Vasile Surducan wrote:
>=20
> On Sat, 8 Dec 2001, Dave Dilatush wrote:
>=20
> > Alexandre Guimar=E3es wrote...
> >
> > >    No getting back to the SMPS. I will have an application where I =
will
> > >have to make a supply with 74 volts DC input and 24 v at 300 ma and =
5 v at
> > >30 ma. I am thinking about using 2 of your 2 transistor designs modi=
fied to
> > >the my specific needs. When I finish it I will let you and the list =
know how
> > >it goes and what modifications are needed.
> >
> > With those voltages and currents, why not use Russell's design to ste=
p
> > the 74 volts down to 24, then Roman's circuit to step the 24 volts do=
wn
> > to 5?  Russell's circuit is probably more suited to the high-voltage
> > part of this thing, especially if you need 300 milliamps out of it.
> >
>   I wouldn't do it like this. For me sounds like a standard switching
> supply using a driver ( tl494, UC3842 ), a mosfet or bipolar switch and=
 a
> transformer, then a good rectifying diode pair and finally a filtering
> stage.
> To increase the efficiency the driver could be supplied directly from
> output voltage after the oscillation has begun.
> I know that many piclisters are afraid about designing feritte core
> transformers for switching supplys. So why many of you are using standa=
rd
> coils. However for dedicated applications we need dedicated solutions.


Vasile, 24v at only 300mA is well within the
range of the cheap circuit with a pre-wound
inductor costing a few cents. Two transistors,
cheap inductor, no need for a chip or otherwise.
I've spent years winding ferrites too and its
nice sometimes not to have to do it. :o)

If REAL efficiency is needed I would PWM the
solenoids direct like Alexandre suggested, as
you can give them full current for just long
enough to "pull-in" and about 1/3 the current
for holding. I have a one R one C circuit
that does this that i've been using for many
years, (but with the added PWM this would get
very very efficient for solenoid driving.)

The non-pwm circuit:

+ ---R-------------
         |        |
         |        |
         |        SOL(+diode)
         |        |
         C        |
         |        |
         |        Transistor
         |        |
- -----------------

This circuit allows full 12v etc to pull-in
the 12v solenoid, and you simply choose the
R value to give enough holding current. You can
hold a 12v 100mA solenoid reliably with about
35mA, and just make sure C is big enough to
provide the pull-in energy. I invented this
when I was about 12 playing with my first
large solenoid. I don't think i've ever used
a solenoid since without doing this, AND its
great for keeping solenoid hash away from your
PSU. :o)
-Roman

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