Electric setup conundrum

[Mounty]

I converted an old Ripmax Tiger Moth to elec, and my question is this, I have to connect a separate battery for the servos otherwise I have a problem with the motor kicking in when operating the sticks, which means I have two batteries, lone a flight battery and the other a motor battery, anybody any ideas what the problem is, thanks guys, Joe

[Mounty]

Sorry for the spelling mistakes the above should read one a flight battery.

[Denis Watkins]

It sounds like you are running everything through a conventional receiver Joe

Where normally an ESC is used, and one supply battery, a lipo

And the ESC plugged in the throttle

Perhaps a photo of the set up and some more info would help us

[Geoff S]

What transmitter/receiver are you using? Presumably you were using the BEC from the esc originally and with just the flight pack? I assume you disconnected the positive lead from the esc when you used a separate receiver/servo battery.

Geoff

[Piers Bowlan]

If you have plugged your motor ESC into the throttle channel of your receiver, only the throttle stick of your Tx should operate the motor (left stick/mode 2, right stick/mode1). If any other stick or switch movement operates the throttle as well then your transmitter has some unwanted channel mixing programmed into the transmitter. (Unless I have misunderstood the question!)

[Bob Cotsford]

2.4 or 35MHz? Receiver mounted to close to the ESC? They can be a bit noisy electrically so best route the receiver aerial(s) at least a few inches away from it and especially if on 35MHz use ferrite beads around the lead from the ESC.

[Peter Beeney]

Joe

This sounds to me as though this may possibly be an electrical supply problem from the BEC. Assuming that in the first instance you are using the BEC supply via the throttle control cable, (normal practise), when you move the servos a resulting volt drop - for whatever reason - is seen at the receiver causing the throttle channel to mis-operate and momentarily fire up the motor. If this is so it might be a fairly serious condition and perhaps needs a spot of looking at.

Connecting a battery to the rx then immediately provides a robust parallel supply which won’t be affected by the sudden current pulses caused by the servo motors starting up when you operate the sticks. Thus the rx supply voltage remains stable throughout and ensures the throttle doesn’t operate.

On the other hand it might also be the case that these same pulses are causing the voltage regulator (BEC) itself on the ESC to fluctuate and so affecting the components on the ESC. Connecting a battery would have the same effect as before; it would keep the voltage constant. But this is pretty unlikely I’d have thought however, that might point to the regulator being seriously under-spec. BECs are usually at least 2 - 3 amps output these days.

If you don’t have any test equipment, voltmeter etc., then one answer maybe is to simply leave the flight battery connected anyway…

Good luck.

PB

[Allan Bennett]

The suggestion that the ESC's built-in BEC's voltage is dropping due to the servo load being too much for it would be my first thought, especially as connecting a dedicated receiver battery (I presume that's what you mean when you talk about a "battery for the servos" ) seems to cure the problem. If you end up deciding that's your permanent solution, you should disable the ESC's BEC by removing the red wire from the ESC plug that goes to the receiver.

Is it possible that your failsafe is not set for zero throttle? If so, a momentary loss of signal from the receiver to the ESC, caused by the voltage dropping too much when you waggle the other sticks, might cause momentary bursts of motor.

If you tell us exactly what ESC you're using, and how many and what servos you have, others might be able to confirm or rule out any particular diagnosis.

Edited By Allan Bennett on 29/07/2018 11:27:53

[Peter Beeney]

Joe

With hindsight I now think it might have been rather more straightforward (and easier!) to do a spot of backtracking and then simply ask - ‘Did you by chance happen to use the same ESC?’

If not it then seems to me it’s beginning to stretch a coincidence a little bit more than somewhat…

See here

PB

[Peter Beeney]

As a small aside to the main question within this thread, I notice there is still a continued reference to it being of some importance to disconnect the red wire in the throttle cable from the ESC when also connecting a battery in parallel to the receiver. I think this also sometimes applies when paralleling two ESCs together. Solely in my opinion this is a bit of a wrinkled old conker that’s been knocked about a bit over many threads in the past and has taken a few hits but as far as I know has never been mortally wounded; or even particularly scratched when push comes to shove. My thinking is that this actually doesn’t matter either way, really, the ESC will function perfectly ok with the wire connected or disconnected; the internal operating conditions do change though, but that is all unseen of course.

