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Never power the rfd900 from the autopilot

I’m looking to power a liquid level sensor that requires 5 volts for power. Nothing on my airframe provides 5 volts without using a voltage step down of some sort. In an effort to reduce weight on an already heavy (48ish pounds) build I’m considering using the 5 volts off the RFD900x. To power the sensor I’ve connected pins 2 and 6 on the RFD to the corresponding ground and power on the Pixhawk 2.1’s Aux 1, providing power to the Aux rail.

I currently have a DC motor set up as a dummy load for testing, but I am wondering if there will be any negative affect in doing this, such as overdrawing the telemetry port on the pixhawk? Everything seems to power up and connect properly on the bench however.

These radios should not be powered from the telem 1 port or any serial port on pixhawk at least that is what @proficnc has stated, I have yet to find an explanation from anyone as to why but it’s just a good idea to not power, power hungry devices from any serial port. Use an external BEC.

I’ve yet to have an issue powering the RFD900x off of telem1 but I imagine profi would know best.

I also had no issues but also didn’t want to go against the manufactures suggestions. Its actually quite nice to have a separate BEC as you will eventually be adding many sensors and having the ability to easily connect new sensors to power is a big benefit.

And note that there is no such thing as the “Aux” power rail…
main and Aux all use the rail…

But connecting to the Second power input adds the Zener diode into the line, helping to protect from power spikes.

In the above application, I would advise connecting the primary power supply as well.

And yes, my advice is definitely “Do not power an RFD900 via the telemetry ports”

This is not only my advice… but the advice of RFDESIGN, Ardupilot, PX4 hardware team, and anyone who has been involved in the core of this project :slight_smile:

It’s not a simple matter of looking at peak current, it has caused brownouts in the past, don’t risk it


BEC it is then. Looks like I’ll be looking into getting the RFD external power too, thanks for the input.

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@proficnc Is this paragraph from the “Pixhawk 2 spec manual” on the proficnc store page for the CubeBlack complete set false?

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But the RFD900 is not to be run on that port.

So then is the RFD900x spec document stating that the RFD900x gets maximum output at 5.5V and 800mA incorrect? We still power the RFD900x through a BEC set to 5.5V in critical roles, but it would be nice to be able to use the TELEM1 port as a power source for quick airframes that are not high risk.

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@TunaLobster is your BEC redundant in power like the Pixhawk? That’s the main reason I’d like to power straight off the Pixhawk.

so can you be specific as to what the problem is with this? Many people say they do it and its fine. We poer ours seperately but have lost telemetry on multiple occasion due to BEC’s dying. Redundant power from the pixhawk seems nice compared to that.

No the RFDESIGN docs are fine.

Maybe someone like Jeff Wurzbach can give the details as to the math involved, but things are not as straightforward as simply looking at voltage and current sometimes.

Basically, this rule… connect external power to the RFD900 has been there for over 5 years! This is not new information and is covered in the Ardupilot wiki. This applies to ALL FLIGHT CONTROLLERS regardless of manufacturer

We have a 6.6V plugged into the servo rail for servo power then take a BEC from the servo rail to the RFD to avoid a ground loop.

I can state from personal experience that the RFD900x draws enough (at full power) to cause problems in certain situations. When I did this with a PixHawk powered over USB, the PixHawk would often lock up.

If you can’t power from an external BEC, you can reduce the transmit power. If you drop the output power from 30 dBm to 20 dBm, the required current drops from ~1.2 Amps to ~.3 Amps.


I’ll be powering my RFD900x off the power cube in my setup. Unfortunately the one on the aircraft doesn’t have 5v out, although you can get a power cube with 5v which would negate the need for a BEC/step down. The power cube provides redundant power as the air frame has redundant power sources, obviously this isn’t applicable on every air frame, but it’ll solve my power issues for the time being.

So, it may be something inherant to our system, but we’ve lost multiple Mauch Power cubes. Some of these have resulted in losing telemetry in flight. I would not consider this a redundant system, and the reason I’m participating in thread is because of this exact issue. I need redundancy outside of the powercube, like that presented in the Pixhawks power system.

So, if this is a known concern, and its something that consumers want, what stands between you and making the Pixhawk compatible with this power output? I would gladly buy a new pixhawk for all my frames if I could ensaure redundant power this way without adding some other redundant methodology to complicate things.

You could hook up two BECs potentially, doesn’t seem very practical or efficient to me but I’m unaware of any BECs with built in redundancy and we’ve ruled out the pixhawk as a safe power source. I agree that it would be very convenient to solely use the Pixhawk.

We build an autopilot, NOT a power distribution system.

Where should it stop? The ESC’s? What power? 500A?

We stated, and have done for actually nearly 7 years that the way to power the RFD900 is as per the recommendation of THE MANUFACTURER OF THE RFD900

By the seperate power pins of the RFD900…

Flight safety is our primary concern.

I actually will not be continuing this discussion.

Topic closed

Mauch has a post-cube redundancy system we have yet to integrate, but, its another piece of hardware.

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