Cutting the (+) wire on ESC Pixhawk data leads...is anyone doing this?

Posted by Glenn Hollowell on February 6, 2018 at 10:12pm

Recently I've come across the suggestion that when you are using the Pixhawk power module, you should snip the (+) wire (middle one) that connects the ESC to the Pixhawk unless you are using a Zener diode in the line to prevent over-voltage. I've got 2 quads with Pixhawks, both have the standard 3DR power module, and both have the (+) wire in the small 3-wire bundle from the ESC unsnipped. I am interested in people's thoughts regarding the merits and risks of leaving this wire unsnipped. Thanks in advance.

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  • UAV Skies February 16, 2018 at 4:01pm

    Your welcome. I wish people explain things in detail so others can benefit from it.

    My pet peeve is always piece part info :)

    CORRECTION: "When you are looking the Pixhawk facing towards you where all the connection pins are, all the TOP pins are ground and all the middle pins are supposed to be +5V.

    Hein du Plessis said:

    Wow, thanks for this detailed explanation!

    Cutting the (+) wire on ESC Pixhawk data leads...is anyone doing this?
    Recently I've come across the suggestion that when you are using the Pixhawk power module, you should snip the (+) wire (middle one) that connects th…
  • 100KM
    Hein du Plessis February 15, 2018 at 11:14pm

    Wow, thanks for this detailed explanation!

  • UAV Skies February 7, 2018 at 11:47pm

    First, you have to look at the schematic of the PixHawk and get a better understanding of how the power design is done. I am assuming you have a basic electronics understanding.

    3702414540?profile=original

    When you are looking the Pixhawk facing towards you where all the connection pins are, all the bottom pins are ground and all the middle pins are supposed to be +5V.

    If you look at the schematic above, there are two diodes inside the Pixhawk connected back to back on the +5V rail. Now the question remains what do they do?

    Here is a fun experiment you can do:

    1. Pick any Pixhawk and power it using, for example, your laptop USB port. Now measure the voltage between GND and +5V pin (Any bottom or middle pin). Write down the voltage. It will be somewhere between 4.79 volts because USB is really not a powerful voltage source. Unplug the USB cable.

    2. Next use the PixHawk standard voltage regulator (2S to 4S) one. Plug the power connector to the Pixhawk. Once it's powered up, measure the voltage between +5V and GND. Write it down, it will be roughly 5.XX volts. Unplug the battery.

    3. Now use a standard RX battery (Not Lipo) and connect it to +5V and GND. This will also power up the PixHawk.

    Confusing right? In step 1 and 2, we were able to measure the voltage from +5V pin and GND, that means those pins were acting as +5V volt voltage SOURCE, not as inputs. Now the question remains how the pins which were acting as voltage source all of sudden became a voltage input source?

    This is where these back to back diodes come into play. If you look at the schematic above (Upper Left corner), When the system receives the voltage from PixHawk voltage regulator (U1001), it provides sufficient power to Pixhawk that makes the back to back diode to block the voltage from coming in on +5V and GND leads, instead it flows the current to devices that are connected to those ports if in need for power source.


    Now in a case, if your Lipo battery dies, and the voltage inside PixHawk drops to the threshold of the voltage regulator, then the +5V pin can act as the secondary power source for PixHawk.


    I hope this helps.


    Next thing to look at is ESC's. ESC is directly wired to the LiPO batteries. This means the red lead on the ESC's is not an input +5V, is actually an output source. Now the question remains what kind of voltage is coming out of the ESC?


    Well, it is always going to be close to 5 volts, however, you need to know if the ESC you have a is a Linear ESC or Switched ESC?. if it's a Linear ESC, the 5V coming out of those ESC can be connected in parallel as someone mentioned before AS LONG AS ALL ESC ARE the same manufacturer and type (to be safe). If they are switched ESC, you do not want to connect the 5V source from switched ESC's together. The reason is that switched ESC's do not provide a consistent source and will introduce electrical noise to your power rail on the PixHawk which is the last thing you want.


    So long story short :), you can remove the red wire as long as you don't care about a secondary voltage source at the bus bar pins, and simply want to power the PixHawk using the Pixhawk voltage regulator.

  • Clifton Hipsher February 7, 2018 at 4:29am

    It also depends on the ESC.  True optically isolated ESCs MUST have the + lead to supply power to the opto isolators.  These ESCs are very rare.

    For "regular" ESCs you need not worry about ESCs that have a LINEAR BEC.  If the ESCs have Switching or "Switch mode" BECs, you need to "snip" the + lead.  The reason for this is when 2 or more switching regulators are placed in parallel, the voltage sensing circuits that control voltage regulation will "fight" with each other for control.  This causes noise and eddy currents that waste power and can affect sensors and other digital electronics.  Further more, the PM uses a switch mode regulator...

  • 100KM
    Hein du Plessis February 7, 2018 at 12:55am

    For ESCs on multirotors, I don't remove the +, it serves as a nice smooth backup power supply to the PH.

    For planes, you'll have to go the zener route to protect the PH from the servo's voltage spikes. I keep the servo + on a separate rail sometimes.

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