nuckollsr(at)cox.net
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 Posted: Tue Aug 22, 2006 5:07 am Post subject: MOV's vs. Diodes for spike suppression |
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At 11:02 AM 8/21/2006 -0700, you wrote:
| Quote: |
Coil Suppression:
MOVs are considered to be better than diodes, although they have higher
impedance. But MOVs typically have a limited lifetime.
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The lifetime issue controls when the MOV is stressed
repeatedly to its maximum rated energy levels . . . in
coil spike suppression, the energies are tiny by comparison
and life-limits do not become an issue.
| Quote: | Diodes are not the best method today. Not even the second-, third-, or
fourth- best method. But in the 1960's they were the way to go.
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<snip>
Measurements on my bench have failed to demonstrate
the suggestion.
A number of papers have been cited over the years, some
written by some folks who work for big name companies
like Tyco-Amp, Teledyne, etc. wherein authors have suggested
that the readily observable effects of plain diodes on
opening delay (no big deal in 99.99% of applications)
directly translates to slower contact spreading velocity
(extended arcing during contact break).
Two separate conditions are in play. (1) slower rate
of decay in the relay or contactor's coil current and (2)
rate of decay in the magnetic attraction force as the
moveable armature begins to separate from its seated
position within the device. The diode has a profound
effect on (1) but a small effect on (2).
Most of the papers I've read accurately observed
and discussed (1) and even did some measurements but
then went on to improperly assume that similar effects
would be noted for (2) as well.
The trace at:
http://www.aeroelectric.com/Pictures/Curves/704-1DelayNoDiode.gif
Shows contact OPENING DELAY of an S704-1 plastic high
current relay when no diode was present across the coil
and the coil current was being interrupted by the "perfect
switch". Note the expected coil spike on channel 2 and the
opening delay of about 2.5 mS. You can see the
arcing across spreading contacts if you look carefully
at the falling trace on channel 1.
http://www.aeroelectric.com/Pictures/Curves/704-1DelayWithDiode.gif
This trace shows what happens when we use the plain-vanilla
diode across the coil. Yes, OPENING DELAY goes up by a factor
of 500% to about 12.5 mS.
Now, let's go take a close look at the arcing phenomenon noted
in the two traces above . . .
http://www.aeroelectric.com/Pictures/Curves/704-1OpeningTimeNoDiode.gif
. . . with no diode, a series of about 10 measurements produced
an opening time (ARC DURATION) that averaged 210 uS. In the
next trace . . .
http://www.aeroelectric.com/Pictures/Curves/704-1OpeningTimeWithDiode.gif
. . . the diode was put back on and we see an average of
230 uS ARC DURATION for an increase of about 10%
I did similar experiments with other relays and did not
formalize the data gathering but got similar results.
Bottom line is that the use of the lowly diode for coil
spike suppression does not deserve relegation to the dust
bins of electronic history as a 5th-rate spike suppression
technique. Unfortunately, some big names working for big
companies have stubbed their toes on significant but error
driven assumptions.
The repeatable experiment has demonstrated otherwise.
Continued use of diodes as suggested in the 'Connection
and on many drawings posted to the website is not a
recipe for failure. Substitution of a more "modern"
technique will produce no observable effect on the
service life of your relays and contactors.
Bob . . .
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