[sdiy] Power Supply Design Questions

[harrybissell]

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.
>
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