Uh, not quite Jack......... Yes, the intermittent relay failed (melted
parts) because it was held in too long, but in the odd Lancair terminology
environment:
S = Switch (+12VDC, while the other side of the coil is grounded through
the case) coming from the pressure switch and thence from the gear selection
switch through the 5 A breaker/fuse that folks forget to pull when they pull the
pump breaker whilst power is supplied from the main bus and the particular
pressure is low enough to allow the pressure switch to make.
I = Indicate = +12 VDC available as a tap off of the contactor plate when
it makes the connection betwixt power and pump used to provide the
positive voltage to the lamp on the console marked "pump in transition".
And, yes, hopefully the 18 gauge wire running from the I-terminals will
never short while the pump is engaged because it will try to carry hi amps
up to those of the 50-60 A breaker covering the #10 wires used to power the pump
through the relay.
OH well.......
Diode stuff is good..........
Grayhawk
In a message dated 4/19/2013 12:46:57 P.M. Central Daylight Time,
jmorgan1023@comcast.net writes:
The
failed relay is an intermittent duty starter solenoid. The "s" terminal label
is for start. The "i" terminal label is for ignition. The "i" terminal shorts
out the ignition ballast resistor in a breaker point ignition from the old
days. The plastic coil melted because the relay was held on too long.
The add from snap jack is aimed at small mass relays. The usual snubber
diode technique common in our airplanes slows the collapse of the relay
magnetic field which slows the contact break action. Slowing the break action
in a small relay will increase contact arcing which shortens the life of the
relay when used near it's maximum rating with an inductive load. The large
solenoid type relays we use will not benefit from the snap jack since the
break is dominated by the mass of the moving contact rather than the collapse
of the magnetic field.
A note of caution with the snap jack decision. The snubber is in place to
prevent the arcing of the contacts which energize the solenoid (pressure
sensor, microswitch, etc.). The snubber used must keep the voltage below 35
volts to be effective so if you decide to use the snap jack be sure it
specifies a clamp voltage below 35 volts. For those with 24 volt systems, I
would suggest not using the snap jack since the clamp voltage is nearly the
same as the the system voltage and no benefit is realized.
As you might be able to infer from this... I suggest the use of the
simple diode with the precaution that the polarity be correct.
Feel free to shoot me an email if further comment will help.
Jack Morgan
On Apr 19, 2013, at 6:00 AM, Lancair Mailing List wrote:
Subject: 320/360 intermittent
duty relay
Date: April 18, 2013 2:56:12 PM
EDT
Had
an opportunity today to dissect the failed relay on the low pressure side of
my system.
Clearly there had been arcing between the load terminals and
the contactor bar but more interestingly,
the plastic which surrounds the
magnet had begun to melt and crack. This prevented the contactor bar shaft
from sliding
freely when the magnet was energized.....
So, at the
risk of displaying my normal electrical ignorance, which diagram below best
represents
the intermittent relays in our 320/360s? My relays are
grounded at the case and have two small terminals
on the side marked "I"
and "S". Voltage applied to the "S" terminal will energize the
relay.
In this illustration from the Perhgelion Design website, it
looks like the 2nd diagram from the left best represents what we
have. So
placing a Bi-directional Zenner diode between the "S" terminal and the case
ground is all that's needed
to protect the relay control circuit, right??
But what about the fat contactor load terminals and the bar inside the
relay, how can or should they
be protected as well? Does it
make sense to place a Bi-directional Zenner diode across these terminals as
well?
See, your suspicion that I don't have EE after my name is
confirmed!
Angier
Ames
N4ZQ
22hrs
<Zener.jpg><zener-1.jpg>
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