nuckolls.bob(at)aeroelect
Guest
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 Posted: Thu Oct 15, 2009 7:18 am Post subject: Bi-Directional Zener vs. Diode and Zener in Series for Coil |
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| Quote: |
So is anyone willing to explain why the diode-zener diode is more robust than the bi-directional zener (SnapJack) method? |
Not sure why "robustness" is being questioned. If one
is addressing lightning issues (like we do on TC aircraft)
then it's a WHOLE NEW BALLGAME. It's not uncommon to select
transient management devices rated to stand off 600V tranisents
with a 100 ohm source impedance (6A short circuit). An that's
for INDIRECT effects of lightning. If your electrowhizzie's
wiring is at risk for direct strike . . . well.
Lightning strikes and load dumps are NOT spikes. These
are high-energy over-voltage events that are managed
with something much more than a sprinkling of diodes or
Transorbs of appropriate "ratings".
| Quote: |
I understand that you have the ability to control the ratings of each diode which might be preferable to a bi-directional diode where you don't. |
I think I explained that electrical stresses placed on
the coil suppression device are limited in voltage to
ordinary levels experienced on the airplane's electrical
system. It's limited in current based on demands of
the energized relay or contactor coil. Total energy
is a time dependent function measured in ten of
milliseconds.
The short answer is that you're worrying about breaking
the handle on your sledge hammer while using it to
squash ants.
The "ratings" of all commonly used devices far exceed
the electrical stresses the device is expect to
accommodate. I.e., electrical ROBUSTNESS so tiny a
a concern, I find it more useful to select devices for
their mechanical ROBUSTNESS.
Please review the documents offered on aeroelectric.com that
speak to "spikes". 99.9% of what is considered common
knowledge about the existence and risks of "spikes"
is urban legend elevated to what ever level of concerns
will sell more/better prophylactics against electrical
gremlins and goblins.
I spent a lot of get-yer-hands-dirty time looking at the best
and the worst of electrical system events on aircraft.
There's a REASON that spike suppression in the Z-figures
is limited to a very few components using very simple
devices.
Bob . . .
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( . . . a long habit of not thinking )
( a thing wrong, gives it a superficial )
( appearance of being right . . . )
( )
( -Thomas Paine 1776- )
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[quote][b]
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