[sdiy] power-question

[Jim Patchell]

    The voltage you are getting sounds about right.  On transformers, you
have to remember that the 18 V is at 2.22 amps.  With no load, you will have
a bit more current.  Also, you need to remember that the 3V you need for the
regulator is the minimum.  So, you must allow for ripple.  At one time, I
had a formula posted for estimating the ripple voltage for a given filter
capacitor.  I will have to see if I can get that up again.

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

    The only real solution here is to make sure everything is as beefy as
posible.  For high current (1.0A) supplies, I prefer TO3 regulators.  If you
look at the picture of the board on
http://www.silcom.com/~patchell/smb1/smb.html , you will see a +/- 15 volt
and +5 volt power supply in the lower left corner.  The heatsinks are
diecast aluminum, 1 1/4" high, (I think, trying to trust my memory).  I am
running it off of a 40VCT transformer at the moment, which is really too big
for the job.  In free air, the heatsinks are staying within temperature
limits (just barely, I need to get something closer to 30VCT).  Another
heatsink I like to use is called the DUDE4 from Jameco.  Cheap, with lots of
fin area.

ulrich kehrer wrote:

> hi!
>
> a power supply problem...
>
> i have a 2x18V/2.22A tansformer and after the bridge rectifier i get +/-
> 28V thus my +/-15V regulators get really hot (i know that the input of
> the regulator should only be 3V higher than its output)
> i suppose the 28V are not normal, are they?
>
> (the rectifier is a 50V/6A one; i replaced it with 4 1N4005 diodes but
> that didn't change anything)
>
> any help concerning the +/-28V is really appreciated.
>
> regards,
> ulrich