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Do I need a UBEC?

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Dave Hess23/09/2018 10:02:45
303 forum posts
18 photos

I'm building a Flitetest Edge 540 and using their power pack kit that includes an EMax BLHeli 30A ESC. I'll be running with a 4S lipo pack. In their build video, the guy uses a UBEC because he says that there's a risk of brown-out when doing 3D stuff. The servos are very small (9g). I don’t have UBECs in my other planes. How do you decide whether one is necessary, or is it just a case of assurance? Is a 3A one enough and how does one calculate that?

Here's the power pack:


and the plane:


Paul Marsh23/09/2018 10:06:32
3956 forum posts
1182 photos

It will probably be ok, as it's not a large aircraft, though you can go another route and have a small Li-fe rx pack running the servos and the BEC disabled on the ESC.

Biggles' Elder Brother - Moderator23/09/2018 12:12:06
15748 forum posts
1460 photos

Basically you have to make an educated guess at how much current the servos are going to draw in a worse case scenario.

So, its a 3D model and I assume you intend doing 3D syle flying with it? So worst case is fast flying with lots of big, and continuous, control movements from all three primary control surfaces. The question then is could these servos between them pull enough current from the ESC's BEC to exceed 3A and so starve the Rx of power?

Now its difficult to definitely answer that question. It depends on how fast and how heavy the model is as well as other factors. At 30A the power isn't that high (max approx 420W) so the model can't be that big and heavy to do 3D on 400W. So, personally, I would doubt that three 9g servos could pull the Rx down in this model and I wouldn't fit a separate BEC.myself, but then I tend to do mainly scale like aeros etc. So what do I know!

Sorry we can't be more definitive but it really is a bit of a grey zone.


Jason-I23/09/2018 12:26:18
302 forum posts
37 photos

Also depends what make & power the servos are.

I have some cheapo 9g servos that will pull in excess of 1.2A each when stalled (worst case scenario)

Whilst other 9g servos will only pull 500mA when stalled. If I remember correctly, HXT900 9g servos pull about 500mA when stalled, so you could have 6 of those connected to your 3A BEC, all stalled, and still be OK (just!).



Edited By Jason Inskip on 23/09/2018 12:27:13

John Muir23/09/2018 20:54:35
379 forum posts
1 photos

Further to what Jason was saying, I have a 30A ESC with a 2A linear BEC like the BLHeli and the instructions state that if using a 4S lipo the maximum number of typical 9g servos it can run is 3. With 3S it can handle 4 servos and on 2S, 5. I'd definitely get a 3A switching UBEC. Only a few quid and safer.

PatMc23/09/2018 21:41:40
4382 forum posts
524 photos

I'd use a 3s lipo that the would be suitable for motor/prop/esc combo as per the links in the OP.

Dave Hess24/09/2018 11:10:13
303 forum posts
18 photos

Thanks for your answers, guys. 3A UBECs are pretty cheap, so I'll get one and fit it just to be sure.

Just one more thing: Has anybody used a capacitor to support the supply from the ESC's BEC to the receiver? Is there any mileage in that? It would only work for instantaneous brown-outs, I guess. What happens if you get an instantaneous brown-out, i.e. how quickly does the system recover? Does the receiver have to completely reinitialise, like when you first switch it on? Does it normally end in disaster?

John Muir24/09/2018 21:04:18
379 forum posts
1 photos

Don't think a capacitor would help. The problem with an overloaded BEC is heat. If it gets too hot the whole ESC goes into thermal protection mode or whatever and shuts down and stays shut down until it cools a bit. I remember that causing issues a few years ago with a helicopter at our place which used to simply fall out of the sky periodically. By the time the owner recovered the model and tested it, the radio worked fine. He blamed Spektrum until he eventually realized what was going on and fitted a receiver battery. I suspect it's quite a common cause of mysterious 'brown outs' or 'lost link' events. Once the ESC cools and starts working again the receiver sorts itself out pretty much immediately I think. Instantaneous 'brown outs', i.e. instances where the battery voltage drops below the safe minimum for the receiver for a moment, woudn't even be noticeable in flight, the receivers recover so quickly. Wouldn't happen with a BEC anyway, would it? It would either be delivering 5V or not working at all I would have thought. Not sure about that to be honest.


Chris Walby24/09/2018 22:01:03
1194 forum posts
297 photos

IMHO I would like to add my experience with cheap UBEC's

I built a model where I was trying out a throttle to two ESC's mix so had the RX battery three digital servos (2xalieron and a elevator) on the building board.

All bound and failsafe checked with the RX and 5A UBEC (no ESC's connected, but a couple of micro servos in the throttle channels) and did the gentle TX stick moves - all nice and no problem.

Then stick bang each channel individually - again no issue, all looking good then.

Lent across the bench and knocked the TX over (was standing on its base) with an instinctive grab I caught both sticks at the same time and saved any damage to the TX.

The odd thing was the RX was now flashing loss of signal so I investigated further with the conclusion that only when all three servos were operated at full speed the UBEC voltage would drop and brown out the RX.

If this was in the model the first time I would need everything working to its full travel would be to get me out of trouble...not a good time to see how long it takes for an RX to reboot!

Replaced it with a good quality 5A version and had no further issues.

Moral of the tale, stick banging on the bench has its place, if only to find issues that would cause a loss of model or worse at the flying field.

Nigel R25/09/2018 09:37:05
3730 forum posts
583 photos

Dave H

That 9 wind motor is going to be too harsh on 4S with the 10x4.5 prop - that's a 3S setup. You'd need a different motor to go with 4S, or a tiny prop and (my opinion) 3d setups work better with big low pitch paddles like the slow fly 10x4.5.

On 3S I would go with the built in BEC. This is only a 1.25lb ish model I think? It's slow flying. Big surfaces, for sure. But slow. I have a very similar size 3d model on the same kind of power train, 4 x 9g servos, BEC in the ESC, no issues (I am aware that is sample size of 1, but there are a lot of folks out there flying this size kit on the BECs with no problems).

RX brownouts are very unlikely with current round of kit. There was (historically) an issue with early 2.4GHz kit (loads on the forums about first gen Spektrum RXs suffering with long reboot times AND a relatively high brownout voltage) but the problems have been completely ironed out as far as I am aware now that we're into the 2nd and 3rd gen of 2.4GHz.

You flown any 3d before?

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