Transponder Power Supply

johnf_1

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Hi: I am installing an HDX800 with SV-261 transponder in my ancient RV4. I am keeping my old Rochester piper engine gauges, so I am not relying on SV for engine instrumentation and specifically oil pressure info on start up. The master switch and starter solenoid are difficult to access. For 34 years, I have switched all the avionics on after start up and off before shut down. I see the installation note to add diodes to the contactors. I would like to omit the diodes until I venture into the solenoid area when access will be easier. That may a year or so. In any event, I intend to continue my practice and to shut the HDX off before shut down and on after start up, so I see no risk of spiking it, precisely as I have with my avionics for these many years without any trouble. That said, the transponder has its own unswitched power supply cable that may or may not be affected by whether the HDX is off or on. I have two questions: does shutting off the HDX also shut off the transponder and isolate it from a start/stop spike? If not, is the transponder affected by a start/stop spike in any event? If the transponder is at risk, then I will switch the power supply to avoid a spike, but it would be easiest if the HDX power switch functioned as an avionics master at least for the transponder. Thanks in advance for your time and comment. John N95JF
 

Raymo

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Power spikes are not a problem for modern avionics. Modern is pretty much anything since 2000ish. No need to turn anything off for engine start or shutdown. Do take advantage of the Dynon EMS for engine monitoring. The audible warnings are fantastic and let you know when something is wrong without having to scan the instruments. All my avionics are on with the master, which is the first thing on when I sit down so the EFIS has time to power up. I have backup batteries connected to both screens so they stay on when voltage goes down during engine start.
 

johnf_1

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Power spikes are not a problem for modern avionics. Modern is pretty much anything since 2000ish. No need to turn anything off for engine start or shutdown. Do take advantage of the Dynon EMS for engine monitoring. The audible warnings are fantastic and let you know when something is wrong without having to scan the instruments. All my avionics are on with the master, which is the first thing on when I sit down so the EFIS has time to power up. I have backup batteries connected to both screens so they stay on when voltage goes down during engine start.
Thanks...so, why do you think there is a requirement for the diodes? My understanding is that the diodes dissipate the spike, and they are required per the installation documents. I am hoping to end run that requirement by simply disconnecting the boxes from the system, but not sure that will be accomplished by turning off the display. J
 

Raymo

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John,

Can you point me to the area of the manual that requires a diode? From my experience, they (Schottky diodes) are only used to separate dual electrical systems, which is not part of the Dynon installation documentation.
 

Raymo

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Thanks, John. I didn't realize your were talking about the master contactor/relay diode. Most come with them installed (as did mine) so it didn't trigger in my head. Turning off the HDx will not turn off the XPNDR, to my knowledge, but you could pull the breaker prior to starting until the diode is installed. You can purchase a contactor w/diode from BandC.aero.
 

johnf_1

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Thanks. When I get to that, I would be inclined to make up my own 50 cent diode...but there still is the issue of whether the transponder is switched by the HDX power switch. It would be great if there is a Dynon monitor who could chime in...??!! Thanks J
 

RV8JD

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...but there still is the issue of whether the transponder is switched by the HDX power switch. It would be great if there is a Dynon monitor who could chime in...??!! Thanks J

Not a Dynon Moderator but ... Apparently it is not. See pages 11-2 and 11-3 of the SkyView HDX Pilot's User Guide (reproduced below):

"The following SkyView HDX system units are powered by a SkyView HDX display / SV-BAT-320 and thus will remain functional if Avionics power is lost during flight:
  • SkyView HDX display.
  • SV-GPS-250/2020 GPS Receiver (MAP functions will be normal).
  • SV-ADAHRS-20X ADAHRS Module (PFD functions).
  • SV-EMS-220/221 Engine Monitor Module (ENGINE functions will be normal):
    o Includesenginesensorsthatrequirepowertooperate.
  • SkyView SV-ARINC-429 ARINC-429 Module.*
  • SV-KNOB-PANEL Knob Panel.
  • SkyView Video Input Adapter.*
  • SkyView Wi-Fi Adapter.*
    *Although this unit is powered by the SkyView HDX display and will thus function during the loss of Avionics power, it is typically connected to units that require Avionics power, and thus that subsystem may not function when Avionics power is lost.

The following SkyView HDX system units are powered by Avionics power, and thus will not function / be powered by an SV-BAT-320 Backup Battery if Avionics power is lost:
  • SV-XPNDR-261/262 Transponder (transponder non-functional).
  • SV-ADSB-470/472 ADS-B Receiver (ADS-B information cannot be displayed).
  • SV-COM-C25/X25/X83 COM Radio (COM radio is non-functional).
  • SV-AP-PANEL Autopilot Control Panel (AP is non-functional).
  • Dynon Avionics Servos (AP is non-functional).
  • Heated AOA/Pitot Probe (AOA / Pitot Probe will not be heated)."
 

johnf_1

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Not a Dynon Moderator but ... Apparently it is not. See pages 11-2 and 11-3 of the SkyView HDX Pilot's User Guide (reproduced below):

"The following SkyView HDX system units are powered by a SkyView HDX display / SV-BAT-320 and thus will remain functional if Avionics power is lost during flight:
  • SkyView HDX display.
  • SV-GPS-250/2020 GPS Receiver (MAP functions will be normal).
  • SV-ADAHRS-20X ADAHRS Module (PFD functions).
  • SV-EMS-220/221 Engine Monitor Module (ENGINE functions will be normal):
    o Includesenginesensorsthatrequirepowertooperate.
  • SkyView SV-ARINC-429 ARINC-429 Module.*
  • SV-KNOB-PANEL Knob Panel.
  • SkyView Video Input Adapter.*
  • SkyView Wi-Fi Adapter.*
    *Although this unit is powered by the SkyView HDX display and will thus function during the loss of Avionics power, it is typically connected to units that require Avionics power, and thus that subsystem may not function when Avionics power is lost.

The following SkyView HDX system units are powered by Avionics power, and thus will not function / be powered by an SV-BAT-320 Backup Battery if Avionics power is lost:
  • SV-XPNDR-261/262 Transponder (transponder non-functional).
  • SV-ADSB-470/472 ADS-B Receiver (ADS-B information cannot be displayed).
  • SV-COM-C25/X25/X83 COM Radio (COM radio is non-functional).
  • SV-AP-PANEL Autopilot Control Panel (AP is non-functional).
  • Dynon Avionics Servos (AP is non-functional).
  • Heated AOA/Pitot Probe (AOA / Pitot Probe will not be heated)."
Thank you very much. It still is not conclusive: it could be true that the transponder power supply (avionics power) could function, but that turning the display on and off would switch a transponder relay to avoid a situation where the display is not turned on and the transponder remains powered. Stated more clearly, the Avionics power can be on and powering the transponder, but the display could be turned off, so the transponder is humming away without any control on the display...that seems undesirable, and I would think I would design around that (by switching the transponder with the display as I had hoped!). I guess I could just plug it all in and see what happens...I gather you are a lawyer with an 8?
 

Raymo

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Just install an avionics master switch to control both the EFIS and XPNDR (and radios). You would not want the display to control XPNDR power in a case where the EFIS failed or needed to be powered off during flight to save the main battery if the alternator failed.
 

johnf_1

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Thanks for the comments. And my solution is an avionics master as you suggest, so all good! J
 
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