On 2008-02-02, Don Foreman wrote:
On Sat, 02 Feb 2008 01:09:12 -0500, clare at snyder.on.ca wrote:

Because a Zener only STARTS to conduct above a certain voltage. Zener
regulators are GENERALLY shunt regulators. The load is in series with
a low value, high power resistor, and the zener goes across the load
in such a manner as to short excess voltage to ground, causing the
voltage drop across the resistance to absorb/regulate to the zener
resistance.
Diodes in SERIES are generally used for minor voltage adjustment (not
regulation) with the forward drop of various diodes generally running
in the 0.64 volt range for silicon diodes, Schottky diodes drop
significantly less, Germanium diodes significantly more.

Actually -- germanium diodes significantly *less* -- as is
supported by the site which you point out below. Germanium diodes at
about 200 mV forward drop, Silicon diodes at about 700 mV forward drop.
(Actually, I had remembered germanium as being closer to 150 mV, but
haven't checked one in years.

See: http://www.kpsec.freeuk.com/components/diode.htm if you don't
want to take my word for it.

By the way, what business are you in?
25+ in the automotive, and almost as long now in computer electronics.

[ ... ]

A Zener would indeed provide a fairly constant drop and they are
sometimes used that way. This wouldn't be a regulator, just a drop
that is fairly constant over a reasonable range of load currents. The
output would not be regulated, but it would be reduced by a fairly
constant amount.

The problem here is that a 12V automotive system goes through a
wider voltage swing than a 6V one does, and if you are using a zener in
series, (assuming that the load draws enough to get past the knee of the
zener's curve) you will wind up with more voltage swing at the 6V load,
by subtracting something approaching a constant from the 12-14V swing of
the charging system and battery.

Enjoy,
DoN.

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