On Tue, 22 Sep 1998, Dave VanHorn wrote:

> >Isn't there a bunch of schematics to make a 7805 a switcher with one
> >transistor ? I've never used this, but it used to be in the National
> data
> >books. Has anyone tried one of these as a fix on an existing design ?
>
> There's something in the databook about using an external follower,
> basically, again, just moving the heat problem, not getting rid of it.

Nonono. I mean the one with the follower, the coil, the recovery diode and
a resistive divider in the output to induce hysterezis. This is one of
those weird designs where the regulator stays linear for low Vi/Vo and
starts switching when current is demanded.

> >OTOH how about using a 555 as power oscillator at 1/4 duty cycle
> followed
> >by a D, L and C to feed a normal series regulator with 8-9 V ? This
> is one
> >that I've used when I had no choice (had to modify an existing design
> that
> >had a closed plastic case and fried regulators on a regular basis). I
> >don't know about passing FCC etc with it however.
>
> I'm not clear on how much power he needs, I was assuming large-ish due
> to his comments on heat.
> Anything with much current demand would blow the 555 approach, plus
> using transistors to switch large caps on and off isn't very efficient
> either. An inductor is the right tool here.

I was suggesting the use of a 555 as switching regulator (series, open
loop) before the actual reg. If the 555 is used to give 100 mA it should
give about 300 mA available at the input of the linear regulator with the
given voltage drop.

Peter