[sdiy] Multiple Voltage Power Supply / design problem!

[Rutger Vlek]

Hi everyone,

I'm working on a nice 8xSID synth based on the works of Thorsten Klose 
(www.ucapps.de), but with the addition of 8 seperate Moog filters (one for 
each "voice"), controlled by 4 PIC's. This design has quite complicated 
power requirements, so I started designing my own PSU, but I think I need 
some help with it...

First lets make a list of the power lines I need and the current 
requirements for each of them:

+5V    (for digital stuff, estimated max. 700mA)
+5V/1 (for analog stuff, estimated max. 800mA)
+9V   (for SID's, estimated max. 320mA)
+12V  (for SID's and filters/op-amps, estimated max. 640mA)
-12V   (for filters/op-amps, estimated max. 320mA)
AGND  (analog ground)
GND    (digital ground)

The design is meant to be used with either 8580 or 6581 SID's. They use 
different types of voltages to run so a jumper is added to select the 
voltage on the SID's power pins. This means that EITHER 320mA is drawn from 
the 9V line or 320mA is drawn from the 12V line. The above list takes shows 
the needed 320mA on both rails...

This is the schematic I made:
http://members.home.nl/r.j.vlek/psu.jpg

A big issue in the design of a PSU for this project is the amount of current 
needed on the +5V rail (both +5V's are joined at the PSU). Regulating this 
down from a high voltage results in alot of heat production at the 
regulator, so my idea was to use a seperate transformer with a lower output 
voltage for the +5V regulation circuit. The second transformer is a center 
tapped 2x15V transformer that drives the +12/-12V regulation circuit. The 9V 
is regulated down from the +12V line.

Two questions came up, during the design that I do not know the answer to. I 
hope someone here can help out with it...

1. Is it alright to join the grounds of the normal and the center tapped 
transformer and the earth pin on the 230V power inlet. Something tells me 
that there will be a difference in potential between the grounds of the 
regulation circuit from the two transformers. Is there a solution to this 
problem?

2. What type of transformers should I use? I'd like to keep down the heat on 
the regulators, but I think that 2x15V is the minimum for the +12/-12V 
circuit and 9V is the minimum for the +5V circuit. Is that correct?

Greetings,

Rutger

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