Pixhawk and RX 5V supply observations/questions

I am setting up an octa, having just replaced my APM with a pixhawk.

As the max current will be over 90A I have made up an attopilot 150A system and a 5V BEC, all wired into the power port. This is working fine.

 

The Receiver (taranis X8R) is plugged in the RC (s-bus in) port and is powered from this connector.

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Contrary to this diagram, there is no continuity between the RC +5v and the other +5v pins in that rail but there is about 4.7V present on the RC middle pin. There is no voltage on the other +5v pins unless a ESC etc is plugged in.

Also (as documented) any 5v on the other (not RC) power pins will power the pixhawk, giving redundancy if the top power connector is also supplied. Any 5v (input)  on the RC pin will NOT power the pixhawk.

 

Finally Im getting to the point.

I am running a small servo and other devices directly from the X8R receiver, therefore these are powered from the pixhawk. They are not drawing much current BUT they easily drag down the voltage to 3v or less.

I have plugged in another BEC to boost the RX supply but am not sure what effect this will have on the pixhawk RC pin output. I may remove the 5v wire from the S-Bus lead and just use a 5v bec for the RX (and also the pixhawk servo rail).

 

Any thoughts or info on the current rating of the pixhawk RC +5v output would be welcome.

 

Also I cant find any info on the small amber b/y led or the blue one.

 

Thanks.

 

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Replies

  • I wonder why the first pins (usially Ailerons) are different from the others?
  • I don't have a Pixhawk yet, so let me start by saying I might not know what I'm talking about.
    In my opinion you should not power the Rx by two sources. If you want to use the Rx to power servos then you must remove the red wire on the PPM cable. Now your Pixhawk will not have any motors on its power supply. There is a chance a servo could lower the Rx voltage but it should've much more resiliant and an Rx brownout probably won't bring down an aircraft. The other option is to split the servo wires and only plug the signal wire into the Rx, and plug the servo power leads into the output bus with the other servos. I think this is a better option.
    Sorry to hear the Pixhawk power redundancy feature isn't working yet.
    • OK thanks for that. I have it sorted now.

      Just be be clear, the above diagram implies the +5v rails go right across all ports BUT the port 1 (RC) is different. Its the only one that gives 5v out. The pixhawk will run if 5v is applied to all ports except the RC one.

      I have 2   5v ubecs. One is used on the top power port and the other is powering the RX and the pixhawk servo rail providing redundancy.

      • I was thinking about this problem too.

        Did you do this because you need more than 8 servos and you were not able to bridge sbus to the AUX 1 and 2 of the pixhawk?

        Indeed, if I have to hang servos off the receiver (I'm guessing you managed to reprogram your receiver so that its own pins do 9 to 16, correct?), I'd cut the 5V line from the RC port and send 5V from one of the servo ports instead.

        Woud that work?

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