John Laverick Posted July 2, 2012 Share Posted July 2, 2012 Hi Guys, don't know if I'm losing it but I've just been testing the throws on one of my planes, switched my TX on connected a 6v rx battery to a spare channel and the motor started singing?? no battery to the motor, I thought "that's odd" moved the throttle and the motor sprang into life, admittedly it wasn't turning fast enough for flight but it was still turning at quite a speed. I've never seen that before! Normally the motor just bleeps to say it's picked up the signal for the RX. Is this normal? After all these years have I been checking servos with my face near the prop and the motor can spring into life with a RX battery? John Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted July 2, 2012 Share Posted July 2, 2012 Hi John, does this set up normally use separate Rx and flight-power batterries? If so then have you cut back the red lead of the ESC? If the model uses a single combined battery for servos and Rx, and you are just using a NiMH pack to servo test, then yes of course provided the package voltage is high enough it will trigger the ESC and hence the motor - why shouldn't it? BEB Quote Link to comment Share on other sites More sharing options...
Olly P Posted July 2, 2012 Share Posted July 2, 2012 I've seen it once before, and that is why I take the prop off or disconnect the motor even for servo setting. Olly Quote Link to comment Share on other sites More sharing options...
ken anderson. Posted July 2, 2012 Share Posted July 2, 2012 hello john - would have thought that if everything is connected up the way it normally is then you connect a batt up via the RX..... then as far as the motor is concerned.....it is armed and ready... if you really want to connect with out worrying about the motor....you should diconnect it.....dont want to spoil your good looks etc... ken anderson ne..1 .... good looks dept. 3 great minds have posted together.......who is correct ? Edited By ken anderson. on 02/07/2012 15:02:58 Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted July 2, 2012 Share Posted July 2, 2012 Most odd indeed......I would have thought there would be some electrical isolation between the BEC circuitry & the power circuitry with the BEC drawing power from the main ESC battery input before regulating it to 5v (or so) so I can't see how this would happen.....What sort of ESC is it? I can possibly think how it might work with a linear BEC but not a switched mode BEC... I've never actually done this though....I might go & try it in a bit... Quote Link to comment Share on other sites More sharing options...
Martin Harris - Moderator Posted July 2, 2012 Share Posted July 2, 2012 I don't know about a motor turning with a receiver battery, but I got a painful whack in the midriff from the little prop of my ST Models ASW28 last Thursday, with the battery totally disconnected! I'd just landed it and was carrying it back to the pits - the wind had spun the prop up silently to considerable rpm until it got close enough to me to give me a nip! Luckily I had a fairly heavy sweatshirt on and there was no damage but unprotected skin might have suffered a bit more badly... Quote Link to comment Share on other sites More sharing options...
Erfolg Posted July 2, 2012 Share Posted July 2, 2012 What seems strange to me is that all the esc's I have will not power the motor, until armed with the throttle closed. If the throttle is open, even a jot, they complain lodly, until the throttle is closed. Not one will start with part throttle or full, without that arming process. I have a range of about 7 badges at least (if not manufacturers). Quote Link to comment Share on other sites More sharing options...
Chris Bott - Moderator Posted July 2, 2012 Share Posted July 2, 2012 John this is very odd behaviour indeed. I would expect that current could only pass one way through the BEC circuit, and that would be from the flight battery through to the receiver. (having been dropped in voltage) In this case it seems to be flowing backwards, from the receiver connection to the power side. It must be an unexpected "feature" of the BEC design. Just goes to show how careful we need to be. Quote Link to comment Share on other sites More sharing options...
Biggles' Elder Brother - Moderator Posted July 2, 2012 Share Posted July 2, 2012 Mmm. Interesting. I would not have assumed that it wouldn't work the other way - why not? Hence why I was not so surprised. I'm not saying all ESC's could be reversed, but I see no fundemental reason why it should never happen? So I'd start from the assumption that it will! (Murphy's first law - if something can go wrong, or work in an unexpected way, it will!) BEB Edited By Biggles' Elder Brother - Moderator on 02/07/2012 15:32:15 Quote Link to comment Share on other sites More sharing options...
