Turn off the radios

Discussion in 'Maintenance Bay' started by brien23, Aug 11, 2017 at 9:55 AM.

  1. SkyHog

    SkyHog Touchdown! Greaser!

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    Everything Offends Me
    What I find unbelievable is that this is still necessary. How has no one found a way to isolate the power to avionics from such brownouts or spikes?

    It's 2017. This should not be a thing anymore.
     
  2. Ravioli

    Ravioli En-Route PoA Supporter

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    :popcorn:

    And some for you guys:
    upload_2017-8-12_10-39-48.png

    Nice to see the avionics master switch thread roll back through. It's been a while.
     
  3. GlennAB1

    GlennAB1 En-Route

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    It is nice to have radios already on while starting engine(s) in case there's a fire, but that would only matter if you were at a real airport, oops, sorry.
     
  4. weirdjim

    weirdjim En-Route

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    Most of us put a 15 volt zener across the input wire on a 12 volt radio but even a zener has a measurable turn-on time (minority carrier lifetime and all that rot) where a really humongous spike can do some serious silicon barbecue. You won't see anything with a voltmeter; an oscilloscope with memory is the only way to see the nanosecond spikes and dips.

    Jim
     
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  5. denverpilot

    denverpilot Taxi to Parking

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    People have. They're not perfect.

    You're also talking some damned big spikes when cranking an engine that doesn't want to start with a lightweight electric motor. Like Jim said, there's no such thing as a perfect zener, capacitor, or any other component, and the devil is in the details.

    Every start with the radio on stresses the components that can't handle the extremely short but extremely big voltage spikes.

    You'll get away with it, until the component has had enough or the old starter motor tosses a particularly big spike today. Then you won't.

    Best way to protect it? Take it out of the circuit.
     
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  6. Capt. Geoffrey Thorpe

    Capt. Geoffrey Thorpe Final Approach PoA Supporter

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    Know one of those Chinese guys that can figure out what this part is (the one with the hole in it)? If I could find the spec's then I could fix my turn coordinator.
     

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  7. denverpilot

    denverpilot Taxi to Parking

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    That does not look new enough to be obfuscated. But whatever blew the hole in the thing probably needs to be fixed before replacing it.

    I bet you post that photo to the EEVBlog Forums someone there would be able to guess what it is, with half of the part number missing.

    If you have the service manual for the TC and it has a schematic, that'd make it pretty obvious what it is, too.

    Definitely let the magic smoke out, though.
     
  8. Capt. Geoffrey Thorpe

    Capt. Geoffrey Thorpe Final Approach PoA Supporter

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    I suspect that would be the old electrolytic cap that blew. There is enough of the case left to get the value off it. That's the easy part :)

    Thanks for the suggestion.

    If only.

    Yea. Blew a trace on the board as well.

    And, it wasn't because I forgot to turn off avionics when starting.
    (It was that way when I bought the airplane. )
     
  9. tspear

    tspear Pre-takeoff checklist

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    Avionics master anyone?

    Tim
     
  10. deonb

    deonb Cleared for Takeoff PoA Supporter

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    How many legs are on the IC?
     
    Last edited: Aug 13, 2017 at 12:02 PM
  11. Capt. Geoffrey Thorpe

    Capt. Geoffrey Thorpe Final Approach PoA Supporter

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    There is a center lug bolted down on each side of the chip and two more reasonably fat legs - if the motor is DC, I would assume it is a voltage regulator, or, if the motor is AC (somehow seems more likely, but that's just a guess, and I am a dumb old mechanical engineer so take that for what it's worth) it would seem to be a power transistor driving an oscillator. The board has the larger resistor, a couple of pots, 3 or so small capacitors, the remains of an electrolytic cap, the motor and that's about it.
     
  12. Timbeck2

    Timbeck2 En-Route

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    Okay, show of hands - how many had to look up "zener" and found two different definitions? One says it allows current to flow both ways and the other says it allows current to flow in only one direction.
     
  13. weirdjim

    weirdjim En-Route

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    Of course it conducts in both directions. It is a plain old silicon diode in the FORWARD direction (and acts as a reverse-polarity protector) and a diode with a specific breakdown voltage (zenering voltage) in the REVERSE direction. It thus protects against both positive and negative spikes.

    My basic electronics class, lesson #3. The person who wrote that it only conducts in one direction is one of my F students.

    Jim
     
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  14. Timbeck2

    Timbeck2 En-Route

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    Well I appreciate the education Mr. Weir.
     
  15. denverpilot

    denverpilot Taxi to Parking

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    Tom Petty wrote the song Breakdown for Zener diodes. ;)

    (Kidding.)
     
  16. nrpetersen

    nrpetersen Pre-takeoff checklist

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    Just look at the circuits & try to figure out where possible transients can come from. The real problem is inductive kickback from something with a lot of stored electrical/magnetic energy. Two major transient sources are the starter and the alternator. Smaller solenoids such as the starter or master operated relays can also kick back, but the worst situations (biggest voltage kicks) will be whenever the electronics are somehow still connected to the system but when the the battery is somehow out of the circuit. This can happen if the master solenoid somehow stutters and the starter kick is dissipated into the electronics. Another transient source could be the alternator stator when the master is turned off or the alternator breaker pops. In that case I believe the voltage transient goes negative but only on the voltage regulator (Jim W check me on this).

    Cars don't have these problems because there is no master relay, and t he the electronics are permanently connected to the battery. By far the easiest is to simply disconnect the electronics whenever there is a chance of the master relay chattering. There is no fuse as fast as a silicon transistor junction.
     
  17. denverpilot

    denverpilot Taxi to Parking

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    Couldn't speak for newer stuff, but on all my decade plus old cars, turning the starter key triggers a disconnect of the accessories like the stereo, audio amps (to avoid huge subwoofer thumps) and other "sensitive" stuff, via a relay that's kicked open during engine start.

    They're sequenced such that the relay is open before the starter solenoid is energized and when the key is released from the start position they're sequenced such that the starter stops and then the accessory relay is closed. If you turn the key really slowly you can see the sequencing.
     
  18. James331

    James331 Touchdown! Greaser!

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    Just find out who it is and let them know in writing that they will be held responsible for any avionics damage if they keep leaving the switch on.




    Lol, man guess I have been to any real airports, actually if it's a evac NOW type fire I'm not chit chatting on a radio.
     
  19. Ghery

    Ghery Final Approach

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    Well, I am that old. My first car was a 1954 Buick Special. Vacuum wipers. Vacuum operated windshield washers (you held down the button to fill the pump against a spring, then released to put water on the windshield). The starter was in the gas pedal. Turn the key to "ON", step on the gas and the engine started. A vacuum cutout prevented it from trying to run the starter motor when the engine was running. I watched a mechanic in the early 1970s darned near break off the ignition switch trying to start that thing until I leaned in the driver's window and told him how to start it. The car had power steering and it's wheel was huge, too. Oh, and until I put one in, it didn't have a radio.

    This was my great grandfather's last car. I got it from my great grandmother when I was in college, so I'm older than the car (not by much). Would you believe that it was 19 years old with 34,340 miles on the odometer when I got it? It wasn't driven much, and then typically by family when we would visit. It got serviced then, too. Papa wouldn't have approved if he had known. "Buicks are greased at the factory!" Yeah, right. :p