[sdiy] power supply qeustions

[René Schmitz]

Hi Sven,

> i've got some questions about this [1] power supply.
> 
> 1) which voltage should the ac->dc transformer output? the datasheets for
> those regulators (78xx and 79xx) sais, one should use 35V max for output
> voltages from 5 to 18V. So, could i use everything between 5 and 35V or
> are there some advantages for a specific voltage?

You need an input voltage for the regulator which is at least 3V above 
the voltage you're going to regulate to. An 18V transformer will be 
enough for a 15V supply. (There is also a factor of 1.41 involved, 
because the caps charge up to the peak voltage. And transformers have 
more voltage when they are not loaded. Thats called the load factor, 
usually something like 1.2 or so.)

> 2) i know, that the ground-sign right of the diodes must be connected to
> the ground-sign left of them. but what is this connected with? i've got
> one of this little blue transformers they sell at conrad (don't know any
> manufacturer). they just have 2 input and two output pins, no connector
> for the ground.

Then you have the wrong transformer. You need a center tapped one, or 
one with two secondary windings. Either take two of these, and connect 
them in series. Or buy a 2x18V transformer. (Be sure to align the phase 
correctly. You should measure a voltage that is twice as high between 
the outer ends than that of a single winding. If they aren't connected 
right, you will read a low voltage.)

> 3) how much VA should such transformer do? i know, it depends on the count
> of modules. is there some thumb-rule about the relation of watts per
> module? ;)

That does vary a lot from module to module. The best thing is to measure it.

> i just found one in my
> stomp-in-all-electronic-thingies-that-could-be-useful-in-future-box.
> (article no. at conrad.de is 506079 ) it outputs 15V with 1,5VA. is this
> useful for, let's say, a sequencer, 2 vco, 2 noise and maybe a filter and
> an adsr?

I'd say thats probably not enough. Especially if you have lots of LEDs....

Cheers,
  René

-- 
uzs159 at uni-bonn.de
http://www.uni-bonn.de/~uzs159