On Sat, 02 Feb 2008 17:42:35 -0600, Don Foreman
wrote:

On Sat, 02 Feb 2008 15:12:48 -0500, clare at snyder.on.ca wrote:

On Sat, 02 Feb 2008 03:52:18 -0600, Don Foreman
wrote:

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

On Fri, 01 Feb 2008 15:32:02 -0700, Lew Hartswick
wrote:

clare at snyder.on.ca wrote:
A zener does not make a good series regulator Lew.

And why not? It's a perfectly good constant voltage drop
over its operating range. I've use them that way over
the 20+ yrs I've been in the business.
...lew...

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.

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.

I'll bet you're very good with autos and computers -- but don't quit
your day job to design power elex!

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.


I said exactly the same thing, did I not? Forward drop across a
junction is NOT a regulator.

You did say that. A zener has about the same fwd drop as any other
silicon diode, but breaks down at zener voltage in the reverse
direction. So, it can provide a fairly constant drop of Vz (could be
6 volts) if used that way. Power zeners are made, but not nearly as
readily available as 400 mW and 1W units. The reason may be because
it is very easy to "fake" a "power" zener using a small zener and a
power NPN transistor. Zener goes from collector to base. The collector
becomes the cathode (K), emitter becomes the anode.

Fwd junction drop in germaninum is less, not more, than silicon
including Schottky.

You are correct - Germanium is about .3 volts. Gallium Arsenide is
about 3.5 Schottky are generally figured as .2 to .3 volt.
Selenium stacks are much higher, and increase with age.
It's been a while since I worked with anything with germanium
transistors - but the lower forward drop makes germanium diodes
superior to silicon diodes af RD detectors (think crystal radios)

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