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.



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


  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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Added an issue about it to ardupilot wiki 


Turns out the zener diode SHOULD be installed in reverse polarity. If it's installed with it's negative pole to the negative of the rail and + to +, it goes up in smoke within 10 seconds.

I now have it back the way I had it and it seems to be OK. Perhaps I had a bad diode that got hot.

Where in the spec sheet does it say it should not go over 5v? Would explain why mine gets hot, but according to the table, the nominal voltage is 5.6v.

I am fairly sure that the intent is not to exceed 5V input voltage when using the1N5339 Zenner. Anything over that and with measurement errors and Zenner tolerance, you are getting quite close to things getting hot :)


For tolerance of the zenner, See the "Electrical Characteristics" section on Page 2

I though the idea of the zenner was to limit voltage to the 5v pixhawk rail by not allowing voltages above 5.7~ volts. A 5v zenner would block valid voltages of 5v to the rail from a 5v bec.

I am confused about the zenner recommended though. It is a zenner in reverse?

I plan on testing all this on the bench before adding to the pixhawk. Have ordered the 1n5339 but have not recieved it yet. The 1n5339 is the recommended diode on the official site and it is 5.7v.

previous comment edited for clarity.

A Zenner diode is intended to be used "upside down" compared to a regular diode.


Note: In previous posts in this thread clearly my spelling was wrong.

Should be Zener not Zenner !

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. 


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


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.



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.


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