Re: [sdiy] power supply qeustions

[Roman]

---- Wiadomość Oryginalna ----
Od: Sven Windisch <mai00fpz at informatik.uni-leipzig.de>
Do: synth DIY <synth-diy at dropmix.xs4all.nl>
Data: Tue, 24 Jun 2003 01:41:32 +0200 (MEST)
Temat: [sdiy] power supply qeustions


>1) which voltage should the ac->dc transformer output? the datasheets for
>those regulators (78xx and 79xx) sais, one should use 35V max for 

less voltage over the regulator, less heat. There should be couple of volts (2-4) to make it work, and have small margin if line voltage varies. So e.g. DC voltage before 7815 sould be 17-19V, plus 1V for diode gives 18-20V, divided by sqrt(2) is 12.7-14V AC.
This is required transformer voltage at load that doesn't cause any significant ripple over the capacitor before 78xx. If the load is big enough for 1000u cap to cause several volts ripple, like 1A, you need higher transformer voltage to avoid dropping below 17V for 78xx at any time. Transformers are specified at full load, so if it says it's 13V AC, it may be 16V at no load.
What I sometimes do is unwind part of secondary winding and lower the pre-regulator voltage to minimum level at known load.
But anyway, unwinding any wire, or any work on transformer is NOT VERY WISE THING TO DO!

>
>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 

at schematic GND connects to transformer center tap. Your transformer doesn't have that, so in order to get bipolar supply, you must get another transformer with center tap, or use another rectifier topology. Cut the connection from transformer to bottom corner of diode bridge, connect this broken line to GND at transformer side, and leave open at diode side. 2 diodes are then not used, so you don't need a full bridge. This is half-wave rectifier and it has some disadvantages:
- ripple is now at half frequency of previous one (full-wave) and is more difficult to filter
- current spikes in diodes/transformer are twice as high
- filtering cap must be bigger, say 2200u

>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? ;)

transformer power = load power + regulator power + 20%
usually modules take 30-50mA per rail, that's 1-1.5W, say 10 modules (15W), 500mA over ragulator at 4V = another 2W, safety margin 3W, so 20VA transformer.
But 50VA is not much bigger and will run much cooler and have capacity for expansion.

Roman

PS. if you're not certain what transformer to use, maybe it's better for keepeng you alive just to buy ready made power supply.