Alright, no laughing, this is a serious linear power supply, not some
wimpy half-amp unit. Not that I'm claiming my design is any good...

I've got a device I built that has 64 stepper motors with associated pic
chip drivers in it. These are extremely cheap steppers,
http://www.allelectronics.com/cgi-bin/item/SMT-57/400600/STEPPER_MOTOR_.html
to be exact. Total current draw averages about 22A-25A but can peak as
high as about 40A. Not that I am totally sure about those numbers,
that's just assuming that the amp meter on my testing power supply
works. The motors are not synchronized to each other. To be exact, they
play Conways Game of Life...

The the problem is as follows. I built a linear power supply using a 28A
6.3V transformer from digikey. 237-1255-ND. 4x LM396K 10A regulators in
parallel through load-sharing resistors regulate the voltage. For my
first attempt I used a standard 50A 600V diode bridge, MP506W-BPMS-ND
That didn't work, the final voltage at the "5V" line ended up being
about 3.5V, causing the master PIC controller to crash.

My second bright idea (ha!) was to try to reduce the voltage drop by
using those fancy schottky diodes I noticed in the catelogs... So I
ordered three 115CNQ015ASL-ND 15V 110A and made them into a diode
bridge. (they are two diode packages) That worked beautifully, giving me
a usable 4.2V line, until after about 30 minutes of running I heard a
loud bang. The main AC fuse had blown, though not the 40A DC fuse, and
the after some testing the diodes were found to have developed internal
shorts. All three seem to have done this at once!

So, any ideas where I was going wrong? One possible culprit I think is
how I soldered them, I made up a circuit board and used a heatgun to
heat the whole packages up and "reflow" the heat-spreader tabs to the
PCB. All this done without much regard to temperature other than go
slowly and use as little heat as need be.

In any case, I just want to get some ideas before I go off and
potentially spent another $40 on diodes, or try to find a bigger
transformer.

-- 
pete@petertodd.ca http://www.petertodd.ca
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