Homebrew PCBs

On Sunday 10 July 2005 04:21 pm, Stefan Trethan wrote:
> On Sun, 10 Jul 2005 21:16:38 +0200, Robert Hedan
>
> <robert.hedan@...> wrote:
> > Hi,
> > I've just uploaded a schematic of a 40V 3A circuit that I put together.
> > I've thrown in every security measure I could find in the DigiKey manual
> > as
> > an exercise.  The file is in FILES / Power supplies.  This link may (or
> > may
> > not) work:
> > <http://f1.grp.yahoofs.com/v1/MHDRQjmKCjHN8hORzw6Lxapg3rQLHR9sg4BZ8ePemrC
> >IKl
> > pX6E8d2Q1QnZ52WUfAGgpe1uV08UyZCt9sgF5IHEcaLLyziJI/Power%20supply/Drilling
> >%20 station%2040V%203A%20steppers.JPG>
> > Please let me know if I have design flaws, is the fuse rating high
> > enough,
> > components in series instead of parallel or vice versa, suggestions for
> > additional security measures, improvements in filtering, etc.
> > This is my first power supply and I don't want to have problems with it
> > in
> > the future.  I also want to make it as robust as possible.  A separate 5V
> > circuit will power the logic circuitry, this is exclusively for the
> > steppers.
> > Robert
>
> You certainly have a few surplus parts in there ;-)
>
> What's the inductor L1 for?

I was wondering about that myself.

> U2/3/4/5 are a mystery to me.
>
> What is the EMI coil, what is the input? mains?

Presumably coming out of a transformer?

> Putting caps in parallel to lower ESR is a nice idea, but usually you'd
> only put one electrolytic and one ceramic.

To the contrary,  data sheets from some regulators do recommend a tantalum cap 
there.  I've done it that way myself on more than one occasion.

> If you want to use several electrolytic parallel instead that is a good
> thing, but usually you'd use the same size. A very small electrolytic
> parallel with a big one is just not changing anything, esp. if you have a
> ceramic cap too.

It _will_ have somewhat better response at higher frequencies,  but a tantalum 
cap will have much more of an improvement.

> I dunno about your fuses, you certainly have a great many. I wouldn't want
> to predict which one will trip first. Also, i wouldn't see the need.

Two isn't what I'd call "a great many",  the first one protects the 
transformer from a shorted rectifier,  the second one protects the regulator?

> What i would have done if i had to use a transformer supply and not a SMPS:
>
> hot->fuse->(NTC)->primary1->primary2->neutral.
> (ntc in case of toroid transformer)
>
> sec->bridge rect. at least 20A->electrolytic cap 6800-10000u->voltage
> regulator->cap 220u or more parallel 100n to 470n->polyfuse->out.

I too tend to over-spec rectifiers.  The difference in price for hobby 
quantities is just not significant,  particularly when you add shipping 
charges into the picture.  I don't know if I'd go as far as 20A, but it sure 
wouldn't hurt anything.  I also notice a couple of 1N5401 diodes in there, 
presumably to protect the regulator.  I wouldn't bother with anything less 
than 1N5408 -- same series,  ten times the breakdown voltage rating,  and 
probably a minimal difference in the cost.  They'll be useful for other 
things as well.