Voltage regulator with alternate voltage source...

Bill Sudbrink wh.sudbrink at verizon.net
Fri Apr 8 13:50:26 CDT 2016

William Donzelli wrote:
> > 1) The 7805 is actually a Motorola MC7805CP, date
> >    code 7308 with gold leads.  Very hard to exactly
> >    replace.
> Is that the big flat plastic package with the wide
> flat leads?

Nope, purple ceramic.  The heat transfer plate is gold
plated too.

> > 1) I have a 12 volt DC supply.  12 volts seems to be
> >    within the VIN range for the 7805s whose data
> >    sheets I've now read.  Can I simply apply 12 volts?
> Yes, but that regulator might get mighty hot! I would
> not do this for fear of cooking the poor thing.

That's what I figured to start with, before reading the

> > 2) Could I place a resistor in series between the 12V
> >    supply and the 7805 to drop the voltage at the 7805
> >    to somewhere around 8?
> Yes, you could do this. Pick an appropriate power resistor,
> or use a big wirewound rheostat.

At this point, I think I'll just wait till Monday and work
on other things.

> > 3) If I was to "tack on" a jumper between VIN and VOUT,
> >    would that protect the 7805 and allow me to power the
> >    circuit with 5 volts?
> I would not do this at all.

I didn't think so.


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