Pixhawk power supply questions on Airplane

Guys,

Was installing the new Pixhawk on a plane and was using the 3DR power module to power the Pixhawk. I also have an ESC (w/BEC) and 4 servos to drive around. Now i noted that without the ESC connected the Servo rails were powered with the 3DR Power module. This leads me to 2 questions : 

A. If i connect the ESC to the Servo rails, will the PMU power not conflict with the BEC and damage either the BEC or PMU ? Or are these diode protected to avoid reversal of current ? 

B> Pixhawk advertises that it has redundant power supply. However I am not sure how this helps. If my ESC's BEC dies out. Then the servos will be driven by the PMU power supply, which is clearly indicated that it should NOT be used since the PMU is not intended to drive the servos.

In my earlier APM 2.5 and 2.6 installs the power setup was explained very clearly and I was able to separate out the servo rail power from the main board power .

Not sure how things are wired here and what would be the best way to install the PMU and ESC/BEC power.

Please help

Thanks 

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Replies

  • So does this mean that the ESC with UBEC is connected to the servo output is enough to power the servos like for APM 2.5?

    • Well that depends on your servos, but yes, it is like the APM 2.x in that regard. The difference with the Pixhawk is that if the PM fails, it can take power from the servo rail as an emergency backup.

      However, in this scenario the servos and the Pixhawk will be sharing the same power source which we all know is a bad thing (see Tridge's post above). This is why the wiki tells you to put a diode on the servo rail. I'm still uncomfortable with this however.

      I think I will power the servos off a totally separate buss, with its own UBEC, it could even be 6v. I'll attach the ESC BEC to to the Pixhawk outputs to serve solely as a backup emergency 5v source in case I loose power from the PM.

      Does anyone have any thoughts on my plan? Please poke holes in it!
      • MR60

        Should be OK but make sure you have the same ground reference between your servo power source and pixhwak outputs.

  • I am doing the same thing!

    I have experimented a little over the past few days. Andrew is right, the PM does not give sufficient power for the servos. I managed to use the power from my sbec (3A max) to power the servo rail but on reading info on Pixhawk I realised that I could use a spare UBEC to omit the PM. This is  5A (7.5A peak) rated and powers both the servo rail and the Pixhawk (I use a simple 2 wire adapter to stop the sbec from supplying to the rail). I had to switch jumpers on the ubec so that it only supplied 5V rather than 6V, if you supply above 5.7V to the rail it will not power the Pixhawk ( a curious decision I do not fully understand).

    There is no redundancy here but as you say the PM cannot power the servos in any case, so there seems to be little point in having the PM as well.

    Cheers

    Darrel

    • Developer

      I have experimented a little over the past few days. Andrew is right, the PM does not give sufficient power for the servos

      It doesn't give any power to the servos at all.

      This is  5A (7.5A peak) rated and powers both the servo rail and the Pixhawk

      That is not safe - the problem is that servos can cause large spikes in voltage during flight. This could cause the servo rail to go outside its safe limits, which could cause the Pixhawk to reset. If you fly with only servo power your Pixhawk may reboot mid-flight causing a crash.

      I had to switch jumpers on the ubec so that it only supplied 5V rather than 6V, if you supply above 5.7V to the rail it will not power the Pixhawk ( a curious decision I do not fully understand).

      The reason for this design is that peripherals like the GPS are powered directly by this supply. If it went over 5.7V you could damage your GPS.

      There is no redundancy here but as you say the PM cannot power the servos in any case, so there seems to be little point in having the PM as well.

      The PM module is vital. You really are risking a bad crash if you don't use it. I've looked at a lot of traces with an oscilloscope of what happens to the servo rail for rapidly moving servos, even with very high current BECs (20A or more) and it is quite horrible. Getting spikes of up to 11V is not unknown.

      Cheers, Tridge

      • Dear Tridge,

        Many thanks for that explanation, which makes total sense.  I am new to this and struggling to get things to work due to generally poor documentation (I'm sure most of the information is out there but it is chaotically organised).

        I started my first thread a few days ago (to which no one replied) when I was puzzled by the 'no response' from my servos on the output rail.  There is nothing in the 3DR quickstart guide to indicate additional power is needed to the servo rail.  The servos did respond when I plugged in my ESC (sbec) - I guess I was a little stupid not to put two and two together at that point. It took me another day to realise this was a power issue which I could resolve also by supplying power from a separate UBEC. I then simply unplugged the PM and to my surprise everything still worked!

        I appreciate that it is reasonable to expect a new user to read the documentation (I am assuming this information is buried somewhere and may be I am just not good enough with my searches), but I do beleive that 3DR need to do better with the guide.

        Thank you once again, you have probably saved me from an unnecessary crash. I will put the PM back in -although I already need to repair one of the wires which stripped[caught by a fingernail] as I removed the connector :( 

        Best wishes

        Darrel

  • Developer

    Now i noted that without the ESC connected the Servo rails were powered with the 3DR Power module

    You need to run that test again. The 3DR brick connected to the PM connector on the Pixhawk does not power the servo rail. Only the receiver connector is powered.

    Cheers, Tridge

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