| 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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