Transformer question for PSU

[WelshWonder]

Hi all,

I'm new to this game, so please don't flame me for posting questions that have been asked before. 

I'm thinking of building my first power supply (all 8 x 9Vdc) and have chosen the GGG Ultra Clean Power supply. Now, lookign at the diagram and the list of materials, the transformer required is a 120/25 volt. I am in the UK and our electric runs at 230Vac so I'm confused on choosing a suitable transformer. I'm not entirely sure what spec I should be looking at when deciding to buy. I use http://uk.farnell.com as my source. Can anyone recommend me one and fill me in on transformers specs? I have searched this site and looked up wiki for info on transformers but my knowledge is still shady.

[Papa_lazerous]

Can you post a link to the schematic ot GGG, most people wont bother to look without a link.  I use Farnell there online catalogue is quite daunting at first.  will have a look for you

[WelshWonder]

sure, there project page is here ---> http://www.generalguitargadgets.com/index.php?option=content&task=view&id=77&Itemid=106

[Papa_lazerous]

Just had a look there already on farnell having a look, I dont know What the current draw is on that circuit though 

[Papa_lazerous]

Not sure if this will work but hit this link its a result of a search I did

http://uk.farnell.com/jsp/search2/browse.jsp?N=500015+1004676+173298+190414&Ntk=gensearch_001&Ntt=transformer&Ntx=&_requestid=349900

[WelshWonder]

that's great thanks...

so when I choose a transformer, do I assume the primary be 230V and the secondary be in the region of what the project is for (in my case, around 9V)?

[Papa_lazerous]

ok I will try and explain a bit for you

230VAC at 50Hz is the UK supply not 240 like most people think!

Thats what goes in the primary.  Dont forget to put a suitable fuse in there too!!  Be safe 

The secondary calls for 25VAC but 24V will be fine.

this gets Rectified the maths is 24*squareroot  2  sorry dont know how to insert symbols

But you get 33.95VDC roughly after you make it DC this gets Filtered to remove noise which lowers the Voltage and also gets put thorugh a regulator I am not going to work it all out for you but with a bit of magic it ends up a nice clean output of 9VDC

When choosing the transformer you must consider its power in this case VA which is V*A otherwise known as Watts.  You will have to think about the amount of pedals you want to use and the consumption plus allowing a little headroom for the current used by the regulator etc.  Maybe another Schem will point you in a suitable direction.  Just off the top of my head I have a korg Power pack for a multieffect that I daisy chain and run 3 pedals off its got 500ma output.  There are others here who will give you a better answer I am sure

the search mostly brought up discontinued items lol, but dont worry it will offer an alternative when you click them the search may be restrictive as I search PCB mount ones.  If you searched for transformers then narrowed the primary voltage and the secondary then the power you would have plenty to chose from.  Hope this helps

[Cliff Schecht]

Your not going to want such a large voltage drop (from 24-9), it makes the regulator heat up more, work harder and it provides less current. It needs at least a 3 volt drop to do its magic (I used the LM317T, the TO-220 package) but will provide the max current at such a low drop, around 1.5A. That's enough juice to power at least 250 10 mA pedals (most don't have much current draw at all), so you'll be good.

[Papa_lazerous]

I wondered about being such a large drop that might explain why they also specced up a heatsink on the GGG site.

[Seljer]

I built something very simliar to that power supply (I used a 7809 instead of a LM317), I power it off a 12VAC wallwart and its dead silent for me
12V on the secondary of the transformer is enough voltage to get a clean 9V after the rectifier and the voltage regulator

http://www.geofex.com/Article_Folders/Power-supplies/powersup.htm

[Papa_lazerous]

Nice find there,  I never thought of looking over at GEOfex, RG saves the day.  I'd build that supply over the GGG one anyday.  I may even build it just for fun

[gez]

Your not going to want such a large voltage drop (from 24-9), it makes the regulator heat up more, work harder and it provides less current.

Absolutely!  Most 9V regulators only have an absolute max input voltage of 35V, so  25VAC is really pushing it!

R A Penfold, Power Supply Projects.  Costs a few quid, explains all.

[Cliff Schecht]

I mounted the 317 directly to my Hammond case, it makes an amazing heatsink if you remember the mica isolator and thermal compound .