Acceptable level of noise on the servo +5V rail?

We are having some odd intermittent problems on our fleet of RVJet/Pixhawks so I took a good look at the +5V servo power rail with my scope to see if that was the problem.  I was quite surprised to see power spikes as high at 400mV when the servos were thrown.  The ESC also puts some noise on the rail of about the same magnitude, even if the internal BEC is not used.   Here's a shot of the scope from one spike (100mV vertical division, 200us horizontal).


Some of the planes had a separate high quality Castle Creations BEC and some used the BEC built in to the speed controller.  Some planes had high quality servos (HiTec or Airtronics) and some had Range Video house brand.  No matter what hardware configuration the noise levels were about the same.  

I put a 680uF electrolytic cap across the +5/GND of the servo power rail and the noise levels dropped dramatically.  The noise from the servos was now down to less than 100 mV and the motor put slightly more noise on the rail.  These noise spikes were very infrequent; I had the scope on single shot mode and often it took about 30 seconds of constant servo movement to generate one.  

So I have a few questions:

1.  Is there a consensus on how much noise is acceptable on the servo rail power bus?

2.  Is a large bypass cap the best way to deal with it?  I don't think a zener diode would work this close to the setpoint voltage.

3.  Could the 400mV noise spikes been the source of odd, but very intermittent, behavior from the Pixhawk?  For example, we've had a couple of our planes porpoise wildly on occasioin.  Sometimes our airspeed indicator rapidly jumps to very high settings when the plane is on the ground (e.g. 12 m/s).  And we find we have to do frequent power-down reboots to get the system working properly.

Thanks in advance for any tips!

Tom McKinnon

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  • MR60

    Hi Tom,

    If you want to use both your ESC-BEC and a separate BEC to power the pixhawk servo rail (being itself a backup of the PM source to power pixhawk), then you can use one Rectifier Shotky diode : MBR1545CT as described very easily in this post:

    The MBR1545 will select and output the higher of the two input voltages. So let's say for example that your ESC-BEC produces 5.3V and your separate servo BEC produces 5.5V, the MBR1545 will select 5.5V on its ouput. If one of your two BEC fails , the remaining one will be aitomatically selected by MBR 1545.

    If one fails "high" and produces a high voltage, then the zener diode you added on the pixhawk servo rail will trim it down to 5.6V.

    And if you add a cap in parallel to your zener diode, it will smooth ripples.

    The big advantage of this MBR1545 is that it is easy to mount in a compact and light way as shown here on mine:


    • Hughes,

      Thanks!  It always seemed like a waste not to use the integrated BEC as a backup but I wasn't 100% sure how to configure the diodes.  This setup is just the trick.



  • My leads are about 6cm. I can shorten then to about 3cm if it makes such a difference. The fuselage constraints prevent me from attaching directly to the rail.
  • Developer

    If you are using the PM and the servo rail as a backup, you need to keep the servo rail below 5.7V.  If you are attaching 5V servos you must use a zener as the noise levels from the servos will exceed 5.7V.  Adding a cap is a good idea too.  If you are connecting >5V servos they cannot be used as a backup FMU power supply.  The IO portion of the board will continue to run up to abt 10V and then it will shut off as well.  At some point in the future we will write some code to take advantage of having the IO remain active in a failsafe condition but at the moment we just have the hardware in place to allow a future feature.

    Basically if you are going to use the servo rail as a backup power supply you better make sure it is a reliable supply.  Back in the days of the APM it was common to use the servo rail as the power supply but it turned out that was a really bad idea as the servos are really too noisy to not cause problems.

    Here is the scenario, if you are only using the PM then the PH will continue to use it as it fails (or under voltages) to just over 3.3V.  If you have a voltage on the servo rail then and you have noise or an undervoltage event on the PM input, the PH will instantly switch to the backup supply. If that supply is not valid then it will shut down the PH to protect itself.


    • Craig,

      Thanks for the detailed explanation.  A few questions:

      1.  Do you ever consider the built-in BEC on an ESC to be a reliable supply?  Or is the environment of the ESC (heat, etc) just a bad place to put a mission-critical piece of equipment?

      2.  If using a separate BEC, does it make any sense to use the built-in one as a backup?  The Castle Creations BEC is programmable so it could be set at 5.3V or so, then a blocking diode could be put on the built-in BEC (often hardwired at 5V) so it would sleep until there was a dropout of the main one.  Or is this just too much trouble for the value?


    • Developer

      1) it is probably not as reliable as the PM.  It is almost certainly noisier.

      >>>  If using a separate BEC, does it make any sense to use the built-in one as a backup?

      I'm not sure what you mean

    • ..

    • Ok, I found a 1000uF 10v Low ESR capacitor and soldered like in the photo. How's this look?

    • Developer

      I can't really comment on the size of the cap.  If you have 1000uF, just make sure the power supply can source the current to charge it.

      Also you need to keep the cap as close to the Pixhawk as possible. you could use another servo connector.

    • Yes, I found that the wire length is very important.  First I used a large cap with long leads (15 cm or so) but I found that a 680 uF/10V with leads about 2 cm did a better job.

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