Pixhawk 1 power redundancy

I have read in documentation that pixhawk can be power from servo rails as well.
But there is also a mention of

“The servo rail can supply servos requiring up to 10.5V (but not also power the Pixhawk). Voltages above 5V cannot be used to power the Pixhawk via the servo rail. In this case the Zener diode must not be used”.

How should the redundancy be disabled in this case ?

Exactly as it written.
5V on servo rail, plug in the zener and you have redundacy.
more than 5V on servo rail , do not plugin in the zener and you don’t have redundancy.
more than 5V on servo rail, plug in zener and you have a burnt out pixhawk for paperweight.

Trying to catch up…

Does it mean the servo rail voltage has to be more than 5.5 from power up ?
I am trying to look at a scenario where my esc is powering the servo rail but not concerned about the overshoots, but pixhawk shouldn’t taken in the servo rail supply.

By this way I am trying to mitigate the pixhawk reset problem which might arise because of possible overshoots.

Since I have only one battery, redundancy is not that important for now, but just want the pixhawk work through out and use power from esc for the 2 servos connected.

Could you please advise ?

Good day,
in my personal opinion better solder the Zener in the reverse biased condition on the capacitor… in this way it will work as power regulator.
So just use an ubec with a 5.30Vout and solder a capacitor with the Zener in the reverse biased condition and you will have 5.32v for power the servo rails

Thank you, but let me follow up on the same again,

How does the higher voltage work then (clear about the fact that zenor should not be used) ?
This is more from an understanding on how does pixhawk know if it is a redundant power or not ?

is there a software or hardware configuration to say to pixhawk that dont use the servo rail voltage as redundant source of power ?

there is no software for this.
You have to consider this… if you will overvoltage your board you will damage the voltage regulator inside the board.
if you want power directly the servo rail using an ubec, in case of malformations of the ubec it will spike an overvoltage and you will will damage it.

The higher servo rail voltage, above 5.7v , will never be used for redundancy, nothing needs to be configured, nothing will be damaged.
The integrated power selector IC will simply ignore that source.

Sorry for the long pause in between.

Could you please let me know if the below mentioned scenarios are correct then ?

pixhawk supply rail powered up with more than 5.7 before the primary - no redundancy but pixhawk executes the code once primary supply is available

pixhawk is powered from primary and then connected more than 5.7 v to the rails (with out Zener) - resets once

pixhawk is powered up from primary and then connected exact 5v to rails, then increased the supply to more than 5.7 (with out zener) - resets once


pixhawk 1 may be on a 2.4.8 clone. This Chinese driver rather has only a diode with a fuse in the direction of the power supply. So connecting a higher voltage than 5.7v on the servo bus will rather heat the driver. that it would not exceed 5.7vi and would not damage the driver. In the original pixhawk (not 2.4.8) there is a power driver that will probably allow more than 6v to be delivered to the rail, but not the fc power supply.

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Thanks for that input @MarcK, this helps. I am having the 2.4.8 clone. Now wondering if this ever will be a problem if I connect the esc power to the servo rails (5v measured from voltmeter) with out encountering a reboot in flight

There is an obvious solution which I perfectly agree, by adding a zener, but wondering should it be must in this case


the zener diode turns on the servo bus so that if high voltage is induced in the motor, it does not get to fc

Thank you @MarcK, I am risking even though its 2.4.8 with out the zener is what I am getting as a feeling. Thank you all for helping me here.

I have a second power supply connected to the servo rail as a backup directly. Dgy you don’t use the servos, you can do that. In addition, in the 2.4.8 controls, I cut the diode and the fuse from the supply side and thanks to this I have a stable 5v, previously it was floating … if you also wanted to do so, you can connect the main power supply not to the power port but to the 5v line on the telemetry socket. stress …