[sdiy] Power Supply Design Questions

[Scott Gravenhorst]

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/