less hum: switching power supply?

[Jim Patchell]

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