[sdiy] Powersupply question

[Seb Francis]

Hi Mikael,

Mikael Mørup wrote:

>Hi list.
>
>I'm planning on building a bench power supply for SDIY work,
>i'm tired of running a long cable out of the back of the
>enclosure of my synth for testing modules under
>construction.
>
>I would like the new supply to have both +/- 12V, +/- 15V
>and +5V to cover all bases.
>
>Can i do this with one transformer and rectifier and just
>parallel the regulators after the rectifier ?
>
>  
>
Yes, although you need to be careful not to draw too much current where 
there is a big voltage drop unless you have very good heatsinking.

Here's a schemo of my bench PSU which outputs +/-15V, +/-12V, +/-9V, +5V
http://burnit.co.uk/sdiy/PSU.gif

Originally it was only +/-15V & 5V which is why the other voltages are 
shown separate (they are on a separate board).  Since I don't use a 
heatsink for the later added 7809, 7812, 7909, 7912, I chose to run them 
from the +/-15V, not the unregulated output from the rectifiers (so the 
power disapation will be shared by the 7815 & 7915 which have heatsinks).

There are load resistors on the outputs of the 7809, 7812, 7909, 7912 to 
ensure they regulate properly (especially the 79 series don't always 
give the right voltage unless you are drawing a bit of current).  These 
load resistors are not included for the other regulators because these 
are driving indicator LEDs via resistors (not shown on the schemo).

Purists would say that it's not the best to use these simple integrated 
fixed regulators, but it works fine for a bench PSU running small 
circuits at a time.

>Also most designs looks something like transformer ->
>rectifier bridge -> large bypass caps -> small bypass caps
>-> regulators -> more small bypass caps.
>
>Can i use the same bypass caps for 3 sets of regulators, so
>it's just the regulators (and the caps after them) that are
>paralleld or must i copy the whole chain (after the
>rectifier) for each voltage ?
>
>  
>
You should have decoupling caps very near to each regulator (the 100nF 
ones), plus 10uF of tantalum (perhaps a bit bigger if electrolytic, 
though 10uF is probably ok) after each regulator.  The big smoothing 
caps are only necessary right after the rectifiers.

One other point: it can be a good idea to have a diode on the output of 
each regulator, between the GND and VO, to protect the regulator (and 
tantalum caps) against reverse voltage (obviously put the diode round 
the way it won't conduct in normal circumstances!)

Wow, I didn't intend to write such a long reply!  Anyway, I hope it helps.

Seb