Zener diode in PIXHawk

   On the power connection page, it is suggested that we use zener diode for protection when using back up power supply. The connection suggested is to connect zener across the +5v and GND of the servo rails directly.

3691142342?profile=original

http://plane.ardupilot.com/wiki/common-pixhawk-wiring-and-quick-start/

 

   However when I read about zener diode, then the circuit to control voltage is suggested as follows.

 3691142409?profile=original

  Notice the resistor in series with the zener.  I understand that this resistor will consume the voltage which is excess to the required voltage. However in the connection suggested for PXHawk, there is no resistor.  In this case how is it able to regulate the voltage? My knowledge of electronics is limited, so I would appreciate an explanation of this.

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  • No, as I understand it the zener must be reversed.
    That's what it shows in the official http://ardupilot.org/plane/docs/common-powering-the-pixhawk.html#co...

    And I also researched elsewhere on the net as i wasn't sure about it.
    Powering the Pixhawk — Plane documentation
  • MR60

    The zener + should be connected to the servo rail +. 

    Chris said:

    Just to be clear. The zener diode and capacitor should be wired in parallel. Therefore the zener-capacitor combo are wired in reverse to the BEC correct?  I am using this youtube video as a reference where the guy wires the zener and capacitor to the BEC.

    https://www.youtube.com/watch?v=e6jDSCXQHAc

    Thanks

  • Just to be clear. The zener diode and capacitor should be wired in parallel. Therefore the zener-capacitor combo are wired in reverse to the BEC correct?  I am using this youtube video as a reference where the guy wires the zener and capacitor to the BEC.

    https://www.youtube.com/watch?v=e6jDSCXQHAc

    Thanks

  • MR60

    The objective of the recommended zener model is to trip (reverse avalanche) at 5.6V, in order to trim any short lived voltage spikes that are created by equipment connected on the servo rail.

    Therefore you should NOT use a 5V zener diode as it would trip all the time with a perfectly valid 5V power source connected on the servo rail.

    A zener diode that is connected correctly does not carry any current and therefore does NOT get hot at all.

    So, please stick to the recommendation of the wiki with a 5.6V, 5Watts Zener diode. NOT a 5V Zener diode.

    @Hein, Justin, you have understood correctly.

    • I feel like I need to jump in here and encourage everyone to not over think this zener thing.  Hugues has nailed this discussion perfectly and there is no need to go any further.  The single purpose of the 5.6v zener in question is to prevent a voltage spike from a secondary back up supply to the rails from triggering a shutdown of the APM.  If an external supply goes above 5.7 volts, that is what will happen, hence the zener clamp at 5.6v.  Nominally you would power the APM from a 5v power monitor board and for insurance, use an external UBEC at not greater than 5.3 volts attached to the output servo rail just in case your power module dies. That's all there is to it.  BTW, the banded end of the zener diode is the cathode and should be connected to the +5v side of the rail.  The unbanded end of the diode is the anode and should be connected to ground.  It is reversed biased in this case and conducts no current until the voltage exceeds the zener voltage and then it goes into avalanche and conducts current. 

       Happy Flying. 

      • Doug,

        Thank you for awesome explanation.

        I have a problem with the zener diode issue. I am using 5339 zener diode with a 10v 1000uf capacitor but I have noticed that when I want to use external usb adapter it does not power up the board while zener/capacitor plugged. If I unplug the zener diode/capacitor from the servo rail then external usb works. I have noticed that the LED on the external usb and my dragonlink receiver require too much current together and when I disable the dragonlink receiver then external usb works again (but this time zener plugged).

        Do you have any idea?
        Thank you

        Onur.

        • Are you running a secondary 5v supply (UBEC) to power the servo rail?  You would have to if your are going to power any other device from the servo rail.  Also, please verify that you have the zener diode in the correct polarity.  The end with the band on it should be connected to the + side of the servo rail.

            Doug

          • Doug,

            Yes, servo rail is supplied by 5v bec. (But you know while connecting the USB cable for downloading logs from the external USB adapter there is no power on the BEC).

            I have double checked the zener diode and capacitor polarity no problem with that.

            Tonight I found something. 2 servo wires are connecting my dragonlink UHF receiver with pixhawk board. One is PPM cable and the other is RSSI. When they are both connected on the receiver I cannot use the external usb adapter, but when I unplug RSSI cable then I can start the board from external usb. (does not change with/without zener)

            (Of course I have no issues with the normal battery startup and everything works.) 

            Should not be this much complicated...

            Thank you.

            Onur.

            • On the RSSI, use only a single wire from the DragonLink RSSI pin to the SBUS signal pin on the Pixhawk.
              • Iskess,
                I think I could not expressed myself well enough. Let me correct it.I use only one cable from pixhawk sbus to uhf rssi. The other cable I meant is from ppm to pixhawk rc in.
                Thank you.
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