i have a few questions about my new APM2, going on to my Y6-Copter so i thought, I'd bundle them here:

1) How come, if i have the Power-Jumper deconnected, and the BEC going into the APMs Input rail, I don't get my reciever to power up, even though it is connected with a Female-Female Cable going to the Input Rail?

2) In the APM2 manual ( http://code.google.com/p/ardupilot-mega/wiki/APM2RCSetup under point 2) ) when talking about the flightmodes, it says mode 6 always has to stay manual, as a Hardware-failsafe-backup. Does that still apply to the newest APM2? Because if i go into the flightmodes menu, i can't even find "Manual" to choose from the list, and it was by default on "stabilize" in mode 6

I'd be happy about any answers!



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First off, don't put power up the radio input rail! there are diodes on board to stop this. The main reason is a clean path of power from the outputs going to the ESC are meant to be the power input to the system. My best advise to give you is follow the wiring diagrams exactly and do not skip any steps in the instructions. 95% or more of the problem here in the forums are by people re-engineering the wiring and not understanding that there are reasons for the way the board is designed and the system as a whole.

That said one other reason is that when you power via USB, the limit is 500ma and even that may trip up a USB port so diodes are in place such that the outputs are not powered when USB is plugged in.

Next, you will never, ever put any type of multirotor (hex, quad, y, octo) in manual mode. You need stabilize just to be able to keep the thing in the air. All other modes are advanced versions of stabilize to add more features (loiter=stabilize+ stay in one place based on GPS), (Altitude hold=stabilize+stay at altitute) , and so on...

Manual mode only applies to planes, thus when you load multicopter firmware, the option is removed.


Again, the best advise I can give you is do not stray from the instructions on any step or wiring. You need to use a power distro system that exactly follows the wiring diagrams so that proper ground path and 5 volt power is distributed per the diagrams. Further, the ESCs are connected in a very specific way to prevent gournd and power loops due to the high currents involved which could damage components if stray currents pass through the pins of the APM.


You need to be following these specific instructions for Arducopter since that's what your are building.


The arducopter APM2 specific info



Again, go through the APM setup first, then follow each direction from the main page and you'll be flying in no time.

Trust me, I've built both a quad and the new hex and both flew perfect the first time when I followed the directions step by step. All a Y is my Hex with motors on the bottoms of the arms. I highly recommend you use the power distro board from DIY drones. This will make your life a lot easier and prevent some fatal wiring mistakes regarding ground and 5v power to the APM. Also, we've seen some home built power distro boards and it gets downright dangerous if they cause a short with LiPo batteries as they tend to catch fire. My stock hexa with the 850kv motors would max amperage 18A per motor X 6 =108A!!! Obviously normal flight hopefully won't hit that high but my first flight was with an older 3 cell 3P pack rated at 6600mah and a conservative 8C rate which got considerably warm during the first 12 minute flight. One guy here in the forum was usign stripboard for power distro and the problem is that it's only copper glued to cheap pcb board, single sided and not plated through with the wires all on top(copper side). The problem is, tension on the wires or heat could cause the copper to peel off the board and then short the battery since nothing prevents them otherwise.


Thanks Barry,
yeah, i noticed my fault of thought with the manual modus shortly after posting it, of course that was for planes only!

However, i don't understand, why i shouldn't power from the input rail, as this is a given option (first paragraph here: http://code.google.com/p/arducopter/wiki/APM2board )
I'd like that, because i have disconnected the Internal BEC of the ESCs, and have an external BEC instead.

I didn't have the USB plugged in, i remember the whole problems with the APM1 having brown outs, when only powered by USB.
When i connect up the USB cable, i make sure, i have the normal powersource connected before plugging in the usb!



Nothing wrong with using an external BEC, but again, put it on the outputs not on the inputs. Without digging into the circuit diagrams, I know there is at least one diode in the path and a few have blown said diode by not following the directions. Hopefully that's not what's happened already in your setup. I didn't design the board so I can't give you the reason for the power path other than they were concerned and trying to prevent the brownouts and other issues on APM1.

1) How come, if i have the Power-Jumper deconnected, and the BEC going into the APMs Input rail, I don't get my reciever to power up

I'm not exactly sure the answer as according to the fact the inputs are/should be a rail, unless you have your radio wired wrong? This also might lead to a burned trace in the board, and it's a multilayer board so the trace might not even be visible. That said, nothing prevents you from soldering an external wire across the row of pins and/or plugging the BEC into the receiver. My concern is that again, you've strayed from the tried and tested wiring setup. Now when the amps start increasing from the motors, you might introduce ground current loops with your BEC setup on the input side, burn out ground traces through the APM and have a real bad day.

