I’m concerned about connecting a 5v BEC to the servo rail. There’s already 4.9v on it when the FC is plugged into a battery through the power module. Will this cause a short circuit?
Rick
Good day, on which fc you need power the servo rails?
I’m using one of the PX4 2.4.8 clones. The funny thing is the RCIn pins are powered independently and powers the receiver. I can power servo’s from the receiver but not from the servo rail unless I attach an external power source. I’m just wondering if this is going to conflict with the 4.9v which seems to be supplied internally to the servo rail?
Rick
- you cannot power servos from the receiver the power circuit that powers the receiver is not designed for that.
- If you measure voltage on the servo rail that means that your Pixhawk 2.4.8 (Which is by the way does not exists, it’s just a common name for cheap crappy chinese clones) either faulty or either the power circuity is messed up by the genius chinese “engineer” who “designed” the board. If you insist to use it then power servos separately and use gnd as signal only from the servo connectors.
You cannot power the servo from the receiver… eventually you can power your receiver from the esc of the motors.
If you want you can add an ubec for power the servo rail… pixhawk is power redundant but don’t exceed 5.30v or you can damage the voltage regulator on the pcb.
For power servo rails just use aux pin 5 or 6.
If you want add an ubec you can choose one from 12 in to 5v out ., on the 5 volt out with a little of diy you can solder a 1000 micro farad capacitor with a zener diode… just check the way you solder the Zener diode coz you can have different results…
Thanks David. Just wasn’t sure if adding a 5v BEC power source to the servo rail would cause a problem with the voltage already present.
Rick
You will not damage it if you will not exceed the 5.30v.
Just consider that pixhawk is 3 ways power redundant… 1 is from the pdb, the 2nd is from the usb and 3rd from the servo rails.
If you are good in soldering you can and in output cable a 1000 microfarad 10v capacitor and one Zener diode 5.6v 1/2w. The capacitor help for remove noise in circuits, to eliminate voltage surges,the Zener diode used in reversed polarization work as voltage regulator.
@Dave84 What you wrote is correct, but only for the reference design 2.4.6. The 2.4.8 is missing most of the redundant power circuity. I would not trust it, especially because there should be no voltage on servo rail if it not powered externally.
Must be checked the circuit in the pcb and what kind of regulator they have solder inside and the rails. There are some versions of the 2.4.8 that you can do it…
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If your FC is a Pixhawk or Cube
“Pixhawk does not supply power to the servo rail . Powering the servo rails is normally done by an ESC or alternatively a separate source such as a BEC (as shown in the wiring overview below). Some power modules explicitly provide support for powering the servo rail , backup power and support for larger batteries.”
This is what I said. If his clone HAS voltage on the servo rail, it means that the power circuity is fishy. So the best course of action is to ignore the power rail and power servos directly from a 5V UBEC.
Actually, it works just fine and now I have a 5v, 5a servo rail without all that extra wiring.
Rick
What can cause power to appear on the servo rail if no BEC connected? I am assuming The FC Electronics is using some high impedance tri state IC’s to turn the pins on or off when used as relays other wise voltage level would stay below +1 volts. That could be the only source I can think off that tri States are failing for some reason.
Well, the 2.4.6 design use an LTC4417 power path selector with appropriate fet switching. Since I dont’ have schematics for the 2.4.8 (and they varies from manufacturer to manufacturer) I can only speculate that they use a simple diode power logic…a faulty diode, or a totally missing diode and connecting power inputs together can cause such power on servo rail.
Eosbandi makes sense…so the open source licensing allows manufacturers to divert from part standards or change schematics? I wasn’t aware of that.
Does Chinese manufacturers care about licenses ?
As Mr.Clarkson said, the term “copyright infringement” does not translate to Mandarin… 
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Eosbandi lol 
Good point…
So I guess everytime we buy an FC, open it, telly it against schematics and see if it follows the rules or not…what a bummer!!
Or you can buy from trustworthy sources, CUBE, MRO, CUAV…
Eosbandi, but for Bench testing and to answer one million Forum questions, it’s fun to learn the BS out there. I am getting there slowly…
My motto of life, More you know, more you realize how less you know"