On 2/15/06, Peter Todd <pete@petertodd.ca> wrote:
> 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.

 A typical situation when better split a huge supply in many smaller than
struggle with noise and spikes.



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.

6.3V ac is not enough to get 5V/25A dc stabilised with any schottky
diode ever built.

>
> 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!

three dual diode diode in a bridge, are you using tri-phase mains supply ?

>
> 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.

 Wrong direction.

best regards,
Vasile


>
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