I totally understand what you are doing and your reasons why, but understand too, I'm trying to help you not burn up a $200 board. And a very hard to get replacement as well.


Hey Barry,

thanks a lot!
Didn't try to be sniffy, i just was wondering, because it was written in the manual, that you can use the input-rail.

Nevertheless, i have now wired everything to the output-rail, and now it seems to be working fine! Thanks a lot.

Oh, i have thought of a question 3)
When i recieved the APM2 board, it had 3 jumpers attached to the pins JP2/JP3/JP4.
Because i couldn't find them explained here: http://code.google.com/p/arducopter/wiki/APM2board (second picture, bottom right, next to the external gps-adapter) i thought I'd ask what they are for...



Let me just add why I think you might create a ground loop with your setup.

If you disconnected the BECs in the ESCs, you still really need to keep the signal ground and power because obviously, ESCs use a ttl level pwm single ended signal referenced to ground. Each ESC can draw 18-20AMPs. Even in the short wire from the ESC to whatever power distro setup you are using, there will now be a hopefully tiny voltage rise at the signal ground now above the battery ground (because the return current raises the ground voltage above ground). The external BEC now will be at battery ground voltage and trying to keep the APM at battery ground (which we already showed is lower than the ground for ESC signal) and that's just for 1 ESC. So if you plug in the ESC cables into the APM directly at the output, the output ground bus will be pulled a few millivolts higher than the input ground bus connected to the external BEC which ties to the Battery ground. This would cause some level of current to flow through the APM ground plane especially if you run all the motors at anything near even half current. This likely will blow something in the APM. You could cut the grounds at the ESCs signal wires but then you are floating the ESCs and using the now high current carrying ground wires which we know have a voltage difference and thus introducing huge noise between the ESCs and the APM. This goes back to why I was saying, follow the original design, don't overthink it. When they said external battery input, they mean either a battery or BEC but it cannot be tied to the main battery or else you will cause a ground loop through the APM.

It's cool, i'm just trying to provide info based on my experience. I try to post info and reasons not just for your question, but also explain so if somebody else has a problem they can learn and see the why behind the answer.

As to your jumper question, I honestly don't know. Maybe they already had a minor board revision? 

> Vernon

“Nothing wrong with using an external BEC, but again, put it on the outputs not on the inputs.”

Should we assume the wiki is completely wrong when it says “The board comes with two pins that have a jumper that determines how it's powered (see below). If the jumper is on, the board is powered from the Output rail (typically the ESC/motor controller on channel 3). If the jumper is off, the board is powered from the Input rail, but the Output rail will need its own power source. This configuration is used if you want to have two seperate power sources in your aircraft, one powering the servos and the other powering the electronics.”

(from this page http://code.google.com/p/ardupilot-mega/wiki/APM2board )

Like Daniel I want to use an independant power source for the APM... I've measured that the BEC output of my ESC (Multiplex MULTIcont BL-20 SD-L) doesn't deliver a stable 5v output when the servos move altogether (it sometimes drops to 4.80v) and i don't want this to power the APM2 !

If the wiki is wrong, it should be corrected asap, especially on sensitive subjects like powering the board with risks of frying it !

Was my post not long enough to explain the concept of a ground loop and what damage it can cause?

I'm not saying you cannot use an external BEC. I'm also not saying you cannot power the system from the input pins. What I am saying is that you cannot use an external BEC that is tied to the main battery,and then power the input pins of the APM because you just caused a dangerous ground loop.

By the way it is worded in the WIKI, yes if you use a second separate battery, NOT in any way connected to the main battery that just happens to be 5 volts, or even use a 7.4 volt 2cell and a 5 volts BEC, you can pull the jumper and power the radio and input side.

Again, for standard purposes if you use only one battery, in order to prevent a ground loop you must connect the BEC on the output pins. I'm just augmenting the directions and explanation, but hey, this is just my opinion and it's just your $200 board at risk.

I want to use an independant power source for the APM

What I'm saying is that if you have a second battery powering that BEC, then fine, but if that BEC is tied to the main battery, so is its ground and thus it's not INDEPENDANT! Thus you must not follow the external power diagram because you are causing a ground loop. The WIKI is correct and your interpretation  is the problem. When they say independant,they mean not in any tied to the main battery.

The reason it's a problem is that we cannot negate the 18amps of current going across the power input wires and specifically the ground side of the ESCs. Just like a current shunt, each end of that wire has a slight voltage difference and by powering the input, even with an external BEC, the ground is straight through the BEC. So the input ground pins on the APM will be at absolute ground (same as the battery terminal), but the ground wires of the ESC signal cables will be trying to pull up the ground pins on the output side to some voltage higher than absolute (battery) ground. This is very very bad. It might only be milivolts but any potential is bad.