In fact it may well be that there is some sort of argument for saying that adding a battery is the answer to a problem, as in Joe’s case. He disconnected the red wire perhaps but it would not have mattered had he not done so; in my view this is rather an infrequent and unusual and situation, though, I’ve never seen this happen, and I’d be endeavouring to find the root cause in the first instance; this could perhaps be a type of fault which might eventually become a disaster…

And it not be the first time that’s it’s happened, either. Long ago I related a story of how disconnecting such a battery ultimately resulted in a total matchwood type crash; and to initially confuse the issue even more so the radio was working perfectly afterwards; and that destructive little gremlin has also appeared out of the blue on more than one or three occasions in the past, too…

Just an individual preference, I guess, in such circumstances I would want to keep the positive supply connected but others may well decide to remove it; and in 999 cases of a 1000 it’s always going to go entirely unnoticed either way…

This is all definitely just my own personal viewpoint and not in any way meant as any form of instruction or advice etc.

PB

[Mike Blandford]

Peter, if the ESC has a BEC, and is then powering the receiver and servos via the red wire, connecting a battery to the receiver as well is then providing two power sources in parallel, with NO protection to either.

It the receiver battery is lower in voltage than the BEC output, then the BEC will be charging the battery, with no current limit.

If the receiver battery is higher in voltage than the BEC, then the BEC circuit will be powered in reverse. These days, many BEC circuits are "switch mode" operating in "step down". This generally means there is a MOSFET transisistor that switches the flight battery voltage through an inductor directly to the BEC output. Such MOSFETs have a (parasitic) diode that would conduct the opposite way to normal (i.e. pass the receiver voltage back to the flight battery. One result of this is if the receiver battery is connected, but the flight battery is not connected, then the receiver battery will power the ESC. If you advance the throttle at this point, then the motor will start, powered from the receiver battery. There is likely to be enough power for the propellor to injure someone, particularly if they thought the motor wouldn't start because the flight battery is not connected. The amounr of current flowing to the motor may also be enough to overload the copper tracking in the receiver, thus causing damage to the receiver.

If you don't have a "power sharing" circuit to prevent these events happening, then I strongly recommend the red wire is disconnected from the ESC.

If the ESC is an "opto" type, then it doesn't have a BEC and the red wire does need to remain connected.

Mike

[Peter Beeney]

Mike, Thanks for your answer but after some considerable tinkering with this stuff quite a while back I found there was little or no evidence to suggest any real issues, at least as far as I was concerned. For instance, in practise I found it actually quite difficult to charge a 4 cell nickel battery at all with a 5V regulator, as soon as the pack is at the 5 volt level (quite quickly) everything stops. The regulator is current limited and thermal protected too; I consider the thermal switch might just be the ‘destructive little gremlin’ I previously mentioned; it protects the regulator just fine but can easily destroy your model in the process!

Previously the fact that I was using a battery/BEC combo for rx power saved my bacon on two occasions. Flying a ‘hot-liner’ powered glider the motor battery went open circuit, most likely due to over exuberant use of the throttle stick. I was still able to land safely but without the receiver battery this would have been a missile with no means of control, something that I personally believe to be a very dangerous (and often unnecessary!) situation.
It also allowed me to do all pre-flight checks before connecting the motor battery in relative tranquility; a 16 x 13 carbon folder turning at around 10k is definitely more than just a bit intimidating when close up; I treated it with more care than I would a demented chainsaw on steroids! Yet another extremely worthwhile safety point in my opinion; taking off with the ailerons reversed is still a trick that can occasionally be observed. Of course, it would still be possible to do all this stuff with the pos. wire disconnected and no motor battery, this is just another case for having a separate rx battery supply

I take the point about the switching regulator but I’ve not used one so far I know, I’ll see if I can find one to play with. With regard to inadvertently opening the throttle, for many years now I’ve assigned a transmitter switch to prevent the throttle opening. I call it ‘throttle lock’. Also helps considerably with the intimidation bit. It would also prevent an inadvertent start up relating to the BEC running the motor too. I’ve found that if I always work to a set procedure when flying it does help to prevent mistakes because actions tend to become automatic but I’d also admit that I can go well off the track on occasions. I have proved that more than once!! Nothing is ever guaranteed but I do try to take every precaution.

So in my semi-conscious risk assessment mode I’ve still decided that the parallel BEC/battery set up is perhaps just the lesser of two evils, as they say.

Regards.

PB