Question about the GGG Ultra Clean Power Supply

[Pushtone]

I've got the GGG ultra clean power supply on my breadboard
and it seem to be working fine with a 12VAC transformer.
I'm getting 9.7 VDC output without a load.

GGG states it needs a 24VAC transformer in the parts list.

Ultimately I want to coax 12VDC  500mA out of it so I know I will have to use a bigger transformer.
But does it really have to be a 24VAC output when all I want is 12V?
I have a 15VAC transformer I would like to use to save some money.



But before I can do that I have a question....

The LM317 datasheet says all I have to do is change two resistors and I can adjust the output anywhere between 1 and 30 volts DC.
Then I found this handy online calculator...
http://www.electronics-lab.com/articles/LM317/

From that calculator I need  270R and 2k5 resistors to hit my target voltage of 12VDC.

But when I look at the GGG schematic I get confused?
http://www.generalguitargadgets.com/diagrams/ultra_clean_ps_sc.gif

Which two resistors should I be looking at?
r1a and r1b?
or
r2a and r2b?


I don't understand that formula at the bottom and I'm just a little concerned about damaging the LM317.

[Pushtone]

But does it really have to be a 24VAC output when all I want is 12V?

OK, I was having internet problem when I posted that and I could not do a search.
The net is back and I have answered this question for my self.

No it dosen't need to be 24VAC.  My 15VAC transformer will work fine.




I found two other posts about the resistor question that went unanswered so I'm not too hopeful I'll get a response.
I'll try subbing resistors to find my own answer and I'll post back to this thread so it can go into the collecting knowledge base.

[Pushtone]

From the answering my own question department....


I have been trying to get 12VDC out of the Ultra Clean power supply.

I have focused on resistors r1a and r1b in the schematic as the ones to change but was having no luck getting close to 12V with a 15VAC wall wart.

Today I went out and picked up a 24VAC wall wart and things have improved but I still can't hit the mark exactly.

With a 2k7 as r1a
and a 4k7 as r1b
I'm getting 11.91VDC output.

With a 2k7 as r1a
and a 6k8 as r1b
I'm getting 13.33VDC output.

Now that seems close enough but I'm concerned because the calculator link above states r1a should usually be 240R.
I was getting nowhere with that value.

It's outputting a steady voltage all right and the LM317 is not hot or even warm
so I guess I'll go with the 13.33VDC output to power the Tone God's Tube Tremolo.

That what this power supply will be used for.
http://www.diystompboxes.com/smfforum/index.php?topic=62049.0

[Pushtone]

Here is a picture because pictures are pretty.
The voltage dropped a little but I think that because the meter probe is putting less pressure on the output lead.
LM317 still as cool as a cucumber.

I'm figuring with a load on it, 13 volts will work OK for the Tube Tremolo.

[Satch12879]

Do make sure the VA rating of the transformer secondary is sufficent to supply that current at that voltage.

[davent]

Hi Pushtone,

I think you need to leave the resistors for R1 alone and work with the R2 values.
If you can, try these and see what you get. For 12v you need 240r for R1 and a 2k064 for R2.
For 15 v;  R1 240r,  R2 2k640 (2k7)

I've found this calculator real helpful for finding parallel pairs needed for odd resistor values, like 2k064 & 2k640.
Parallel Resistance-Reverse Calculator

To get the above values for R2 I used this formula: R2 =[ [V(out)*R1]/1.25] - R1

Take care
dave

[Pushtone]

Success! Thank you very much davent.

I built up the GGG pcb using sockets for the r1 and r2 resistors and tried some values for r2.
Of course I have limited values on hand.

I only have a 270R and not a 240R for r1a.
That might have thrown off the calculator because to get the 2k064 it told me to use 2k2 and 33k.
But with 2k7 (all I have) and 33k I was getting 13.54VDC.

It did get me going in the right direction though.
As you can see I hit 12.01VDC using...

r1a = 270R
r1b = 2k7

r2a = 2k7
r2b = 12k

(although I should meter the actual values because I'm using my cheapo bulk resistors here)




So credit goes to davent for answering the "what resistors do I change to get a different voltage" question.
Thats a question that went unanswered in a few threads that died out without a response.

Thanks again

Dave S.


BTW, For the knowledge base...
I didn't have a 1000uF cap on hand and used another 470uf.
I'm sure that 1000uF would be overkill fersur.

[davent]

Hi Dave ,
Glad you got it worked out.

dave