Hello all. I have built this circuit
http://diyfan.blogspot.co.uk/2013/03/adjustable-lab-power-supply-take-two.html (version three)
and have tested it on an 15v transformer. I have used only 6800uf cap but will add another parallel to make up the difference. I hooked this up to a scope and at 13v I am seeing a 10v 50hz saw tooth. any help to reduce this please?

 

Ok so I just chucked a 470uf cap on the output and it seems to have flattened it right out but as I am building this supply as part of various supplies both regulated and plain old rectified so I can use them to power my learning I am wondering what else I should be doing before the output? I see part of the circuit takes ac right from the transformer and I wonder if that is the cause but I really have not a clue. I get that the op amp is set up as an error amp and the 2n3005 is the pass trany and I get what zener diodes do but as a whole I am not sure what the hell is going on there really. Can someone help please

 

Here is the schematic, so people don't need to go to someone's blog page to answer your questions;

(forum failed? would not link in the schematic?)

This is a very complex circuit, and you are asking very simplistic questions.

You say about "adding a 470uF cap to the output" but you also talk about "only using a 6800uF cap"?

Do you want us to help you debug the entire complex project? Can you show a photo? Seeing your construction might help us advise you.

 

Ah yes I see. I use a 6800uf cap after rectification instead of 10000uf as couldn't find the larger size so am adding now another 4700uf cap in parallel to make it up close to 10000uf for smoothing. I blindly stuck a 470uf cap on the output of the supply just to see what would happen and the output seems so have flattened right out. Maybe questions will not work as I have a very limited understanding of whats going on in this supply and as it is in your words "very complex" I should maybe put it to one side and play with it in future when I know just what the hell I am talking about. I am at moment unable to post pic of the build so I thank you for your reply THE RB and will find a simpler circuit to use for now. thank you

 

the schematic says the input transformer should be higher voltage than 15 volts.

 

For a 0-30v supply at 0-3A I think it is overly complex for a beginner build.

And fussy, it will easily break into oscillation (maybe matching your fault symptom?) if the layout etc is not good or if you have a minor wiring error.

For a beginner adjustable PSU, I suggest a LM317 and a few resistors and caps. It will do 1.25 to 30v adjustment, and has inbuilt overcurrent limiting at 1.8A and inbuilt thermal protection.

A very nice beginner adjustable PSU.

You can add current limiting to it with another LM317 before the voltage regulating LM317.

Here is a basic example of a beginner level LM317 adjustable bench PSU;

As you can see it is VERY simple and the LM317 is a wonderful device that does all the hard stuff for you;