Pixhawk 2.1 power questions

So while on the bench im noticing that the pixhawk seems to send power to the servos even when just hooked up to the usb? Also, im running HV to the servo rail for my servos, and the power module that came with pixhawk for the unit. Now no matter what i have hooked up, HV to the receiver, power to the Pixhawk power module or USB everything gets powered. I thought the servo rail was seperate from the Pixhawk power. Also i have some concerns that it’s trying to run 4 power hungry HV servos off the USB port on my computer.

Mine won’t power servos unless a bec is connected to servo rail.
How do you connect receiver?
Pixhawk 2 will provide power to receiver without power on servo rail.

The receiver goes from the SBUS out on the X8R to the RC in on the pixhawk. I have 2s power running into the receiver then to the servo rail for the servos. Is it back feeding through the RC in to the SBUS port on the X8R i wonder?

Try removing 2s from receiver first and see.

Then it is like you apply power to the servo rail. This is why you see power on the servo rail. Beware of your receiver max amp capacity as the servos might draw quite a bunch. It would probably be better that your connect your battery to the servo rail of Pixhawk directly. The receiver will get its power directly from the RC in port. In all cases, ground must be common between all of your power sources.

Yeah, i remove the 2s first, and the servos keep running, thats what worrys me. :confused: And yeah, i intend on powering the rail directly, it was just a convience thing to run a lead to the Pixhawk while bench testing. Not going to fly that way, ill make up a heavy gauge dual lead ec3 to two servo plugs like i do all my helis.

I don’t have any idea.

If I were you I would take a look at servo bus voltage reading, it is under status tab below HUD in MP, the numbers are in millivolt. See if servo bus voltage corresponds to USB voltage and also what happens when you connect 2S.

Its just weird l, after reading the tech specs i assumed the Pixhawk 2.1 only used the servo rail to keep the imu active if the input was under 10V. I totally dont get why its powering the servo rail with only usb hooked up? I have watched the voltage readout and it indeed does go down after i remove 2s from the servo rail. :confused: stumped… ill look over everything again later and see if i missed something. I do know all the grounds are common, that is for sure.

IIRC there are about 3 voltage readings.
When you plug in 2S, does all readings go up?
When you plug in USB only, what is the servo bus reading?

Hi Tim,

After reading the Pixhawk2.1 specs you can download from hex.aero, i think there is no contradiction between what you observe and the way it is supposed to work. Indeed, you can’t power the Pixhawk2.1 (FMU) via the servo rail as an input, but a USB power source will power peripherals (as an output) up to 250mA; and “peripherals” includes the PWM servo outputs.


I read the following in the documentation:

Servo Power
Pixhawk V2 supports both standard (5V) and high-voltage (up to 10V) servo power with some restrictions.
IO will accept power from the servo connector up to 10V. This allows IO to failover to servo power in all cases if the main power supply is lost or interrupted.
FMU and peripherals will NO LONGER accept power from the servo connector

I understand that

  • we can no longer use the power rail as backup Vcc for the Pixhawk 2.1.
  • the IO board (separate from the FMU board) WILL use the servo rail as backup power.

However, I don’t understand how the second bullet is helpful: how can the IO work separately when the FMU is in black-out. Perhaps it works as a direct pass-through in those cases?

Yes Georacer, in case the main FMU is not powered anymore, the idea is that you can at least keep a manual RC control on your ship with the IO being powered. In such situation you loose the autopilot function, you’re purely manual.

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I read thorugh those spec sheets as well, and hadent caught that it would also back power the servo rail off USB. Thats clearly what its doing, and i guess they can be considered “peripherals”, but in my particular case, since they are 4 very power hungry servos involved, i think there is cause for worry… Under that scenario, the servos noticibly change their tone when being powered off the usb, so its cause for worry.

Yes, I also consider it bad design for the servo rail to be powered off of USB.

I have yet to read the documents in detail, to find out what happens when you have high-voltage servo (>=6V) supply at the same time with USB voltage.

If I understand it correctly, and verified from my testing.
If you have 2S lipo going to the rail even with USB hooked up on the desk, it reverts to using the lipo “8V”. The issue is when you unplug your lipo and it swaps to using the USB power, servos “do not sound happy”.
-I’ve gotten into disconnecting the USB first before unplugging my receiver pack for the servos.

USB doesn’t power servo rail on mine as well as power module, only BEC plugged in any of servo rail input will power servos.

I’ve seen many people posting about their OPTO ESCs not working because those ESCs don’t have built in BEC.

This is probably faulty unit or your wiring.

Please post your wiring (photo or diagram) it is the only way for someone to accept it is hardware fault.

I will do a diagram of my wiring when I get a minute.
The quick version is that the servos at plugged into the rail, a lead goes from RC in to the SBUS port on my receiver, a lead goes from the receiver to the Pixhawk to power the rail, and I am plugging 2S lipo into the receiver. Later on I will move the Lipo closer to the pixhawk and power the rail with heavier gauge wire with a EC3 to dual servo connector harness as I do all my helis, but for bench testing with light load I wasent worried about amperage.

When I unplug the 2S lipo, if it is still hooked up to the computer, the USB being the only power source left will power everything.


This is it!
Pixhawk 2.1 supplies power to the receiver via RCIN port.
RCIN port is semi-separated from servo rail, it has its own 5V to power receiver but will use anything from servo rail when it can.
This power got a jumper to servo rail.

Solution: plug your 2S directly to Pixhawk servo rail.

I don’t think this is correct:

S.Bus and CPPM receivers powered directly from the servo rail, and must
the servo supply voltage

which means that PXv2.1 does not power the receiver; it should be powered off of the servo power supply.
Of course, I don’t own a PXv2.1, so I’m just reading the documentation. YRMV.

…aaaand the above is contradicted further down in the document: