# [sdiy] smoothing the AC ripple out of a rectified DC power source

Jim Patchell patchell at silcom.com
Thu Jun 27 21:30:02 CEST 2002

```    I got the answer for that one... goto:

http://www.silcom.com/~patchell/powersupply/powersupply.html

I scanned in some notes on how to calculate (estimate really) filter
capacitor size.

As far as doing +/- 12 volts from a non centertapped transformer...not
recomended.  You will need to use half wave rectification.  Here is a
description of how to do it.  Ground one side of the secondary.  You will need
two diodes on the other side.  The diode that is connected from the transformer
winding at the anode will provide the + voltage on the cathode.  The diode that
is connected from the transformer winding at the cathode will provide the -
voltage at the anode.  Paia did this in their 9700 synthesizer  (I am not really
fond of this aspect, but I understand why it was done).

Using a center tapped transformer is by far the best way to go (because you
can use full wave).

Jon Darby wrote:

> OK, all you pros can feel free to ignore a juvenile electronics question,
> but if you haven't done your good deed for the day now's your chance.  I
> need help figuring out AC-to-DC rectification with a full-wave bridge
> rectifier. My electronics books show it's easy to get +12v DC and -12vDC
> from a 12v AC transformer through the rectifier with the center-tap of the
> secondary winding connected to ground, but what do I do when I do not have a
> center-tap on the transformer? is it as easy as connecting the negative side
> of the rectifier to ground? And also, the book mentions the value of the
> smoothing capacitor(s) and resistor depend on the impedence of the circuit.
> I have completely replaced the old amplifier circuit with an LM386 and have
> no idea how to calculate this impedence or current needs, can someone point
> me in the right direction to determining the values required? Thank you
> kindly!
>