raymondj(at)frontiernet.n
Guest
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 Posted: Mon Feb 21, 2011 9:13 am Post subject: Permanent Magnet Alternators - Over Voltage Protection |
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do not archive
THIS email is why I love this list. I didn't understand a word of it!!!! 
But if I keep reading this list and studying one day I will.
Thanks again, Bob.
Raymond Julian
Kettle River, MN
On 02/20/2011 11:13 PM, Robert L. Nuckolls, III wrote:
| Quote: |
<nuckolls.bob(at)aeroelectric.com>
At 07:31 PM 2/20/2011, you wrote:
>
> <fran4sew(at)banyanol.com>
>
> I read about the Silicon Bilateral Switch on the internet today.
> Evidently it is not necessary to connect the gate because the SBS will
> start conducting when the voltage reaches a certain value without the
> gate connected.
> I am still wondering if a zener diode could be used instead of the
> SBS. If not, it would be interesting to know what the disadvantages of
> the zener diode are, or if it would even work at all.
It MIGHT work sort of if you used a sensitive gate
SCR but that offers a potential for other problems
like nuisance tripping due to noises on the bus.
The non-sensitive crowbar SCR needs a swift kick in
the gate to insure a good trigger at all temperatures.
The simple zener has a soft conduction 'knee' in it's
V/I curve so is a poor voltage reference at the
threshold of conduction. Further, the current that
can be delivered to the gate of the SCR due to
zener conduction is limited by the source impedance
of the calibration voltage divider.
The diac has a more predictable trigger voltage
which produces an avalanche conduction mode that
dumps the time delay capacitor into the gate
of the SCR. That pulse is hundreds of mA to
over 1A. This same characteristic is also desired
for the triggering of triacs in dimmers . . . the
#1 commercial use for diacs of any voltage.
See:
http://aeroelectric.com/Pictures/Products/9003_B&C/OVM_14_Prototype.jpg
This was the first crowbar OVM we offered many moons
ago. This configuration was modified early in
the production experience to add the 10 ohm gate
to cathode resistor to negate nuisance tripping.
The diac in this version visible on the left was
a really cool part. An SD10 as I recall with very
good tolerances on a 10v trip calibration. This part
went obsolete and we went to the MBS4991. That part
went obsolete and I synthesized the diac functionality
with the pnp/npn pair. At this time, I think B&C has
both the diac and transistor versions in production
on the fleet of products. They made a life-time buy
on the MBX4991. I bought a couple hundred too so that
I can support my own production though it's probable
market life. It is similar to this configuration
http://aeroelectric.com/Pictures/Products/9003_B&C/OVM-14_A.jpg
http://aeroelectric.com/Pictures/Products/9003_B&C/OVM-14_B.jpg
except that we now offer a 15 turn calibration potentiometer
that can be accessed in the field.
This is why the DIY project features the transistor
pair. But it appears that the new kid on the block
from Digikey would allow one to fabricate the diac version.
Your choice.
Bob . . .
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