Simple Power Supply Stuff

[gstopp at fibermux.com]

     I'd say off the top of my head that the power resistors are the reason 
     that your power supply is giving off so much thermal activity.
     
     The power supply design that I use all over the place with great 
     success consists of the following:
     
     * 36 VCT 3A transformer, center tap to ground
     * Diode bridge to create +/- DC voltage
     * Two fairly big electrolytic caps (2200 uF 50V for example)
     * 7815 regulator for +15V
     * 7915 regulator for -15V
     * Misc stabilizing caps per databook
     
     That's it. No trimmers, but the outputs are usually pretty close to 
     15V.
     
     - Gene
     gstopp at fibermux.com


______________________________ Reply Separator _________________________________
Subject: Simple Power Supply Stuff
Author:  Bob Zimmer <bzimmer at voicenet.com> at ccrelayout
Date:    6/28/96 1:39 PM


I'm building a simple power supply for the ASM-1 and am wondering about a 
couple of things.  First a little background.
     
The PS uses a transformer from Electronics Goldmine which has three 
secondaries: 
28 VCT @ 2A, 18 VCT @ 1A, and 16 VCT @ 2A.  I'm running the 28VCT into a 
bridge and into a LM317 and LM337 trimmed to +-15VDC. Yes, it does have the 
necessary caps and resistors.  I know that the ASM-1 doesn't need +5V, but I 
have the 16VCT winding running into a LM7805 (just using the center tap and 
one of the other wires for an actual 8V).  I have small heatsinks (5W or so) 
on the three regulators.
     
I'm using a pair of 10 ohm 10W power resistors in series to load the +15 
supply and another pair for the -15V supply to .75A each.  My DMM reads 
about .73A so I know this is close.
     
The questions are:
     
1.  When running like this, each supply is dropping about 4.3 Volts (1.3V 
over the minimum) at .75A.  After a few minutes, the heatsinks seem to get 
too hot to touch, but the regulators do not shut down!  How can I tell if 
this is too much heat for normal operation?
     
2.  The power resistors also get very hot and actually put off an odor from 
the heat.  Is this normal?
     
As far as my math allows, everything should be within tolerances for current 
and wattage.
     
  >=== Bob Zimmer -- Philadelphia PA ===< 
  >=== bzimmer at voicenet.com          ===<