I'll be building a Trex 500 soon. I'll have a 6S flight battery, the stock ESC w/ integrated BEC, a FrSky RX, the Spirit FBL, and the stock servos

I plan to connect BEC -> RX -> FBL -> Servos. I understand that I'm supposed to connect two cables between the RX and the FBL so they can share the power required for the FBL and servos behind it. All the power needed to run the servos runs from the BEC through a single cable to RX, through two cables to the FBL, and on to the servos.

My question is this: If two cables between the RX and the FBL are needed, won't the single cable between the BEC and the RX be the bottleneck? Or am I thinking about this wrong? Should I be using a Y-Cable from the BEC, then connecting on leg to the RX, one leg to the FBL, then the SBUS cable between the RX and FBL?

Hi,

for 500-size helicopter (Trex 500 / Protos 500) one cable is mostly enough.
Mostly ESC for this class has one BEC/Throttle cable - there is no such big current (only around 3-6A in peaks).

So described connection should be safe.

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Spirit System developer

Thanks for the reply! Let's say it's not a 500 helicopter. Is my concern valid? I'm trying to understand the principle and what I perceive as a bottleneck between the RX and BEC? In case of a larger setup, how do you recommend connections are done? I'm ok doing two connections to the FBL, but I want to understand that it's actually helping and not just moving the problem somewhere else.

you have two connections to the FBL because the two wires are carrying your rudder and gyro channel from the Rx. The Spirit just also uses the pos and neg of those two ports for power.

When you go 500mm blade size or larger, then you'd get BEC that comes with 2 power wires (a primary with ESC throttle and a secondary which is just power, or maybe a RPM signal in come cases) or go to a rx pack that has 2 connectors. So you'll have two power sources carrying current from BEC to Rx, and then 2 wires to carry that current from Rx to FBL. You go to two connectors because each connector is rated for 5A continuous. Larger heli will pull more than 5A continuous so you need to add a second connector so you can safely do 10A continuous, because the two connectors will share the load.

The fact you only have 1 BEC wire now but 2 to FBL doesn't matter. Your servos will not draw enough power across the single BEC line that it chokes and the voltage drops.

Thanks for the reply, that was exactly the explanation I was looking for!