less hum: switching power supply?

[jhaible]

... and there was some voltage spiking mentioned.
I don't know if this is the same phenomenon I have noticed
or not, but what I have seen is that 78xx regulators produce
a higher output voltage when two conditions are valid at the same
time:
1) Input voltage way more than necessary (but still withing the
specs)
2) Output current too low (no minimum load).
The tricky thing is that normally you do not need a minimul load,
and normally the input voltage can be rather high, BUT not
both at the same time.
This error is a static one, btw. I'm not sure if it's valid for
all manufacturers of 78xx or just a few. (See archives
for more)
Maybe a condition like this can occur for a short time, resulting
in the mentioned voltage spiking, even when the static case
would be ok ? Reactive loads - anything that can make the output
current of the regulator approach zero during a transient ?
Switching power supplies rapidly on and off is a very severe
test condition, after all !

JH.




-----Ursprüngliche Nachricht-----
Von: Jim Patchell <patchell at silcom.com>
An: <synth-diy at node12b53.a2000.nl>
Gesendet: Montag, 5. Juni 2000 21:24
Betreff: Re: less hum: switching power supply?


>
>
> Paul Perry wrote:
>
> > At 11:45 AM 5/06/00 -0400, Jay Martin  wrote:
> > >I beg to differ,
> > >The 78xx series of regulators do NOT have built in protection for
shorting,
> > >overheating or reverse-voltage damage
> >
> > Hmm.. there is certainly overheating protection on the National Semi
78xxs.
> > And, I've used a hell of a lot of them & never seen '8 volt spiking'.
> > What do you have to do to get these spikes?
> >
>
>     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
>
>
>