less hum: switching power supply?

[Harry Bissell]

in or around 1982 I had a "bad" experience with the 7805...  with a particular
brand of chip (SGS-Thompson). With a regulated 12V "bench" supply feeding it,
everything went
well... but with a standard "non-regulated" supply voltage the thing went
haywire...

Voltages from 8-13 volts made it erratic. Took two days to find it, after
suspecting
EVERYthing else in the circuit. I put a Motorola chip in... problem fixed. Tried
10
other SGS chips... problem restored.

So watch out. All chips are NOT created equal.  I think the 78xx and 79xx
regulators are just fine. Use the protection diodes Jim Patchell describes. USE
a ceramic cap at the output as recommended in the data sheet... and locate them
AT the regulator pins.
Use an adequate heatsink.  Do not rely on thermal protection alone (think fuse
or external current limiting circuit) and if you are absolutely sure you cannot
stand a spike... an inline
PolySwitch or similar positive temperature coefficient circuit protector, and a
shunt Zener
diode are the way to go... the zener shunts the spike, the PTC goes high
impedance in case the spike doesn't "go away" before the zener heats. These PTC
elements can have
fractions of an ohm of DC resistance at normal current levels. Size the Zener
above the
actual output voltage of the regulator...

For the ultimate in "safe" operation... shunt regulators are very good. But they
are so power hungry they can't be used except in the smallest (current)
circuits.

H^)  harry (I'm a "Power One" man also.... Problem solved)

jhaible wrote:

> ... 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
> >
> >
> >