Lots of smoke!

[Tim Kearsley]

Well, I consider myself to be careful and reasonably savvy in matters electrical and electronic but..... I managed to cause a nice cloud of smoke while setting up a new model this afternoon!

 

I had connected up the servos and the throttle lead from the ESC (a FrSky Neuron) to the Rx (a FrSky SR8).  For testing, I thought no point in powering up the motor so connected a 6.6V LiFe battery to an unused servo port on the Rx.  Everything came to life, followed a few seconds later by aforementioned smoke as the wires from the battery and the throttle cable got extremely hot!  A quick disconnect and no real harm (apart from a burned finger!)

 

So.... I'm still slightly puzzled as to what happened.  I know very well that you don't run a separate Rx battery with a BEC and have always disconnected the positive lead from the BEC in that situation.  But in this case I didn't have the main LiPo in circuit at all.

 

Anyone explain precisely what happened?

[Phil Green]

23 minutes ago, Tim Kearsley said:

I know very well that you don't run a separate Rx battery with a BEC and have always disconnected the positive lead from the BEC in that situation.  But in this case I didn't have the main LiPo in circuit at all.

 

Are you saying that 'in this case' the BEC positive was connected Tim?

So you connected a 6.6v very low ESR battery (capable of several tens of amps) to the output of a 5v regulator ?

[Cuban8]

I wouldn't speculate on the exact cause without having the gear in front of me and a DVM to hand, however if you say that both the battery lead and the ESC lead were very hot and issuing smoke, then it's not unreasonable to assume a short on the battery via the ESC. This also raises the issue of short circuit current going via the receiver pins and RX internal printed circuit, which will need to be checked very closely. Same thing with the ESC, although with much more difficulty due to the sealed nature (heat shrink) of same. Check the connectors are not showing signs of melting, they're not designed to take those sort of loads. Fine to run the RX direct from a battery, but not sure why you plugged in an otherwise unpowered (no Lipo) ESC.

Sorry, but your "no real harm done statement" might be wishful thinking - when the magic smoke appears, it usually only ever brings tears!

 

[Nigel R]

You've fried the regulator part of your esc, my guess. At a minimum. Probably a new esc is needed. 

 

C8 is right about checking the rx internals. They're probably OK - but probably isn't definitely. 

 

Lesson is, don't connect a battery to the output of a regulator...

[Tim Kearsley]

Thanks chaps.  As I said, though I'm sure you'd think otherwise, I'm normally very careful!

Yes, the thought of what might have happened to the Rx circuits does worry me slightly.  I've since connected everything up as normal - 3S LiPo connected to the ESC and throttle cable connected to the Rx - and everything works as expected.  But I'm left with nagging doubt....

I might just have to bite the bullet and replace both Rx and ESC.  Trouble is, the Neuron ESCs are currently unavailable and I do like them because of the built-in telemetry.

I HAVE learned a lesson!

[Tim Kearsley]

25 minutes ago, Phil Green said:

 

Are you saying that 'in this case' the BEC positive was connected Tim?

So you connected a 6.6v very low ESR battery (capable of several tens of amps) to the output of a 5v regulator ?

Yes Phil, BEC +ve connected.  And to your second question, yes, I did.  I have a wire-shaped burn on my finger to prove it!

[Don Fry]

Tim, you actually know the answer. Bite the bullet. 

[Nigel R]

It should only be the pin block in the rx and the tracks between the relevant servo connections that need any checking. Rest of it will not be affected.

 

Not sure I'd care to speculate what may have happened inside the esc though. Sometimes with electrical damage things may initially appear to still work ok, but the damage may reduce component life to the extent that a complete failure may be just around the corner. 

Edited August 11, 2023 by Nigel R

[Simon Clark]

I agree that the RX should be OK if the pin block area of the PCB is checked and there is no visible damage or solder splats etc. However, be very careful with the ESC. One very likely failure mode will result in the ESC BEC outputting the full supply voltage. If this has happened and connect up with say, a 3S pack, then you will probably blow your RX and all servos attached to it. If you want to check the ESC, then try first with your 2S LiFe as the main supply and measure the voltage being supplied to the RX. If it is anything other than the expected 5V then the ESC is toast and should be thrown away. It is also possible that he BEC output will have shorted out, so when you connect your battery, be ready to disconnect immediately if anything gets hot or doesn't work as expected.

 

Good luck

Simon

[Tim Kearsley]

Thanks for all the replies on this chaps, it's much appreciated.

Bearing in mind that I've just paid out approximately £290 in buying and kitting out this model,  I don't want to risk the sudden failure of the Rx or BEC caused by my carelessness.
So I've decided to:

a) Replace the SR8 receiver
b) Leave the ESC installed but disconnect the BEC and install a separate BEC
My reasoning is that if the ESC is damaged it's likely the BEC that's been hit and if the ESC does fail then it will only affect the motor and not the Rx, allowing me to practice my deadstick landings!

I would add that I've powered everything up as normal and it all works as normal but I'm very aware that there may be problems lurking, as pointed out by several people, and as Don alludes to, it isn't worth the risk.