wallwarts and regulation..

[Scott Gravenhorst]

>Or another way is to use a 7805 regulator with an as-close-to-8V zener diode (REVERSE biased - >opposite from the 1N914 Jay recommends).

I'd think a higher current capable diode like 1n4001 would be better
since at low load currents, you'll have higher current to ground thru
the regulator's ground connection.  A 914 isn't good for alot of
current.

>Does anybody know if using a zener like this introduces noise?

Hmm.  I'd suspect yes.  how much?  Don't know, but it's possible to
filter that away.  However it seems that you'd be relying on the zener
to do a whole lot of the regulating task, if it would work at all.
The zener, too, would have to be pretty big, watt wise.  And at that,
it may send that 7805 doing wierd whacky things.

All in all, my impression is that there are better ways.  I know there
are regulator ICs that are 'programmable' voltage wise and can be
beefed up with a pass transistor for higher currents.  This will
result in a higher component count, but also in a better regulator.
I'd hate to introduce problems into a system using a questionable PS
design that could be eliminated with an extra 2 hours of work and a
few extra dollars worth of parts.

My $0.02, FWIW.


-- Scott Gravenhorst