nuckolls.bob(at)aeroelect
Guest
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 Posted: Mon Oct 12, 2009 8:27 am Post subject: Diodes vs. Bi-Directional Zeners for Contactors |
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At 09:57 AM 10/12/2009, you wrote:
| Quote: | Bob,
So I believe there's a decent argument to do nothing. If the spike is small, |
"small" isn't very descriptive. While total energy
is measured in a handful of millijoules, the voltage of
the unsuppressed spike is hundreds of volts and does
produce observable erosion of starter push-buttons,
battery master switches and landing gear control
switches (hydraulic pump control contactors).
For years, no suppression was used in any automotive
contactor control until the two-stage, high-inrush
solenoid/contactor was developed for starters. See:
http://aeroelectric.com/articles/strtctr.pdf
| Quote: | | and the switches are robust enough to handle far more than the spike, |
don't know what you mean by "robust". They are
32 volt DC devices for full ratings. The unrestrained
pulse from a contactor coil is on the order of 10x
that amount. I.e., arcing does occur with energy
limits bounded by the [I(squared)L/2] energy stored
on the coil.
| Quote: | | and the current through the contactors is far below the contactor rating, |
current being switched by the contactor isn't
the big driver. We decided this after learning
that suppression of all types has no appreciable
effect on contact spreading velocity.
| Quote: | | and the number of cycles of the switch and the contactor are far below the advertised life... |
yes, VERY much lower . . .
| Quote: |
...then leaving off the diodes is a viable alternative. |
. . . and indeed, until the mid 60's this was
how contactors were treated in light aircraft
although industrial and military users of DC
contactors had been "taming the little tigers"
for years. But many starter contactors are
now offered with built in diodes. LOTS of
mil-spec relays are offered with built-in
coil suppression. Yeah, we could leave them
off, but why not put them on if they're
cheap, easy to get, and choice of technology
is not a significant decision?
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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