Sorry if this sounds like I'm upset, it's that I want you to truely understand the why behind my statements. I don't want anyone to damage the boards or cause an issue. I want to help you fly and understand why things are set up a certain way.

Yes exactly Vernon... Then it must be a misunderstanding, i must have missed the fact that you were talking about the SAME battery... Of course i assumed when i talked about independent power source that the BEC must have been powered by a separate battery ! I'm wondering what would be the point of powering both ESC and BEC with the same battery anyway...

Since you seem to be highly qualified in electronics, how would you recommend me to power the input rail ? Should i connect the 5v BEC straight into an unused channel of the APM2 or should i connect it using a Y cable to one channel of the Rx (if i remember well it's the recommended method in the manual of my Graupner radio) ? I've seen some BEC with two outputs would it be of any use ?

(i'm using a 3S 5000mAh lipo with the ESC and a 2S 1000mAh lipo with the BEC)


Collin, you jumped into the middle of this thread:

I'm wondering what would be the point of powering both ESC and BEC with the same battery anyway...

The original question centered around powering from the input section, and a valid question of why the radio wasn't powered. This led to further investigation of the entire wiring system, why things were set up a certain way, and further why was someone using a non-standard wiring setup on a multirotor. In case you haven't noticed, I'd say 90-95% of peoples major problems in threads here stems from not following the proven wiring guides thinking they are smarter and see something the rest of us don't see. In this case, Daniel never mentioned a second battery and or BEC, his question simply centered around trying to power a hexa (I know he's building a Y6 but same thing) probably using a BEC or the ESC from a single main battery. I know that is an assumption but at some point, it's all assumptions right? Anyway, with his setup and single battery, powering via the inputs is a bad idea and I think I've made a case for that. Further, even from just an EMI RFI reception problem (every wire is an antenna), Using a separate battery to power the inputs and then depending on the ground plane at the outputs to the ESCs and servos has some level of risk unless you definatively star ground everything. Being that so many people are having problems with power, signal, and noise, I'm attempting to educate on the importance of looking at wiring as a system from top to bottom and doing the "what if" questions from a signal, power, noise, EMI, and RFI perspective.


So, now let's answer your question, and to get there, I need some more info.

Like Daniel I want to use an independant power source for the APM... I've measured that the BEC output of my ESC (Multiplex MULTIcont BL-20 SD-L) doesn't deliver a stable 5v output when the servos move altogether (it sometimes drops to 4.80v) and i don't want this to power the APM2 !

I'm going to make some more assumptions here. I think  you are building a plane with a single ESC and multiple servos and the original setup using the ESC BEC, you feel you have too much voltage drop. You want to use a second battery and BEC, power the input side of the ESC via that, and then (assumption), let the ESC BEC power the servos. On paper, according to everything written, this is exactly what the APM2 manual is talking about and sounds legit. But, let's shoot some holes in this, only because I want to solve your issue. The assumption is that either the BEC in the ESC is marginal, thus the 4.8 V output under the load of the servos. If you don't trust that BEC, and I see somewhat why, then don't use it at all. Now in a plane or any flying craft, extra weight is a problem. Your solution to a weak BEC is to add a second "better" BEC and a second battery (AKA 2 more failure points). Why not just use that external BEC, from the main pack, power the outputs pins of the APM normally and just run the entire system that way? Or if you want, you could still use the second battery and BEC, power the APM and the servos, such that when the main pack goes flat, you still have control of the flight surfaces. Again, you're not happy with the ESC BEC, but powering the APM does you little good if the servos are not powered as well. I'm just trying to look at your reasons for doing this and want you to have the best solution. 

Vernon you're absolutely right about the first part, i couldn't agree with you more.

About the second part your assumptions are correct. I thought about the possibility of losing flight control surfaces in case of main pack discharge but i haven't found a solution to this potential problem.

The main goal is not to use the second battery as a backup in case the main one goes flat, the reason i wanted to use a secondary smaller battery was mainly gain more fly time from the main pack, and also don't have to power the board from a battery that will receive voltage drops as the motor draws. Maybe the external switching BEC will handle those voltage drops better than the internal linear BEC...

I trust the ESC's BEC to power the servos but not to power the board, and i still need the ESC to control the motor, and i would like to avoid the fact of having to power the servos from the secondary small battery.

Apart from losing the control surfaces when the main battery goes flat and the fact that adding an external BEC causes extra weight and another potential source of hardware failure, do you think of any possible issue of using this schematics for wiring ? (from an electronic point of view).

Thanks for you time and the way you look deep into the problems Vernon, i appreciate that.

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