Byron A Jeff wrote:
>
> On Tue, Jul 17, 2001 at 08:57:40AM -0400, Robert E. Griffith wrote:
> > I have a design that uses a 12v wall wart power supply because an LCD needs
> > the 12v.  The main circuit runs on 5v and 3v.  In the prototype I have a 5v
> > regulator (LM340TS) dropping the 12v to 5v and a 3v regulator (TPS7230QP)
> > that drops 5v down to 3v. This device will be on all the time.
> >
> > I have a heat sink on the 5v reg and it gets hot. I want to put the finished
> > circuit in a project box with vents but no fan.
> >
> > My question is: is there an easy, low cost, solution to implementing a
> > switcher to drop the 12v to 5v? Is this done? The 3v reg does not provide
> > much power so it is not a problem.


Hi Jeff, since you didn't specify the currents at the
3 voltages, I can only guess, but one way to handle
this problem is to get a cheap multi-tap transformer,
like 3v-12v, these are common from most hobby type
electronics shops, and similar sizes and costs to
your wall wart.

This can give you a 12v rail and a 7v rail, ready for
the 3 pin regulators. Cheaper and simpler and more
reliable than going to switching regulators, and more
efficient as a transformers runs about 90% efficiency.

After servicing many cheap CD portable hifis and CD
players I notice all the manufacturers doing this,
cheap multiple tap transformer and six diodes (two
full rect bridges) to give the 12v rail for the
mechanicals and power amp and the 5v rail for all the
control circuits.
-Roman

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