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.
Then again, nor is a series resistor, which has been used in many
applications over the years to run 6 volt radios, instruments, etc on
12 volt cars. The resistors needed to be the right value for the job
at hand or the voltage would be too high or too low.
I've done design work too. I designed and sold several lines of
equipment for towing 12 volt trailers with 24 volt tow vehicles (in
particular Toyota BJ Cruisers) including units to adapt 12 volt
trailer brakes, using 12 volt Tekonsha brake controllers, on the 24
volt cruiser.
Also built and drove an electric powered Fiat128, with a single
contactor solid state series parallel switching mechanism of my own
design.
I DO understand the issue at hand. Very well.
I have also installed MANY 7805/LM350 replacement instrument
regulators. I like the LM350 best - I used a small trim pot mounted
directly on the terminals of the chip as a voltage devider to set the
output voltage calibration and locked it in place with Hot Glue. The
T03 case unit was mthe one I used most, fastened to a 5 square inch
aluminum heat sink, which was screwed to the back of the instrument
panel mounting brackets (steel)
Never had one fail.
These were installed to eliminate noise in radio equipment caused by
the original equipment thermal/electric instrument regulators used in
the seventies and eighties.
I still have the prototype of that unit floating around my office
somewhere.
--
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