Simple Power Supply Stuff

[Tom May]

Bob Zimmer <bzimmer at voicenet.com> writes:

>The main filter caps are 1500 uF 35V.

At .75 amps with full-wave rectification this will give you a ripple
voltage of approximately (.75A * 1/120 second) / 1500uF = 4.2V.

>At the .75A load, I'm measuring 19.3VDC input to the LM317T regulator which
>should be be fine to deliver 15V (15V output + the 3V needed by the LM317).

If 19.3 is the average input voltage, at the bottom of the ripple the
input voltage will be 19.3 - (4.2/2) = 17.2 volts which is too low for
the 317.  You will need to approximately double the filter caps to keep
the input voltage above 18 volts.

Tom.