Vecchio Austriaco Posted July 2, 2012 Share Posted July 2, 2012 I would suggest to either disconnect the bec (throttle connector) and use a RX battery now or disconnect 2 motor leads and use a flight battery. Both would fulfil H&S requirements for testing your servos and I leave the discussion of reverse BEC to the more clever guys... VA Edited By Vecchio Austriaco on 02/07/2012 15:56:42 Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted July 2, 2012 Share Posted July 2, 2012 Well I've just tried it on one of my ESCs...I connected a 4 cell NiMH battery to the Rx & the radio powered up fine....servos all working as expected....but the ESC started beeping too & although the motor didn't turn there was definitely SOMETHING going on because the motor "brake" seemed to be enabled. What I can't see is how power from the Rx battery can get back into the "Motor Power" side of the ESC.....the BEC will draw power in this way but I would have expected a semi conductor or two to stop it going back the other way..........as Chris says it just shows how careful we need to be.... Quote Link to comment Share on other sites More sharing options...
PatMc Posted July 2, 2012 Share Posted July 2, 2012 If it's a simple linear BEC such as an LM7805 there's only the "series path element " between input & output of the positive rail. & of course ground is common. I don't think the problem would arise with a switched BEC. Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted July 2, 2012 Share Posted July 2, 2012 78xx schematic....... I can't see an obvious path....... (Picture courtesy of Chris Bott by the way....) Quote Link to comment Share on other sites More sharing options...
PatMc Posted July 2, 2012 Share Posted July 2, 2012 Additional to the above - a 4 cell nimh would always give a voltage below the ESC's LVC threshold (except for single cell indoor ESCs) so should never be able power the motor. OTOH a 5 cell nimh could present a voltage above the LVC, depending on the ESC settings etc. Quote Link to comment Share on other sites More sharing options...
PatMc Posted July 2, 2012 Share Posted July 2, 2012 Posted by Steve Hargreaves - Moderator on 02/07/2012 16:19:18: I can't see an obvious path....... Output - R17 - R21 - D2 - R13 - input. Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted July 2, 2012 Share Posted July 2, 2012 Posted by PatMc on 02/07/2012 16:25:13: Additional to the above - a 4 cell nimh would always give a voltage below the ESC's LVC threshold (except for single cell indoor ESCs) so should never be able power the motor. OTOH a 5 cell nimh could present a voltage above the LVC, depending on the ESC settings etc. 'Twas a very simple ESC Pat with 2 setting...."ON" & "OFF"...... I tried it with a 2 cell LiFE battery into the Rx too because I realised that the 5V or so from the 4 cell pack might be too low....same result.... Quote Link to comment Share on other sites More sharing options...
PatMc Posted July 2, 2012 Share Posted July 2, 2012 Steve, it doesn't really matter how simple the ESC is there's always some LVC setting even if it's automatic. If the voltage drop due to the resistance of the backwards connected BEC in parallel with the motor windings drops the voltage seen by the LVC the case you described of a beeping ESC with the brake active would be the result I'd expect & probably more likely in most cases than the OP's experience. Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted July 2, 2012 Share Posted July 2, 2012 Yeah I know Pat.......tongue was thrust firmly into cheek whilst typing...... I have to say I am surprised by this though as I didn't see how it could happen. I must try it with a switched mode BEC equipped ESC......I would be really surprised if that did the same...... Quote Link to comment Share on other sites More sharing options...
John Laverick Posted July 2, 2012 Author Share Posted July 2, 2012 Thanks for all the reply Gents, Looking at your replies it seems it shouldn't happen so just to make sure I've not imagined it, I've just done it again with the same result. the ESC is a Black Mantis 50 amp SBEC programmable, there is world of info about programming it on the Giant Shark site. perhaps some of you more technically minded might be able to glean something from that. Biggles I don't use a seperate battery only for testing the servo's, I "thought" it was safer than using a Lipo attached to the motor Thanks John Quote Link to comment Share on other sites More sharing options...
PatMc Posted July 2, 2012 Share Posted July 2, 2012 I'm surprised that this could actually cause a motor to turn, I suppose it's most likely with a small, low KV with high res windings motor or maybe a small indoor brushed type. Something similar - occasionaly I've charged a model's battery in the car with the charger plugged into the lighter socket & had the radio playing. With my car it's necessary to have the key in the ignition & turned to the first position for the socket to be live, ditto for the radio. If I've switched off the ignition before removing the charger & battery the radio stays on. This means that the battery that been charged is powering the radio & any other ancillaries else that might be connected to the socket. Quote Link to comment Share on other sites More sharing options...
