The Back-to-Front Power Supply..

[Peter Beeney]

This is a little story about a quirky ESC/UBEC situation on an electric model which, at the moment, has got it’s owner slightly confused. Although he doesn’t have a problem with it, as such, indeed, on the contrary, it seems to have fixed an annoying condition.

At the moment I don’t have the full details of all the components, but hopefully I will be able to have a detailed look at it, because it really does seem rather intriguing.

This a a Touch and Go model, I think it has either a 40 or 60 amp ESC, I think 60. This doesn’t have a BEC supply, thus necessitating a separate UBEC rx power supply, the input to this is wired to the ESC/battery terminals, at least I think so! When he started flying this model, it appeared the motor tended to cut at random intervals, I’m not sure if this needed to be reset, or if it just reset itself. I think it was just a temporary stop. Then one day he donked it, repaired the damage to the fuz and carried on flying. Very soon he noticed that the motor glitch had completed disappeared, so he assumed that had been shaken up in the uncontrolled landing. It had, but not as he thought, because later whilst he was having a closer look around he noticed that the the negative supply wire from the UBEC to the ESC had been cut close to the connection. The fuselage had broken at that point, it’s most likely that somehow in doing this it had cut the wire. It’s a very lightweight model, built for flying, not crashing.
Naturally he rejoined the wire and straightaway the start/stop procedure was very much back in evidence. So he explained all this rigamarole to another bod at the patch at the time, who promptly took a pair of cutters and cut the wire again. Instantly back to the much better start-and-keep-going state of affairs; or at least until I tell you to stop, that is.

So it’s remained in this disconnected way ever since, but he has wondered why this should be so. One day, when it’s a bit warmer perhaps, we’ll have a little poke around in the innards and try and at least have a guess at what is going on. Or coming off, perhaps, in this case!

As he said to me recently, had this not happened, he’d never in a month of Sunday’s have thought of this as a remedy to attempt a cure for this infirmity, and I don’t think many others would, either. It’s certainly an obtuse slant on what is possibly a decidedly hazy enigma. As are some other occasional electrical problems!

We will, with luck and a following wind, have at least a semi-reasonable explanation in due course……

PB

[Seamus O'Leprosy]

One question is replacing the suspect parts cheaper than replacing the model?
Electronics don't usually crash themselves better.

[Bob Cotsford]

some form of ground loop through the two -ve connections? Might be that the UBEC is switching the -ve rail giving differences between the two -ves?

[Dave Bran]

Sounds more like it IS a BEC ESC and not an Opto as thought!

 

Power it up with the ESC and UBEC leads unplugged, and measure the leads for output.(well, the lead up to the break in the negative case).

[Peter Beeney]

Certainly the original crash in this case was not down to the motor stopping, rather it was one of those - ‘the - ground - leapt - up - in - the - air - and - bit - my - model!’ types of occasion. Here, to the pilot, it appeared at first glance the bang had effected a cure. But then it was found to be down to the broken wire, seemingly for once a case of, albeit temporarily, perhaps, two wrongs making a right!
Also there is definitely no power supply from the ESC to the rx, it can only come from the UBEC.

I think I may have confused the supply issue too, sorry about that, it might help if the sentence - ‘… look around he noticed that the the negative supply wire from the UBEC to the ESC had been cut close to the connection. ’ actually reads “… look around he noticed that the the negative supply wire from the ESC to the UBEC had been cut close to the connection.’ - hopefully this makes it somewhat clearer, it’s the negative wire that connects the UBEC to it’s battery supply, Vin, that’s broken.

I don’t have a problem with the UBEC, it’s establishing why the motor is cutting out in the first instance that might be a headache. Although if we can get to oblige on the ground it might give as a chance. Then perhaps we can surmise why what might be a small voltage change on the ESC seems to effect this. The ESC does appear to be favourite as the instigator here, but that might not necessarily be so, I suppose, it might be the receiver, but that does seem extremely unlikely.

Certainly a little bit different, at least for the moment, anyway.

PB

[John Cole]

Peter: If what you say is correct and the ESC has no BEC then as things are now the UBEC must be powering the Rx. The UBEC must therefore be taking its negative power connection through the negative line to the Rx, and back out of the Rx down the throttle lead negative line to the ESC, and then down the ESC battery line negative to the battery. A bit of a long way round. But if the connections are sound then the UBEC output voltage will be unaffected.

 

Bob: that means the UBEC is chopping the positive line, I think.

 

I think to make any guesses on what's causing the motor cut and some ideas on how to test to find if one is correct we need some more information: make and model of ESC, make and model of UBEC, make and model of Rx, and some more info on the symptoms: does the motor restart on its own (and if so after how long) - or does it require a throttle-close then open, and does it happen at just full throttle, just mid-throttle or both. But if you can't replicate the problem on the ground I don't think you'll find it.

 

I would not fly the model as it is now. I would at least replace the UBEC with a battery.

