Read: 3G Servo Bus Concern

[impi62]

I received my 3G today and noticed that the Aileron wire from the receiver is the only wire carrying power to supply the servo bus. That means that the current for all 3 cyclic servos passes through only a single wire and servo connector plug which is not even of a substantial guage! We all know that a some digital servos can draw some serious amps and that combined amp draw of 3 servos could exceed 10 amps quite easily. Am I missing something here? Some type of external power bus might be a good idea with the receiver and 3G unit supplying onl the signals.. Any thoughts on this??

[harryboe]

My 3G is still on it's way so I can not personally check the gauge myself.

But from all the threads i've seen here that's the only one that really scares me. This one cannot be addressed with a software only! Some peak currents can be covered with internal capacitors but that's not enough to deal with that amount of current. We do not really talk about bad connectors here ether.

Maybe there are ways to plug a Fromeco Kiwi or Reactor in between to limit the current to signal only?

I will check my pictures at home from Alan's prototype (without case) if they have some capacitors in there and if the prototype was using the same servo cabling concept ???

[impi62]

I opened my one up and theres no "beefy" capacitors inside. Theres a small electrolytic one on the supply to the 3G electronics but not on the servo bus. Even if there were capacitors they might help momentary spikes but in the case of a bad connector or a loose connection, bad crimping on the plug etc,to trust one single small wire guage lead to carry power to the 3G electronics AND 3 high current demanding cyclic servos seems a bit stupid. If you loose power along that one lead you lose your heli too!! The VBAR and SKOOKUM that I have use all the servo leads to carry power between the receiver and flybarless controller, spreading the load and providing redundcy.. I think I am going to make some kind of external BUS like the one supplied with the new SK720.. I think this is seriously an issue and needs to be addressed

[harryboe]

Well here is the pic from Alan's prototype back in oktober last year

The wiring is obviously different and i would not be surprised if most of the 700N testing was done with that prototype. The cabling might be fine for 250 up to 500 class heli's but it is way to dangerous for a 700 Nitro on HV setups. Stall current of a regular digital servo can easily exceed 4A. That is already the limit for a single servo connector! No way that going to work with 3 of those servos.

I can imagine that this connector is going to melt on the first "chicken dance". Really wonder what the Align team tough on that one ??? I agree, this issue needs to be addressed

I the mean time a external power bus or power management device is the way to go. No chance i'm going to fly like that.

[syn90]

Out of all the threads I have been reading for months about the 3G system, this is easily by far the most concerning. I purchased two units, one for my 600 and one for my 700 and installed them this past week. I can't believe I didn't pick up on this. I am running 7100R's and the Align 3G flybarless system with regular 8717 cyclic servos on both. I was going to maiden them this weekend, but not anymore. In my eyes, this would be a huge blunder on Align's behalf. Why would they even consider doing this? There is no way I am trusting one servo connector and one set of power and ground wires to provide the entire current demand for all three of my 8717 cyclic servos, that's just begging for trouble. Someone at my field is running the scott grey reactor regulator and that works great, but the problem is that I would have to roll back to a regular receiver and introduce an external governor, defeating the purpose of me investing in the 7100R and having a nice clean setup with minimal wiring. For all of us that are running the 7100R with the 3G system, is there any way to get the current demand to be alleviated from that one aileron connector? I was thinking about opening up my control units and seeing if I can solder in a positve and gnd wire to the power bus where the aileron wires are soldered and put the pins into the elevator and pitch servo connectors so the load would be distributed over the 3 connectors? Anyone have any thoughts on this or any other ideas? I am grounding my two birds until I can come up with an acceptable solution. I am beside myself with disappointment in Align, this wiring configuration on the 3G systems makes no sense whatsoever.

[mlucia]

This is interesting and I am about finished with my install.

How does Mikado and others have this setup for the power? Just curious.

[harryboe]

Well here is what I try to do!

It's based on the idea of a external power bus. For the suspect channels (PIT, ELE) I will create a special Y cable to crate a power link from the RX to the Servos and provide a additional power input for the 3G unit.
Such a cable can be done out of two servo extension cables.

On the wiring chart below you see the 3G in which is connected to the "one wire plug" and the 3G out that should go into the 3G servo out. Kind of simple but should do the job.

Still cannot imagine what kind of black-out they had when they designed that unit ???

[harryboe]

Originally Posted by mlucia

This is interesting and I am about finished with my install.

How does Mikado and others have this setup for the power? Just curious.

Mikado, Skukum and others just have pins on theyr devices and it's up to the user to choocse and connect proper cables (e.g with more than just signal).

Sad thing about the V-Bar is that they don't support a second voltage for the tail servos. That makes it verry hard to go with HV setups as it reuwires some power regulators behind the v-bar to bing the tail servo voltage down. Guess that's due to the fact that they sell EP helis only that use BEC circuits with limited voltage.

[syn90]

That's a great idea harryboe! I have come up with a wiring scheme that I'm going to implement on my 600 and 700 that won't void my warranty by opening the units or anything and it is based on harryboe's idea. Check out the attachment. Let me know if you guys think this would work and if you have any ideas for improvement.

