less hum: switching power supply?

[Harry Bissell]

All 78xx parts (except the 7824) have 35V max input.  Dropping this with a
"simple" resistor is a BAD idea, unless you are very sure you know the current
under ALL LOAD conditions.  My company has learned this the very hard way !!!
If I were looking at that high of an input voltage,  I'd use the LM317 and
adjust the output. Its good for a 40V
differential... so thats output voltage plus 40V (max.)

BTW the transistor/zener preregulator in the data sheet is OK.

Short Circuit protect ??? How "mission critical" is your system ? If you can
stand the regulator blow-up, then don't worry.  But the regulator might get hot,
short itself,,, and then your supply could throw full voltage against the
shorted load.  I like the PolySwitch
from Raychem, or equivalent. Good stuff !!!

H^)  harry



Roel Das wrote:

> Hello,
>
> In the 78xx datasheet, there's a schematic given for high input voltage
> regulators. They just put a resistor on the input of the regulator (before
> the cap!). Isn't 35V quite high to feed to a 7815/7915? Wouldn't it be
> 'cooler' if you put it there?
>
> What is the preferred input for for example a 7815?
>
> The protection diodes are also covered in the datasheet. And a shortcircuit
> protection using 2 transistors; is this worth the trouble? Anyone of you
> ever got a shortcircuit from normal use of equipement? (Not development,
> just use...)
>
> And I thought I would know it all by the end of the year...
>
> Roel
>
> >
> >     You are correct Paul.
> >
> >     The biggest cause of failure I have noted in both 78xx and 79xx
> regulators
> > has been missuse by the designer.  Probably the single biggest crime is
> the
> > overheating protection.  While the regulators will shut down when they get
> too
> > hot, I have seen many engineers use this as an excuse to inadequately
> heatsink
> > the regulators.  Overheating protection or not, one rule of thumb I
> remember
> > (perhaps a little dimmly) is that for every 10 degree C rise in temp, you
> double
> > the failure rate (this is for semiconductors, don't really know how true
> this
> > rule of thumb is).  78xx/79xx parts basically should not fail.  If you are
> > seeing a significant number of failures, something is wrong.
> >
> >     Also, for the price of two more diodes, you can protect the regulators
> > against reverse voltage damage quite easily (as per National Semiconductor
> > Application Notes).  If you wish to see how to connect these diodes you
> can look
> > at http://www.silcom.com/~patchell/powersupply/powersupply.pdf for an
> example.
> >
> >     -Jim
> >
> > >
> > > perhaps this is time to note that not all chips with the same type #
> > > are always identical.
> > >
> > > paul perry Melbourne Australia
> >
> >