Brushed ESC for LEDs - Wiring Question

[aclysma]

I am using a brushed ESC to generate voltage for the lights on a Radian Pro. This will allow me to adjust intensity with a receiver channel. My wiring is a little odd (see aside below for why) and I am finding that the wings are always max brightness (~11.5 volts) and the fuselage is dimming with the brushed ESC properly (~8.8 volts).

I am including a diagram with measured voltages. All voltages measured are with respect to ground of the battery.

My question is: Why is the ground voltage coming out of the brushed ESC raised to 2.6, rather than the positive voltage lowered to ~8V? How can I fix this? I have tried tieing the Brushed ESC ground to a ground on a servo channel - this causes the ground coming out of the brushed ESC to be 0V wrt. battery ground, but the voltage on the positive lead remains at 11.5, causing the fuse lights to be bright, rather than dimming the wing lights.

ASIDE: I am setting up my Radian Pro as a night flyer (yeah it's a plank, but I know there's people here that can help!). I am trying somewhat of a crazy experiment with wiring. I'm hijacking ethernet cable to carry two servo signals, servo voltage, LED voltage, and ground over one connector. It'd be a real pain to connect 6 wires in the tiny space for wiring every time I put the wings on and I don't have room to store it assembled. I also am putting in a bigger receiver than it's designed to take, so I'm moving the receiver up to the ESC/motor wiring area where there's much more room. I will post more details about the ethernet cable mod - my testing so far looks very good for the low current draw required for the servos/lights. My problem is I believe completely unrelated to my ethernet cable mod. But this cable mod is the reason I'm tieing the grounds of the wing lights to the servo ground. I am combining 2 wires in the Cat6 cable as the wires are only 24awg - giving me a max theoretical ampacity of 4A, of which I spike to about 1.5 and usually stay below 1A. Since I'm using 2 of 8 wires per GND, V for servos, and V for lights, this leaves 2 wires, each for a servo PWM signal.

Any ideas on how to get the wing lights to dim to the brushed ESC out voltage? Thanks in advance! If I can't get it figured out, I will make the wire pair currently pushing LED voltage carry a ground and voltage isolated from the servo ground.

[aclysma]

Here's an idea I just had. Could I tie the voltage out on the brushed ESC to the ground on the receiver? Would this push the brushed ESC positive out to 0, and then push the negative out to something like -8V? Then I could reverse the wires on the LED strips to see -8 and 0, which would be the proper voltage difference. Or would this cause a fire?

[TheBum]

It's possible that the ESC ground could be offset from receiver ground, depending on how it's designed. I would avoid trying to connect receiver ground to ESC ground.

The way to fix it for sure would be to hook the wing ground to the ground of the ESC and not to receiver ground.

[aclysma]

A friend let me know via email that the positive is a pass-through on the brushed ESC because "n-FET's have the lowest resistance and are easy to drive with a microcontroller." Still not sure if I can fix this by tieing the positives together and having two different ground voltages.

[TheBum]

Originally Posted by aclysma

A friend let me know via email that the positive is a pass-through on the brushed ESC because "n-FET's have the lowest resistance and are easy to drive with a microcontroller." Still not sure if I can fix this by tieing the positives together and having two different ground voltages.

I've designed FET switching circuits and your friend is correct. I just didn't think of it before.

You can verify whether it'd be OK be checking the ESC's positive voltage relative to battery ground at several "throttle" levels. If it's always the same as the voltage at positive terminal of the battery relative to battery ground, it should be safe to tie the positive terminals together.

So, are you considering running the ESC's negative output over the Cat 6 cable wire instead of the positive output?

[aclysma]

Apologies for my cell phone English.

I've thought through the problem some more. First of all, I took the two wires I was carrying voltage for lights, and split them to carry the brushed ESC two out voltages. Considering the lights are well under .5A it's not a big deal. I just figured that was a clever thing to do, and it would have been if it had actually worked. I am now using a trivial solution: One of the 4 pairs for lights, two of the pairs for servo voltage, and the final pair for the two servo PWM signals. It worked very well for a reasonably long flight (7 minutes or more) but the cat6 keystone jacks are a bit big. But the general strategy of using cat6 in a low amperage situation to run less cable is at least technical sound. Remains to be seen if the hassle to set this up and added weight are worth it. I wasn't do concerned with weight as it has to carry lights for night flying anyway.

I expect tying the positives together would not have worked. The servo voltage would have gone to 11v and the switching brushless esc BEC would have turned off. It's interesting that on the brushless ESC ground is apparently the pass through based on voltmeter readings. Even if tying positive output had pulled down the voltage on the positive out down, no amount of switching could have brought the ground below 0. I would have ended up with limit to the brightness.

My idea to connect the positive out on the brushless ESC to ground on receiver would have more than likely been a short and would destroy electronics.

So the short of it is I bit the bullet and resoldered to a more stupid-proof plan and it worked

[aclysma]

Unfortunately I didn't get pictures of it lit up but this picture of cat6 sticking out the wing is pretty funny.

[aclysma]

And here's the inside of the fuselage