Erfolg Posted July 2, 2012 Share Posted July 2, 2012 I have two Black Mantis ESC's, to date no trouble. The one I use the most is on my Foam Park Flyer. I know definatly that it just beeps at you if the throttle is open just a little. In my case it sits there beeping, until you close the throttle and then completes the set up beeps. I was using last week, in the park, and yes the throttle was not at the absolute lowest. Quote Link to comment Share on other sites More sharing options...
Peter Beeney Posted July 2, 2012 Share Posted July 2, 2012 Another take on this might be that you just need to consider it as a multiple parallel feed problem. The BEC is not reversed or anything similar, it’s not even fired up without the flight pack connected. If this were connected the BEC would simply be shut down by the increased voltage from the rx battery pack. What is happening is that the rx battery is now powering the ESC, that’s all the components that the BEC would normally supply. This is because the output from the BEC is in parallel with the receiver pack. So when the throttle is opened the FETS must be operating as normal and there must be a path for the current to flow from the receiver to the motor, albeit at a reduced rate, because of the lower, 6 volts, which I suspect is reduced a little bit more by the low resistance motor and higher resistance of the smaller size wiring. As John remarked in the OP. This has to be a bit of a backward supply, via a reversed bias diode perhaps, otherwise the flight battery pack voltage would always appear on the receiver. That could be interesting, but not for long! As Pat says, there is one such path in the regulator, which also seems include resistors 11 and 12, too. Although for the moment I’m not entirely convinced this is the only route the current may be taking. What is the value of that total resistance? More than just a very few ohms and surely the motor wouldn’t run? I’ve always though that a good safety measure would be to incorporate a receiver power switch. I’ve never really understood why this has never seemed to have been considered. The cost angle again, maybe? It would have to be bespoke, but fairly simple I’d have thought, just break into the positive BEC - receiver wire and extend it to a switch. Make it a double pole type, paralleled together, for maximum security. Or read Safety instead perhaps, again. No apologies for that! If the receiver is not powered it’s most unlikely the motor will ever start in any circumstances. It would also stop this particular situation, too, by allowing the receiver power, but not the ESC. With respect to Pat’s car radio that continues to play, I’m not exactly sure how the voltage from the model pack, presumably a 12V lipo?, gets back on to the 'power in' leads. But it obviously must be able so to do. In my case it’s slightly different, the car’s ‘I must put you into Economy Mode at the earliest opportunity’ computer switches everything off after a random short time. Very diligent. I have a 12V battery in the back, which I plug into the auxiliary power socket in the back. Computer switches this socket off, too. However, when plugged in the back feed keeps the computer permanently awake so the radio and everything else stays on. Great, but it gets it’s revenge later, now it won’t let me lock the doors! I guess you can’t win ‘em all! PB Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted July 3, 2012 Share Posted July 3, 2012 Well....purely in the interests of experimentation & because I had it on the bench last night I tried this with my Seagull Extra. The model has a 60A ESC with a switched mode BEC. I connected a 2S LiFE battery to a spare Rx output to power the radio....the servos sprang to life as expected... AND THEN...... The ESC gave two beeps to recognise the 2S pack & a longer tone to say it was "armed" & ready to go.....advancing the throttle saw the motor spin into life......it even responded to the throttle but had a very low top speed.....even so I wouldn't want my fingers to get tangled up with a spinning 11" APC prop...... So there you have it......it seems ESCs can be "powered" via the radio. I have to say I would not have expected this at all & am very confused. The lesson however is very clear.......if you are setting up your electric model & intend to have any sort of battery connected to it ......TAKE THE PROP OFF FIRST.... Quote Link to comment Share on other sites More sharing options...
PatMc Posted July 3, 2012 Share Posted July 3, 2012 That's interesting Steve. I must admit I just assumed that it wouldn't happen with a switched BEC, in fact I think John's Black Mantis also has a switched BEC. If I've ever want to use a 4.8v battery in a model with an ESC I've always un-plugged the BEC as matter of course but wasn't aware of this potential problem. In fact I keep a couple of spare 4.8v batteries that I wouldn't trust in flight just for adjusting control throws etc at home. Quote Link to comment Share on other sites More sharing options...
John Cole Posted July 3, 2012 Share Posted July 3, 2012 Or disconnect the throttle cable from the Rx; that's what's powering up the ESCand (in turn) the motor. Quote Link to comment Share on other sites More sharing options...
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