Power supply schematic review

[this]

Hi.
I'm trying to build power supply, it should be regulated +12V(LM317), -12V(LM337) and fixed +5V(7805).
I have 18-0-18 transformer.
Based on datasheets and various schematics I found on the web, I created this schematic:



Could someone please take a look and point me errors or confirm that this will work.
TIA.

 

[alexan_e]

your schematic seems fine but I have noticed that in the two variable resistors R2 and R3 you have not connected the wiper so you will not be able to vary the resistance, you should connect it to one of the two sides.

Alex

 

[this]

Thanks.
Is it ok now:

**broken link removed**

Also, I said before that i created this "Based on datasheets and various schematics I found on the web" but it should be more like:
"Based on datasheets, various schematics I found on the web and help from edaboard people"

 

[alexan_e]

Yes it is fine.
Your 18v transformer will give you a DC of about 25v , I suppose you are using the 18v transformer on purpose (or you already have it) so that you can get an output of up to about +23v/-23v , if your intension is to have a permanent 12v output then you should go for a lower transformer of about 12v to reduce the power consumption on the regulators , for 12v/1A output it will be about 12w in each LM317 and for the 5v/1A about 20W for 7805

 

[this]

I didn't know that rectified voltage is equal to AC peak voltage, thanx
But it doesn't really matter for me what will be the regulated output max voltage, I need at least 12 regulated(20 is OK) and 5 fixed.

How to decide if I need a fan or is heat sink enough?
Thanks

 

[jpanhalt]

Are you grounding the center tap on your transformer, like shown in Figure 2-7 here: https://www.ece.ucsb.edu/courses/ECE002/2B_W11Shynk/Lab2.pdf ?

John

 

[alexan_e]

A heatsink would be enough but you will be able to use a smaller size if you add a fan or keep the temperature at a lower level.
If you want to do the calculation you can take a look **broken link removed**.

The rectified voltage is the peak voltage but as you start pulling current the output voltage ripple increases and the DC voltage drops, the amount of ripple depends on the capacitor value because a higher value will have less ripple in the same current.
you can take a look at Power Supplies

Alex

 

[this]

I have a pretty big case that I'm planning to use so I guess I don't need to aim for small heat sink.
Thanks for a links, I'll read them during my morning coffe tomorow morning

@jpanhalt - yes, exactly. Center tap = ground.