[sdiy] PSU problem

[Donald Tillman]

On May 28, 2014, at 12:40 PM, Carlos Portela <carlosrochaportela at gmail.com> wrote:

> Don - The transformer only has 2 cables, not ground. the ground its
> the dashed line.

And that's exactly the problem; you are missing a ground.

For a transformer without a center tap secondary, you should ground one of the secondary wires and run the other wire into two half-wave rectifiers for your positive and negative regulators.

Dashed line for ground?  Ground should be the most important concern.  Note that in the drawing the transformer never makes it to ground.)


> Julien - the transformer its 16VAC but I measured 18.6VAC. Also tried
> with 18VAC tranformer (measured around 21V)and got the same problem.


18.6 Vac is a reasonable no-load voltage for a 16 Vac transformer.

16 Vac will peak at 22.4 V, with a little droop and rectifier drop, that will come down to about 20 Vdc which is completely fine for the 15 V regular chips.

  -- Don

--
Don Tillman
Palo Alto, California
don at till.com
http://www.till.com
650 888-9632