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Thread: Strange Ammeter Readings

  1. #1
    Senior Member jiott's Avatar
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    Default Strange Ammeter Readings

    I have a Dynon Skyview in my Kitfox with 912ULS engine. I use the Dynon EMS (engine management system) module with all Dynon supplied engine sensors for the Rotax engine. The amps sensor is a Dynon supplied shunt feeding into the EMS module. My amp readings on my Skyview engine page are always normal and consistent EXCEPT for one situation: When I press the push-to-talk button to transmit on my Garmin SL-40 radio, the amp readout goes wild showing negative values winding up to 40-50 amps! Of course these values can't be real and I have ignored them for 2 years now. It has done this since day one, and the readings immediately go back to normal after I quit transmitting on the radio. No breakers ever trip and no smoke or damage of any kind has occurred.

    I submitted this "problem" to the Dynon tech support people and they think it is due to the RF energy from the radio interfering with the very sensitive ammeter sensor wires (unshielded). The sensor uses only a few millivolts/amp. My radio antenna is mounted on the bottom of the fuse and I use the hi quality RG400 coax cable from radio to antenna. This cable does run within about 6" of my EMS module. Dynon suggests I try moving the antenna cable routing farther away, but I am hesitating because it is not an easy job and that cable is very well shielded.

    So here is the question for you electrical gurus out there: Have any of you heard of this before or known anyone who had to deal with it? What did they do, or what would you do. I am very strongly inclined to continue to ignore it as I have been for 2 years. Do you see any potential problem with doing that?
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  2. #2
    Super Moderator Av8r3400's Avatar
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    Default Re: Strange Ammeter Readings

    Do your push to talk leads pass near the shunt?
    Does the antenna lead pass near the shunt?

    What does pass near the shunt for wiring?
    Av8r3400
    Kitfox Model IV
    The Mangy Fox
    912UL 105hp Zipper
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  3. #3
    Senior Member Av8r_Sed's Avatar
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    Default Re: Strange Ammeter Readings

    This could be a good application for a snap-on ferrite bead located on the sense lines near the shunt. They kill RF while letting low frequency signals pass. image.jpg
    -- Paul S
    Model III SN910
    582 IVO Med

  4. #4
    Senior Member jiott's Avatar
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    Default Re: Strange Ammeter Readings

    The PTT leads do not pass anywhere near the shunt. The radio antenna cable also does not pass near the shunt. The items/wiring that are near the shunt are: voltage regulator, transponder and its antenna cable, capacitor.

    Where do you get the ferrite beads/what brand are they-looks interesting.
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  5. #5
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    Default Re: Strange Ammeter Readings

    G`morning Paul and others

    I have Vixen 912UL. All older round gauges. A210 radio and Terra transponder. Avshop after market Lightspeed headsets.

    Occasionally I get feedback when on transmit to the point that radio is unusable. I try an old DC and all`s well. Then go back to my Lightspeed and all`s well. When flying last week with a friend and 5 headsets, kept changing them around and everything was fine. (the Bose won, hands down!!). Anyway just wondering if these "ferrets" would help this situation and if so does anyone have an idea as to what type and where I would place them.

    Radios and headsets are a total mystery to me and there appears to be quite a bit of knowledge and experience on this forum.

    Right now everything is just fine but you never know "it just might start squealing" again. May not happen til next year.

    Thanks
    Don

  6. #6
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    Default Re: Strange Ammeter Readings

    That should "ferrite"

  7. #7
    Senior Member Av8r_Sed's Avatar
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    Default Re: Strange Ammeter Readings

    Ferrite cores are carried by the major electronics distributors such as Mouser and Digi-Key. You can get split cores that can be snapped around an existing cable / conductor or plain cores where you have to thread the conductor through them. Effectiveness can be increased by making multiple turns of the conductor through the core. You probably want to select one that's effective around common com frequencies like 122.8 MHz. Make sure the hole in the ferrite will accommodate the conductors you want to pass through it. For the ammeter application, you might want to pass both sense leads from the shunt through the ferrite.

    You'll have to sort out minimum order and shipping prices.

    Here's a link to one datasheet: http://www.mouser.com/ds/2/212/KEM_E...-SR-372794.pdf

    Note: I don't have any noise issues on my Model III and haven't used these in aircraft service.
    -- Paul S
    Model III SN910
    582 IVO Med

  8. #8

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    Default Re: Strange Ammeter Readings

    I agree with Paul's advise. Another couple ideas you could try would be to twist the wires together that go to the PTT switch. Twisting together allows cancellation of magnetic flux generated by the collapse of the field. Another idea would be to place a schottky diode across the PTT switch which would allow any field collapse to be dispersed within the wiring.
    Kurt A

    Kitfox II,
    Rotax 912,
    1100 gross
    Fixer Upper Project

  9. #9
    Senior Member jiott's Avatar
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    Default Re: Strange Ammeter Readings

    Kurt, my problem occurs from the time I push the PTT switch until I release it. As long as I hold the switch my ammeter readings go wild. Your suggestion would seem to apply only if the problem occurs momentarily when the PTT switch is pushed or released. Do you agree or still stand by your suggestions?
    Jim Ott
    Portland, OR
    Kitfox SS7 flying
    Rotax 912ULS

  10. #10

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    Default Re: Strange Ammeter Readings

    Jim,
    I did indeed get the wrong impression from your first post. The suggestions I gave would solve only for wild amp meter readings during opening or closing of the PTT switch. Not while holding the switch depressed.
    Kurt A

    Kitfox II,
    Rotax 912,
    1100 gross
    Fixer Upper Project

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