[sdiy] power supply qeustions

[Michael Buchstaller]

>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?

To do a good regulation, those ones need to have approx. 3 V more at the input
than you want to them to output.
A higher voltage does not harm, but increases the amount of heat that the
regulator produces, and thus increases the size of the required heatsink.

Also, keep in mind that if there is little or no load at the output, the input capacitor(s)
charge up to the peak value of the transformer´s AC output (not only to the RMS value,
as under load). So, you can see 1.41 times the voltage at the input (plus some more because the transformer
idle voltage is also some percent higher than the rated voltage)
This limits the max. transformer voltage to approx. 24 V.

For 15 V output, i usually go for a 18...20 V transformer; for 5V i use 9V.

>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.

Sorry, i do not really understand the question...
If you want to make a bridge rectifier yourself from 4 diodes, you can simply look at the
schematic drawn of any rectifier bridge.

If you have only one output (no center tap), you will probably want to connect the ground to
the "-" output of the rectifier.


>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? ;)

I have no idea how many power you will need.
But keep in mind that the transformer needs to be larger than you might expect.
There is a ratio of 1.61 to 1.8 (depending on which book you read) for a bridge rectifier
between continous rectifier output current and transformer AC output current.
This means for 500 mA current to the modules, the transformer must deliver between
805 and 900 mA.
The Watts are simply calculated as Transformer output voltage * Transformer output
current. This will give you a hint of how big the transformer has to be.

Bot also do not forget the heat sinking for the regulator. Under usual load conditons, the
DC output at the rectifier´s cap is approximately the AC input voltage (plus some ripple).
The difference between that and the regulator output voltage must be converted to heat
in the regulator. For example, if you have a 20V transformer and want to regulate 15V, you
have a difference of 5V. When you draw 500 mA, the dissipated power in the regulator
will be 5V * 0.5A = 2.5 Watts - that must the heat sink be able to handle continously.


>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?

Maybe not, 1.5VA translates to 100 mA at 15V, so with the Rectifier factor of 1.61 (optimistically)
you can get roughly 62 mA out of it.
3 standard LED´s alone will draw this amout of current.


Hope this helps a little...


-Michael Buchstaller