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Sonar Drop Cam PoE Issues

Hi there,
I’m in the process of developing a drop camera.
I need to run the following:
1 x Pi
1 x Pixhawk
1x Fathom X
1x BR Low Light Camera
1x BR Camera Servo
1x Continuous Rotation Servo
1x Bar 30 Pressure Sensor
1x Ping 360º Sonar
No Thrusters and No Lights

I have the whole lot hooked up to a 50m standard 26AWG tether. I’m using 2 pairs positive and 2 pairs negative and I’m trying to piggyback the comms between the two fathom x boards on the blue pair.
Running this on the bench from my power supply set to 24V and 4.4A I’m getting 7.6V to the Bus Bars at the bottom of the 50m Tether.
I am currently only trying to run the Fathom X, the Pixhawk, The Pi and the camera.
I haven’t yet hooked up the sonar, either of the servos or the pressure sensor
I’m getting an intermittent yellow flash from the main LED on the Pixhawk, A flickering light on the subsea Fathom X power LED and no light at all on the Link Active LED.

First of all, does this set up sound like it could be achievable and if so, am I missing something glaringly obvious?
All help greatly appreciated.
Many thanks in advance.
A.

So, 1 strand of 26 AWG for 50 meters has 0.127*50=6.35 ohms of resistance, you have 4 strands in parallel, but both ways, so the final resistance is 6.35*2/4 = 3.175 ohms.
Passing 4.4 Amps through it, and you will lose 14 V on wires alone… (which should give you 10 V at the bottom, but I guess there are some other losses along the way).

Note that this voltage actually depends on what the peripherals are doing. If the Pi does any heavy processing, for example, the voltage could drop even more.

7.6 Volts is very close to the minimum voltage for the fathom X, and I don’t think you are supposed to have a shared pair for power and fathom-x.

Hi there William,
I really appreciate you getting back so quickly with a response.
I’ve just rigged the system with some fairly beefy 2 core marine cable. I’d guess at about 60 meters just as a wee test. Anyway, there was zero voltage drop using this with 16V and 4.4A. I did piggyback the comms over this pair and I am getting video but QGC said awaiting connection.

I had been working on the basis of this response to a question titled Drop/Handheld/Mounted Cam Project back in November 2017 from Rusty -

"Hi Mark,

We haven’t done much with PoE, but you can send power in parallel with the Fathom-X boards if you are interested. For instance, you can have a two-wire tether to the camera and connect those two wires to the Fathom-X terminal as well as the 12v power. That would allow you to accomplish the same thing but with a smaller cable.

-Rusty"

Looking at the drawings of the job for which this is being built, it looks like I might get away with a 35M tether. If I can piggyback the comms does this sound like it might be a goer?
I am looking at ordering a bespoke tether for the job but I’d like to exhaust all avenues first.

Thanks again,
Andrew.

Hi Andrew,

Your fine off using the tether with two power conductors and piggyback the signal on top of the voltage.

After all, thats what this two wire ethernet standard is all about and the fathom supports a quite high voltage on the signal line. However, any DC/DC converter will create noise and drag down the ethernet signal, so if you struggle with low bandwith add a ideal diode or schottky(if you can afford the heat generated) before and after the DC converters, which can act as a cheap filter.

But with your setup and distance I think you should be fine with a twister pair cable. Have you done any bandwith & delay test with only the cable for reference?

Your voltage levels will haunt you and a thicker cable will just be a hazzle. What about going up to lets say 48V which is a nice standard?