Building power supply questions

[zener]

Is there a way I can do this out of a 15V transformer. I have a LM317 lying around but not a 25V transformer. If it's possible, I know some components, probably resistors should replaced/removed. So what are those?
http://www.generalguitargadgets.com/v2/diagrams/ultra_clean_ps_sc.gif

Also, how do I wire 2 12V to get a 24V PSU. Do I just wired it in series before just ONE diode bridge or I need two sets of diode bridge and just wired the output after the bridges in series?

Thanks for any help

[ejbasses]

Hey man

You can get 9.5 volts out of a 15V transformer using that circuit. I have something that uses the LM317 and it can do variable voltages. Check out the data sheet to find the proper way to adjust the volume.

To get 24V out of a 2X12 transformer jst get the output from both twelves...

to be more precise the out put looks like this=> 12 0 12 get the wires coming out of the two 12s get it?

use big caps for the filter to reduce ripple. I use the biggest ones i can fit without covering other components on the board.

have fun building!

BTW you have to use only one diode bridge

[zener]

Ok, a correction. Sorry :oops:  

My actual plan is to get 12v and 24v at the same time.

I only managed to get one center tapped 12v transformer, the thing with three lugs that says 12-0-12. I was informed that I can get 24v out of it? How do I do that?

I got info from Kleber's thread but one problem is I don't know which of the two 12's are positive or negative. how do I know that?

Hey ej, thanks for the help. I just want to have a clearer picture. In this thread http://www.diystompboxes.com/sboxforum/viewtopic.php?t=22572&start=15&sid=d853733a9d98a14460883c26ae222369 two diode bridge were drawn out

You said that I just need one diode bridge. If that's so, I'll just get the 12v from the 24v, like putting 7812 and 7824 in parallel, right? What is the difference with that of niftydog's drawing. How do reconcile both of them?

Well, I can just settle for the 2 regulators in parallel. I just don't exactly know how can I get 24v out of a center tapped 12v.

Sorry if i'm messing up everything and thanks for bearing with this ignoramus  :oops:  Just want to learn :wink:  

Thanks a lot.

[Paul Perry (Frostwave)]

A 15v transformer is plenty, to make a 9v regulated supply.
But, personally, I'd rather just use a fixed 9v regulator and be done with it!
If you ARE going to use a LM317 and dick around wiht the resistors, then first measure what voltage is actually going to the input of the 317 (prepare to be suprised!!) then look at the National Semiconductor datasheets for the maths.

[zener]

Thanks Paul.  The 15v thingy is quite resolve now. I've edited my previous post for some corrections.

anyone?  :roll:

[Kleber AG]

I got info from Kleber's thread but one problem is I don't know which of the two 12's are positive or negative. how do I know that?

Hi Zener,
Thanks to the guys here (niftydog) I got mine working perfect.
About wich is +12 and -12, it doesn't matter, you have 12V potential from one outer tap to the center tap and another 12V potential from the center tap to the oposed outer tap but inverted from the first. So, if you "ignore" the center tap you're gonna take both outer taps and have an amplitude curve from each other that has a total of 24V. Remember it's still "AC".
I wired mine exctly like this. Just make A and B points together, they are now your GND point common to both voltages. I really believe you need 2 bridges.
You have a 24Vdc regulator? It will need at least 26 or 27 Vdc to work, although you'll "probably" end up after rectification(diode bridge) with enough voltage for it...

Hope it helps, and if someone can correct me, please do it
Kleber AG

[zener]

Hey Kleber,

I've just figure it out from your drawing. Tell me if my analysis is correct:


12--------0----------12

From a 12v center tapped transformer, you can get 24v out of the two 12's and you can get 12v from EITHER of the 2 12's and the 0 center.

Yeah, I'll get a 7824 and a 7812. From what I got from your thread, it says that I need at least 2v more for a fixed regulator to worked, right?

Whew!(sigh) I think my mind is settled now. BTW, I'll only use this PSU to power up my LM1875 20w power amp with an opamp preamp and EQ. The 24v is for the power amp and the 12v is for the preamp. My 15v PSU is for my "stereo" TDA2003 power amp. It is actually TWO separate power amp, each with 15w speaker. There would only be a common preamp for these two power amp.  I'm still not sure to do it but of course, that deserves another thread :wink: .

Thanks for the help, Kleber, ej, niftydog... everyone

[Paul Perry (Frostwave)]

If you have a 12-0-12 tranny, and you want 9 and 18 regulated, you tie one of the 12 terminals to ground.
Now the other terminals (relative to ground) are 12v AC, and 24v AC.
just use a single diode rectifier & cap from the 12Vac to generate the raw DC for feeding a 9v three terminal regulator to get your +9.
And again a single diode and cap from the  24v AC point for your +18 regulator.
But careful how much current you are taking, because, 24v AC rectified gives about 33v peak, which may exceed the input for the regulator!! and, in any case, if regulator is putting out 18v, trhenb ther is a drop of around 15v across it, so multiplying by only 100ma for example gives heating of 1.5 watts and so on...