Thanks,
-J

[harryboe]

I have done a servo-bus cable and it turned out to be a easy task.

See the attached picture for details. If you carefully look at the pictures you can actually see in the last picture that:
input and output signal path is as it would be without cable.
ground and power does go from the RX to the servo as well as to the servo out of the 3G unit.

Not yet tested but i'm confident it will work

[syn90]

Thanks for sharing the cable assembly harryboe, that looks really easy and it does certainly seem like it should work fine. Once you have it tested, please report your results! Thanks.

[MrMel]

That makes it verry hard to go with HV setups as it reuwires some power regulators behind the v-bar to bing the tail servo voltage down. Guess that's due to the fact that they sell EP helis only that use BEC circuits with limited voltage.

Actually you need only to make a special cable if you run a AR7100R, takes less then 10 minutes.
https://www.helifreak.com/blogs/willjames/548-ar7100-v-bar-setup-8v-cyclic-servos.html, there is a solderless version too, couldnt link to that right now since the page is currently inactive.
(this setup works on any setup that has a "shared" powerbus)

[bestan]

just a silly question: how do you connect your bec (external or bec of esc) to your receiver? probably with one servo connector i guess. so its pointless to connect your power supply better between receiver and servo as long as you have that one only connector at the beginning of the supply

[MrMel]

BEC's /ESC's with BEC's for bigger machines has 2 cables normally.
(Western Robotics, Castle Bec Pro, Jive ESC's)

[syn90]

Any updates on testing your cable configuration out harryboe?

[kinosoo]

Hi everybody, I have been thinking about this thread for the last few days and while I am not an electrical engineer I can't figure out what the design flaw is. The servos draw current from the control unit not the receiver. The control unit draws current from the receiver. Unless the servo bus is directly hooked up to the power wire from the receiver that wire should never carry enough current to melt. The servo bus in the controller is the item that has to carry all the current from the three servos. I also have a question about the rewiring jumpers. Wouldn't any potential difference between the receiver and the controller cause reverse current into the controller?

Cheers

[impi62]

You are correct in saying that the servo bus in the control unit has to supply all the current for the 3 servos.. Whether the servo bus in the controller is supplied by the receiver or directly by a battery or a turbine generator for that matter, the fact remains that the feed to the servo bus is via a single small wire guage wire and therefore the 3 sevos will have to get their power via this small wire too. No doubt abourt it this IS a design flaw. I am busy working on an external BUS and will post a diagram soon..

[kinosoo]

I understand your concern impi62 about this set up, and believe me I am too concerned because I was ready to purchase FBL 3G system. I do not want to spend 400 plus dollars on a system that I have to modify. But before I declair something Flawed I like to get all the facts. By now I am sure hundreds of Helifreaks are using the 3G system, perhaps if we start a poll on how many of these have the wire melt between the receiver and the 3G controller. Than we can make an educated conclusion on how widespread the problem is. Here is my take on the system, considering that most gyros run on 5.1 volts, the servo bus is probably regulated down to 5.1 volts the sensor is probably hooked up to the servo bus. I do not have a 3G system so I can't confirm this. My guess is that unless you have an external bus like you mentioned you can't run high voltage setup.

PS. Here is some interesting facts from the Align site:
3G system spec.
●Suitable Voltage Range DC 4.5~8.4V
●Current Comsumption:<80mA @ 4.8V

I do not think <80mA is going to melt a 22 gauge wire.

Cheers

[harryboe]

@syn90 . sorry, was busy the whole we assembling "Swedish" furniture. Not time to test yet. But i don0t see a reason why i shouldn't work. However I noticed the i leads to some messy wiring on the bird. That's why I'm working on better solution. Check the attachment for the idea.

@kinosoo
The 3G has two independent power channels. One for the cyclic and one for the rudder servos. There is no regulator. Means what you feed in is what you get out.

I'm not saying the 3G unit will not work. It's the same as running a ESC/BEC setup through the throttle channel only. But we learned in the pasts that this is sometimes not good enough. The AR7100 comes with EC3 (3,5 mm) connectors, other RX have redundant battery connectors! That is not for no reason.
IMHO a 700 class heli is one of the most dangerous RC things you can put in the air. That's why I'm not going to put everything on a tiny servo plug if i have a better option.

[kinosoo]

Hi harryboe.
Hows the weather in Switzerland? Its bitterly cold here in Alberta, CANADA. I just finished building my Trex 700 and in the ground run phase. I agree with you this heli can do a lot of damage if something goes wrong. To bad FBL version just came out. I just do not believe that a company like ALIGN would make such a miscalculation in electrical design. They make awsome vacuum cleaners
Anyways the specs on the ALIGN site says that current consumption is <80mA @ 4.8 Volts.
To me, that means that the controller works on 4.8 Volts so if the input Voltage can be as high as 8.4 V
So the controller must be powered by the rudder wires? and... this is giving me a migraine
I guess I will hold off on purchasing a 3G FBL until more info is available.

Cheers