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Old 07-22-2017, 07:40 PM   #21
Jackob
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Thanks for the picture, it does help me.

This is a "standard" relay that has both a set of NO (normally open ) and NC (normally closed) contacts, plus the coil. The NO contact is unused, most likely terminal #3

The radio master switch in the off position, when you turn on the aircraft master switch. The avionics relay "picks" and prevent power to the avionics bus. When you turn the AV master to on, you are removing power from the AV relay and power is supplied by way of the terminal #1 to #2 (NC). This is considered a failsafe circuit, if the wire breaks or the relay fails, power is provided to the AV bus. If the relay contacts should fail, then power can be provided by way of the emergency switch that sends power to the AV bus from the main bus.

Now for the bad news. The solenoids sold by Sly Tec do not have the NO and NC contacts. They have only NO. Providing power closes these NO contacts to complete the circuit. Finding a relay of your form factor with both NO and NC contacts will be difficult.

You could buy one of the Sky Tec master solenoids with 4 terminals and rated at 100% duty cycle. You could wire it into the circuit and move the radio master switch wire from the Off terminal to the ON terminal and everything would work. Turning the AV master switch ON would pick the solenoid and power the AV bus. With the AV switch OFF, no power would go to the AV bus. The emergency switch would still work as it does now.

This would require a sign off from your mechanic; I would hope he could do it as a minor modification. The mechanical mounting would need to be investigated. I also believe this is the way Piper implemented this function in the Cherokee Archer, with this same type of solenoid.
Ray , this is most helpful !!
Just to be sure , are you referring to this kind of solenoid ?


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Old 07-22-2017, 08:14 PM   #22
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Default avionics master relay

As was noted, what you need is a single-pole, double-throw (SPDT) relay with the appropriate ratings for the coil voltage (battery voltage in this case) and contact ratings for the avionics load (add up all the avionics circuit breaker ratings). These types of relays are extremely common in automotive applications. They no longer come in metal cases and are somewhat smaller than the one in the picture, but their electrical ratings are as good or better, and being automotive, are rated for longer lifetimes. Here's an example for a 12 volt application:

http://www.jameco.com/z/SARL-112D-Sa...m_2094135.html

There are similar ones for 24 volt applications, but they are typically from the industrial world, where 24 volts for the coil is more common.

I don't know about the legality of using something like this...whether it falls under the obsolete parts replacement or the "common electrical parts" dispensation.


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Old 07-22-2017, 10:57 PM   #23
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Ray , this is most helpful !!
Just to be sure , are you referring to this kind of solenoid ?
Yes, that is the one. 12 volt, assuming your aircraft has a 12 volt system. Must be rated for continuous duty, it will be ON 100% of the time while flying. This is the same as the master solenoid at the battery.

The one Tweety referenced would also work. Can you talk your mechanic into substituting a $5 part or a $70 part?
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Old 07-23-2017, 02:11 AM   #24
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Default avionics master relay

If the diagram in Post #17 is correct, the coil of the relay is actually only energized when the master solenoid is "on" and the avionics master switch is "off". The normal state of the avionics relay is de-energized and the normally closed relay contact activates the avionics buss. At startup when you turn the avionics switch "off" the relay is energized and the normally closed contact is broken, protecting the avionics from voltage surges. With the master off, nothing is energized. In a sense, this is a "fail-safe" system such that if the relay coil fails the avionics buss will still be activated.
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Old 07-23-2017, 01:29 PM   #25
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Thank you Tweety and Ray , looks like you found the solution.
I'll contact my mechanic and forward your recommendations, I'll try to go with Skytec as the price is not that expensive although big difference...
I'll update
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Old 07-23-2017, 04:47 PM   #26
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If the diagram in Post #17 is correct, the coil of the relay is actually only energized when the master solenoid is "on" and the avionics master switch is "off". The normal state of the avionics relay is de-energized and the normally closed relay contact activates the avionics buss. At startup when you turn the avionics switch "off" the relay is energized and the normally closed contact is broken, protecting the avionics from voltage surges. With the master off, nothing is energized. In a sense, this is a "fail-safe" system such that if the relay coil fails the avionics buss will still be activated.

All true. The problem is the Sky Tec solenoid does not have those NC contacts. Therefore you have to reverse the logical function of the avionics switch. Not a big deal, the solenoid is rated for 100% duty cycle., just like your master solenoid at the battery. The by-pass switch will work if there is a failure. I still believe Piper whet to this setup in the Archer. Most likely due to the supply of the original relays. The master type solenoid will be available forever.
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Old 07-23-2017, 06:14 PM   #27
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Default avionics master relay

If it was me, I'm not sure I'd go with the Skytec solution. With the Piper circuit, there is only one single point of failure in the avionics buss in operation...the contact in the relay. And if there were a problem there, you'd probably see it on the ground when you turned on the avionics after startup. With the Skytec solenoid, you'd have the solenoid coil, contact, and the avionics switch as points of failure, and any of them failing in flight knocks out your whole avionics set. Of course, you could say the avionics fuse or circuit breaker and the master solenoid are other points of failure, I guess.

I suspect the desire to minimize potential in-flight failure points led Piper to design the circuit the way they have.
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Old 07-23-2017, 07:55 PM   #28
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The Piper Avionics master emergency switch provides a total independent power path to the avionics bus. ANY failure in the primary circuit can be bypassed with this switch. This would include any wire, the inline filter or contact points.

While the original Piper circuit is "fail safe" in the sense any wire or switch failure will maintain power to the avionics bus by way of the NC contacts, failure of the relay contacts or mechanical failures of the relay can still happen. In fact this is the problem posted here. Just like having a wire break in your mag circuit will not shut down the engine; having the mag fail could.

The Sky Tec contacts are rated for much higher current than the old relay and will last a very long time. It will last longer than the part it is replacing. The fact the old relay is no longer available, other than as a used part, it is time for something different.
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Old 08-01-2017, 01:34 AM   #29
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I have a spare one removed from a 1980 Seneca. We changed the way the avionics master worked because the Piper way sucked. When you turn the airplane master on, there is a brief millisecond of power to the avionics before the avionics master relay pulls into the off position. Then when you crank the starter, it is possible for the voltage to drop low enough so that the avionics come on while cranking (bad starter or battery I know). I'm sure you could find a better commercial or aerospace relay to do the same thing. Or remove all of it and replace it with a simple 35A or 50A switch / circuit breaker just like the emergency avionics. Aerodon
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Old 08-01-2017, 11:49 AM   #30
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I have a spare one removed from a 1980 Seneca. We changed the way the avionics master worked because the Piper way sucked. When you turn the airplane master on, there is a brief millisecond of power to the avionics before the avionics master relay pulls into the off position. Then when you crank the starter, it is possible for the voltage to drop low enough so that the avionics come on while cranking (bad starter or battery I know). I'm sure you could find a better commercial or aerospace relay to do the same thing. Or remove all of it and replace it with a simple 35A or 50A switch / circuit breaker just like the emergency avionics. Aerodon
Have some difficulties with my mechanic to change the item .
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