Ok, I already know the basics of powering the board, currently I am running a 3S battery through a UBEC set at 5V, and that is plugged into one of the spare servo inputs on the APM, which in turn passes the voltage to the reciever and servos.
I have a tail servo rated at 3S, direct. That one servo has power and ground directly connected to the battery, and only the signal wire goes to the APM. It worked fine for a while, but on the weekend I had an issue that led to a crash. As best as I can tell, there's some kind of interference related to my Xbee which I just added. Basically, the servo pulses whenever the Xbee is transmitting.
I suspect the issue has to do with a voltage drop on the APM side of things reducing the voltage peak of the PWM signal going to the 3S servo to drop to the point it doesn't see it anymore.
Anyway, I am considering making a few changes:
I want to de-solder the jumper which will allow me to power the APM directly, instead of using power from the servo rail. Question is, can the APM really take 3S power direct? 11.1V? I have seen mixed things about this. BTW, I am talking about a APM 2560 bought 1 month ago, if it matters.
Second thing is, I want to reconfigure my UBEC to output 6V instead of 5V, and pass that power to the servo rail. Is this OK? I know my receiver can take it as I operated like that previously. And the servo will benefit from the added speed and torque. But I have seen nothing saying if the APM can take 6V on the rail? I read the thing saying to only feed it 5V, but I assume that was when the solder was jumpered for external power?
I would then continue to use the 3S servo. Hopefully this will clear up the power problems. If I did this, what would be the voltage peak of the PWM signal going to the tail servo be? 5V (internally regulated APM level) or 6V?
It should be okay for the APM; the AVRs can handle up to 5.5V, and the MediaTek GPS board has a regulator on it. The IMU shield is iffy though. I'd suggest going through each of the 5V components on it and checking their datasheets. Some of the components are 3.3V, they should be fine as the regulators likely handle 5.3V.
Where would I get a listing of the components? The eagle files? (never used that software before, so I'm not sure what's in it.)
Yeah, the EAGLE files should have part numbers on the schematic drawings. You might be able to read them all off the top of the board. Or you could ask the designer (Jordi Munoz) -- he'd probably know offhand.
It turns out another guy is already flying his quad with a setup similar to this, running 5.3V to the board, no problems.
However, I have taken another slightly different route now. I performed the Jason power hack. And I hooked up an L7805CV 5V voltage regulator to power the APM and receiver. It seems to work perfectly fine. This is rated to provide up to 1A. Previously I measured the APM and receiver drawing 0.2A. So it should be fine?
I tested the setup, and all looks great.
Am I missing anything? Any reason why it has to be more complicated than this? Anything wrong with the L7805? (reliability?)
IMHO there is no unreliability issues using the L7805 (very much tryed and tested). I use a similar set up. ESC set at 6V powering the servo rail on a diy board, (designed with EAGLE) then thro a Micrel MIC 4940 LDO regulator this provides power at 5V to Dragon link reciever, Ardupilot (Legacy) and V2 IMU.
Thanks for the feedback. It just seems too simply to be true. ;) I was going around and around looking at all these different setups with two BEC's, etc...
The thing I'm loving about this hobby is learning new stuff. I never knew about this type of thing before.
One thing that somebody else pointed out, and I'm glad they did, was that the L7805 actually prefers getting 3S direct voltage. I was originally thinking about feeding 6V from the servo BEC to the APM voltage regulator. That would have left the APM still vulnerable to brownouts. But, the L7805 is actually happier with a 5-10V drop, so it's perfectly fine direct on the 3S.
Thats why I use the LDO (Low Drop Out) MIC 4940, The L7850 is not accurate enough for such a small diferential i.e. 6V down to 5V. Conversely this makes the L7850 ideal for 11.1V to 5V conversion. The only down side, as with all regulators, take care how much current you draw at 5V with such a large voltage drop!
If its only for Rx, Ardupilot etc no problem. but if for instance you try to draw 500 mA for a hi output video Tx such as Airwave you really need to consider the impending heat issue!
Ok, thanks for the info. Yeah, I only plan on drawing about 0.2-0.3A with it. And it's bolted to a giant chunk of aluminum with lots of air moving over it. And having it draw directly from my 3S battery is my preferred method anyhow. I'm glad it doesn't need to go through a BEC first.
I'll just buy a second one when it's time for a video system.