[sdiy] Multiple Voltage Power Supply / design problem!

[Needham, Alan]

Rutger,
Answering your questions (or trying to)...
1/ The 0v lines are fine connected as you show, each transformer's output would be floating relative to earth without a connection to earth, each transformer supply can have any one point earthed, so that's all OK.

2/ Maths - I hate maths !
By my calculations, the 5v regulator needs 8v minimum supply (datasheet) and the 4700uF cap (C158) will allow 320mv of ripple (with a 50Hz supply)
so a DC line of 8.3v is required -- a full-wave rectifier will give a peak output of 1.41 times its input RMS voltage (but drop 1.4v accross the diodes), so a transformer secondary of 6.1v+1.4v = 7.5v should just cope.
You specify a 9v transformer so the regulator will be dissipating about 11 Watts of heat - nothing exessive there.

The +12v supply, if I understand the requirements, will be supplying 640mA at 12v AND 320mA at 9v -- 960mA total.
I calculate 455mV of ripple accross the 2200uF cap (C242) and a running voltage of 20v so the regulator will dissipate about 8 watts of heat.

The only changes I would make is to drop C11 - the reservoir capacitor for the 9v regulator - it is just excess load for the 12v reg on start-up and, as the 9v reg is on a stabilised supply, is not required.
The 5v supply LED series resistor (R236) will only allow about 2mA to the LED - maybe recalculate this and the other LED resistors ?

This should give nice, clean supply rails - a smpsu from a computer may be cheaper but could generate some high frequency hash noise ?

	That's my take on it,
			Alan


-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl
[mailto:synth-diy-bounces at dropmix.xs4all.nl]On Behalf Of Rutger Vlek
Sent: 14 May 2007 4:28 PM
To: synth-diy at dropmix.xs4all.nl
Subject: [sdiy] Multiple Voltage Power Supply / design problem!


Hi everyone,

--EDIT--

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)

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

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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