[Peter Beeney]

Thanks for that, John. As I see it, as with all common parallel negative return path circuits, this can sometimes give rise to a spot of confusion. Under normal conditions the parallel path negative wire from the UBEC to the ESC would in fact carry some of the return current from the ESC as well as from the rx and UBEC, it depends on the various impedance's of the different parts of the circuit as to actually how much. If this wire is disconnected the only return path is now the main negative power wire from the ESC to the battery; when equipment uses a common return, such as this circuit, I tend to think of the various components making a series of small impedance's, and thus forming some very minor potential dividers, which may have the effect of changing the positive supply voltage to some components. I think this is the reason why telecoms equipment, and I’m sure many others, including some consumer high-class hi-fi gear, I believe, have separate current return paths back to a common point at the supply; to lessen the effect of electrical ‘noise’ in the circuit; in the case of the telecoms apparatus, the wires could often be physically quite big, cross sectional area wise, in relation to the current they were expected to carry; this was to try and make sure of avoiding this sort of situation arising; and would all came under the general heading of reducing volts drop.

So, as a result of the broken wire and the fact that now partly the negative return path has been diverted, if you like, I think it’s possible that some voltages are now being subtly changed on the ESC PCB, it seems maybe enough to reduce the error that something was seeing before. There may be other explanations for this, but this seems the most plausible to me so far. In fact, the output voltage from the UBEC might be slightly higher, too, as a result of the wire being broken.

I think the pilot is quite happy to carry on flying; I also would have no problems with this. It’s done quite a lot of flying in this state anyway, after all, had he not not noticed the broken wire he would have never been any the wiser, and indeed, the negative wire could become disconnected in the same way on any similar UBEC set up and the pilot would be unaware of this. Or, only by seeing it, that is.

When we get a suitable opportunity, we will get our grubby little mitts on the entrails and see if we can make anything of it. I have no details this far, other than that he uses Futaba 2.4, probably that's not significant. The original motor hassle will be interesting, however, it's easily possible we may never be able to sort it ; and sod’s law says that we will put it all back together and it will refuse to play anyway, just to spite us!

Still, at least if we ever get a similar motor randomly stopping condition on a model, we now know one adjustment we can make to try and fix it!

PB

[John Cole]

1. Yes it is potentially significant that he is using Futaba 2.4 GHz, as with Futaba systems the Rxs powers down the motor if the RX voltage falls below 3.8V (to the failsafe setting). It will then throttle-up (for 30 secs) if you bring back the stick to idle and then push it forward. I guess you will now see why I asked the last couple of questions.

 

2. I wouldn't fly it as it is, as there is obviously something wrong. The motor should NOT stop as described with the UBEC correctly wired up. I would seek to find out what was wrong, and fix it. I guess I'm cautious.

[Peter Beeney]

John, I’m afraid I take a slightly different view also. I’ve done numerous checks on this low voltage phenomenon in the past, many of the results I’ve posted on the forum, and I’ve long concluded this spectre is not quite the mischievous little imp that he’s made out to be. In fact, he only appears about once in a blue moon, and that’s only because a pilot has entirely ignored the health of his batteries! Low battery/BEC/UBEC voltage might conveniently take the blame for many a hesitant motor, and other such glitches, but this would be low down on my list of priorities.
In this case it seems as though it’s entirely random, and it would certainly be difficult to see what could be dragging the UBEC voltage down to this level, short of half the servos being stalled solid.

However, this is all speculation on my part. When I meet the gentleman next we will look into it, even if the view is a bit fuzzy. As you say, more info. is necessary before we can really go any further, but whatever I find to cause this, or even if I’m unable to reach any satisfactory conclusion anyway, I will post the results.

PB

[Steve Hargreaves - Moderator]

Peter I wonder if you are barking up the wrong tree here....many ESCs will cut the motor if they are overloaded & get hot. Also if yer man is overloading the battery & pulling the voltage right down the LVC of the ESC may cut in & stop the motor......

[Peter Beeney]

Steve, with respect, I’m quite confident that this is not a case that anything is overloaded. I gather it’s entirely random in nature, but I will endeavour to see for myself. And there are no reports of anything getting warm even, it’s a very lightweight model.

Also I know it’s very tempting for the aeromodelling fraternity to sometimes try to slightly adjust the electrical theory, but I don’t think that simply removing one parallel negative return wire is going to make that much difference to any overheating or overloading problems anyway. If it did, that knowledge might be worth a small fortune! Off the top of my head, I’d say that that it generally makes very little difference to anything at all! But, little as it may be, it’s enough to fix whatever is causing this man’s grief. I think it has to be associated with the ESC’s ability to shut the motor down, this maybe getting some sort of spurious trigger, or a momentary loss of throttle signal from the rx, perhaps, although this is most unlikely, I really cannot see the supply dropping this low; and again, why would this broken wire situation affect that?

i think that’s what makes it so interesting, trying to work out the reasons first, and then finding yourself proved entirely wrong!!

I will persevere……

PB

Edited By Peter Beeney on 30/01/2012 16:48:07

[Steve Hargreaves - Moderator]

H'mm....Maybe I need to read things a bit more thoroughly before posting.....

 

Perhaps some learned words from Albert Einstein can help us here....

 

"Insanity: doing the same thing over and over again and expecting different results."

 

"A man should look for what is, and not for what he thinks should be."