After a number of in-flight failures of the same type of servo under different conditions, I started a thread in another vendor forum asking for help:
Servo failures—what's going on?
In that thread, a number of people have wondered if and how ESCs used under certain (recommended and non-recommended) conditions may have contributed to servo failures without themselves being damaged (i.e. a servo fails, but the ESC is undamaged). That thread seems to have reached a point where more insight on the functioning of ESCs and BECs would be very helpful.
Here are the two set-ups:
1. Pixie 7P, 3 micro servos, 3s lipo, dual IPS ~2 A average draw
2. Phoenix 10, 3 micro servos, 3s lipo, CD rom motor ~ 2 A average draw
In the first case, it has been rightly pointed out to me that the BEC on the Pixie 7P is overloaded with 3 servos on 3s lipo. What would the expected failure be in this case?
In the second case, it is my understanding that the ESC and BEC were not overloaded. Or am I missing something?
It is well worth pointing out that my one and only goal here is to avoid future crashes from servo failure!
I think that servos can be damaged if BEC fails and feeds not 5V as it is designed to do, but full battery voltage-11.1V with 3S LIPO for example.
If it is overloaded(as in too much amp draw from servos with too high voltage) -BEC will shutdown, or loose voltage output to servos, then you loose control of your aircraft. But I don't think that voltage going low can damage servos.
So if you are continuing to use your ESC's and no more servos died it would mean that BEC output is in normal range, you could measure it with voltmeter to be sure.
CC representatives will give you better explanation I hope, as I am not sure how exactly BEC shuts down
Not to stray from Andrew's initial question, I'd like to know the different ways a BEC can fail and the consequences of those different BEC failures. Whether natural or user induced. David
#1 set-up - you are correct, it is simply out of spec and will not work at all, or for long. The BEC chip will be almost constantly overheating and shutting down, until the point of failure (closed circuit) with no output.
#2 is fully within specs, and does not pose an issue for even the BEC to shut down from overheating at all, UNLESS a servo is stalled, or the pot is damaged/partially burned/has dead spots and so on. Even good small servos when stalled can draw surprising amounts of current, and with pot damage can basically be a direct short.
The only real (or induced) damage any controllers BEC can cause (we all basically use similar linear regulator circuits) would be from providing less than 5v in an overheating and shutting down situation. Mike from Hitec would know if a constant shutdown-reboot cycle would do anything negative to a servo. From what I know about them, I don't see how it could be harmful#1 set-up - you are correct, it is simply out of spec and will not work at all, or for long. The BEC chip will be almost constantly overheating and shutting down, until the point of failure (closed circuit) with no output.