[sdiy] compact solutions for power supplies?

[Seb Francis]

Michael Bacich wrote:

>
> On Mar 20, 2006, at 3:43 PM, Roy J. Tellason wrote:
>
>>>
>>> Can I make a +12-0-12 power supply using an ACin AC out wal wart?
>>>
>>
>> Yeah.  Use two half-wave rectifiers...
>>
>
> How does one calculate the necessary transformer voltage for that 
> configuration?


I don't know about calculate, but here's what I've found from trial and 
error ..

If you want +/-15V you can get away with a 15VAC wall-wart if you will 
only draw up to about 1/2 rated current from the PSU.  If you need more 
current you need to use something like an 18VAC wall-wart.  Sufficiently 
big smoothing capacitors will help.  But wall-warts do vary, so be sure 
to check your PSU output (under load) on a scope for ripple.  Also, 
(under load) look at the unregulated voltage from the smoothing caps and 
see how close the voltage is dropping to the regulated voltage - you 
need to have at least enough difference to comply with the regulator 
IC's spec (and some margin for error - mains voltages can vary, and 
capacitors degrade over time).

Seb