Three things I can think of for +40V to +5V...

(1) Since you're considering a linear reg, perhaps try to do this in two =
steps=20
=2E.. drop to say 15V then again to 5V with a second reg.  BUT that assum=
es=20
that you can find a higher-voltage-output regulator that will allow 40V i=
n.

(2) Use a series of diodes (like 7 or 8 of them) on the input to drop the=
 40V=20
to 35V?  Silly IMO, but will work.

(3) Better option ... I know that you can do this with one of the Nationa=
l=20
Semi dc-dc converters.  Look at the datasheets for the LM2677-5.0,=20
LM2671-5.0, LM2674-5.0, LM2670-5.0.  If you go to www.national.com, there=
 is=20
an online tool that will specify workable parts based on your requirement=
s=20
and design the circuit for you.

They may also have something for -40 to +5, but if not, you may be able t=
o do=20
+40V to -5V *isolated* and connect to the other parts of your circuit=20
accordingly.  This last one is a guess, so don't hold me to it.

One last thing, and actually a question for the experts ... the 35V max i=
nput=20
voltage specification for the linear regs should be with reference to it'=
s=20
ground pin, right?  So shouldn't it be possible to raise the ground refer=
ence=20
on a regulator to >+5V, so that the 40V could be dropped to an intermedia=
ry=20
voltage (between 35V and (5V+dropout)), then use another linear regulator=
=20
with a "proper" ground reference to drop it to an accurate +5V?

Cheers,
-Neil.



On Wednesday 07 May 2003 21:31, Andy Shaw scribbled:
> Hi Folks,
> Help!
> Can anyone suggest a good way of creating a 5V supply (max 250mA) from =
a
> 40V supply. Most of the simple regulators don't seem to like this high =
an
> input voltage. Oh and if this is too easy how about generating a +5V su=
pply
> again 250mA from a -40V supply, needs to be able to share the same grou=
nd
> as the +40v and -40v supply and must be +5V not -5V.
>
> What's it for well I'm building a controller for my 3 Axis milling mach=
ine.
> It uses chop mode bipolar stepper motors. I've found a supply from an o=
ld
> HiFi amp that provides -40V 0 +40V I would like to use this as the powe=
r
> for the steppers. I would also like to drive the logic (PIC based) from=
 the
> same supply. Ideally I'll use the +40v for the stepper and the -40V for=
 the
> logic to reduce noise, hence the questions... See
> www.gloomy-place.com/cnc.htm for pictures of the milling machine...
>
> Thanks
>
> Andy

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