed into an integrated study of AC rectification rather than a "splice" of right but incomplete posts..

[GibsonGM]

So, next step should be to calculate/estimate diode conduction phase angle..

P.S.
I presume, it should be better for OP to proceed into an integrated study of AC rectification rather than a "splice" of right but incomplete posts..

Yes.  It is too involved for one person to post here in a way that OP will understand, I believe!

[whomeno]

OK. I built the rectifier like merlinb posted.
useing a 20 volt transformer putting out 21.9 volts ac. after rectifier I'm getting 30.5 volts DC.
Then putting it through a regulator layout I'm getting 28.5 Volts. I'm trying to get it down to 18 volt DC
here is the layout I used



I used 240R for R1
And 3.3k Foir R2.
Why can't i get it down to 18v?
yes I have read the stuff on the link at http://www.skillbank.co.uk/psu/trfrec.htm
Thanks

[antonis]

Probably due to wrong pin-out connection..

Correct pin-out is:
a-12 -> Vin
b-12 -> Vout
c-12 -> Adj

[whomeno]

antonis,
THANK YOU. that was the issue. I now have 21 volt Dc. This will work for the tda2030A which max volt is 22.
Thanks alot!

[antonis]

A 5k multi-turn trimmer instead of R2 could save you a lot of calculations..

https://www.tme.eu/Document/46e9ecf29426285f0ed14cb1dbb4a9ca/SR_Passives-T910.pdf