[sdiy] Power Supply Design Questions

[patchell]

    Here is the rule of thumb I use for selecting transformer voltage...I
need 12 VDC out, I use a 12VAC transformer, although, I prefer using
centertapped full wave rectifiers, so I would actually select a 24VCT
transformer, but rectified and filtered, this would end you up with about
17VDC.  Allowing for 2 volts of ripple, the input to the rectifier would then
go between 15 to 17 volts.  To get 2 volts of ripple at 1 amp, with a half
wave rectifier, you will need 8333uF, for a full wave rectifier, you will
need half that (4200uF)...

Scott Gravenhorst wrote:

> Ok, maybe a silly question... I have a couple of 1200ma 16 volt
> transformers, (technically wall warts, but these are big and heavy).
> Anyway, 16 volts is really too high for what I want.
>
> I clipped together a 1/2 wave rectifier and I see 24 volts.  I'd like to
> use this with 12 volt regulators.  It seems like these transformers
> aren't going to cut it.
>
> harrybissell <harrybissell at prodigy.net> wrote:
> >You heard all (most of) the good answers already....
> >
> >There is a factor in transformer size... smaller transformers are run at
> >higher
> >flux levels (closer to saturation) to make them ...er... small.  They
> >will run hotter
> >and suffer from poor regulation (voltage change from no-load to full
> >load).
> >
> >This can be 20-30% in a small transformer... as opposed to maybe 5% in a
> >larger
> >unit.
> >
> >This means you have to allow for this in the regulator design... the
> >voltage could be
> >30% higher because of small load, and higher still because of a high
> >line voltage condition...
> >so the bulk DC voltage will be much higher, and the regulators can run
> >hotter.  Ripple
> >will also increase in high-load cases.
> >
> >That said... its easy to underspecify  a transformer for a small power
> >supply.  A bigger
> >core (higher current than you really 'need') might actually make for a
> >better, more stable
> >design, if you can afford the size and weight....
> >
> >(guess why a lot of wall-warts run so damn hot ???)
> >
> >H^) harry
> >
> >Scott Gravenhorst wrote:
> >
> >> Some questions I have:
> >>
> >> At what input voltage should a LM78xx or LM79xx type
> >> regulator be run for minimum heat?
> >>
> >> I read in data sheets that 2 volts is the usual value for
> >> dropout voltage (at 1A).
> >>
> >> Would this be the point (well, somewhat above it) at which
> >> the regulator would heat up the least?
> >>
> >> Operated so, does this also mean using a transformer with
> >> better current capabilities?
> >>
> >> Thanks all.
> >>
> >> =========================================================
> >> - A smoking gun is one that's already been fired.
> >> - That which gets rewarded, gets done.
> >> - What good are laws that only lawyers understand?
> >> - Government: The other religion.
> >> - The media's credibility should always be questioned.
> >>
> >> -- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
> >> -- Linux Rex         | RedWebMail by RedStarWare
> >> -- FatMan: home1.gte.net/res0658s/FatMan/
> >> -- NonFatMan: home1.gte.net/res0658s/electronics/
> >
>
> =========================================================
> - A smoking gun is one that's already been fired.
> - That which gets rewarded, gets done.
> - What good are laws that only lawyers understand?
> - Government: The other religion.
> - The media's credibility should always be questioned.
>
> -- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
> -- Linux Rex         | RedWebMail by RedStarWare
> -- FatMan: home1.gte.net/res0658s/FatMan/
> -- NonFatMan: home1.gte.net/res0658s/electronics/

--
 -Jim
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