To UBEC or not?

[Nick Rigg]

In my yet to fly Pulse XT40, I have a Turnigy Plush 60 ESC that is labelled as a "UBEC - 3A/5V ver 3.1". The plane has 4 standard servos and is powered by a 4S 5000 Zippy LIPo. The receiver is Futaba 2.4ghz

In this month's RCM&E, Nigel Hawes says he will not fly electric unless his receiver is powered by a separate battery, regardless of using UBECs, S-BECs or 2.4ghz radio gear.

I assumed my set-up would be safe to fly but now I wonder whether it is right to err on the side of caution. Anyone agree? 

[Bruce Richards]

I have the same ESC in my Wot 4 running on 6S and driving 3 servos and it has been fine now for in excess of 20 flights. I think your set up on 4S should be fine if you are using standard (not digital) servos.

[winchweight]

I now always use a seperate UBEC after my Sea Fury crashes. They cost a couple of pounds and only weight very few grams. So why not use one anyway? I think Nigel is being overly cautious.

[Robert Bromwich]

I always use a Ubec for anything more then 3s.

This gives you the option to use digital servos etc and of course you are not then using the esc's bec so will run cooler. This is something that i do but there are new esc's that can cope with it now also for the price of them i dont think its worth taking the chance of overheating the esc

Rob

[Frank Skilbeck]

Nick I have the same ESC in my E Flite Beaver running 6 servo's off a 4s lipo, no problems todate. I even used my E-logger to meaure the temp of the ESC when in flight , it went up slightly at partial throttle. See below

RX is a Multiplex IPD (35 mhz).

[David Ashby - Moderator]

Sounds like you're ok Nick.

I'm just putting together a Multiplex Mentor which has 4 servos (2 std and 2 mini) and a 3S batt yet despite no mention of UBEC precautions in the instructions I'll be playing safe and using a separate receiver pack as the ESC is a bog standard linear type.  

[David Ashby - Moderator]

Hi Peter - only 250 flights eh? early days  

It's an MPX BL-54. The manual that came with it states that it's rated up to 4 micro servos and 3S - so with 2 std and 2 micro on 3S I'm drawn towards playing safe. Here's the spec from the website ..

Specification:
ZCell count, NiXX / LiPo, with BEC: 5 - 9/2 - 3S
Cell count, NiXX / LiPo, excl. BEC: 5 - 12/2 - 3S
Permissible continuous current: max. 54 A
BEC voltage: 5,0 V
BEC current: max. 3,0 A / max. 3,0 W
Pulse frequency: 13 kHz

[Mike Rolls]

Like Peter I run Axi 2820/10, 3S packs with Jeti 40-3p and 4 servos - usually HS 85MG - with (fingers crossed etc) no problems in several models.

HTH

Mike

[David Ashby - Moderator]

Yup thanks Peter, confirmed my feelings.

[Tim Mackey]

The following is of course, only my personal opinion and not based on science, engineering degrees or anything else quite so sophisticated, but will doubtless cause "debate".

Nigel is of course completely entitled to his own ideas, and I do NOT wish to appear contradictory.

I have stated the following before on another thread far away in another galaxy......but here goes again.

  " BECS are actually safer than a  separate battery" 

Let me explain why I think this could be so.  BECs ( whether separate or part of the ESC ) rely on getting their power from the main flight battery - a relatively meaty great big powerful beast. The ESC also features a low voltage cutoff circuit ( LVC ) designed to give visual but safe indication that the battery is running low. However even if the battery approaches this low level, it will still have more than enough juice left to continue powering the BEC / UBEC which in turn powers the receiver and brown out should therefore not happen ( assuming the BEC circuitry is "up to the job" ). A seperate Nixx battery pack on the other hand has the following potential problems.

1) It needs to be capable of supplying the correct voltage and current necessary - and many are not, especially smaller cell AA type.

2) It needs charging ( and by different means to that of the flight pack ) and this could be / has been forgotten.

3) Nickel based cells have a bad reputation for black wire disease - which most decent evidence seems to point to the chemistry of the electrolyte used. No such BWD has been found in Lithium cell based packs.

4) Perhaps most critically of all - if, for whatever reason, the separate pack runs low, or stops altogether due to a bad connection etc there will be no prior warning via the motor stopping / slowing / pulsing or whatever the ESC would normally do. The first signs of trouble will be the receiver stopping working altogether - including the flight controls! This will not be a "failsafe" situation - this can only occur with RF signal loss / interference

This of course is dependant on the BEC / UBEC being reliable and capable enough for the job...which frankly these days the majority of them are. A separate circuit UBEC should indeed run cooler ( provided it is within capability and has airflow around it ) than one built onto an already warm, hard working ESC PCB

So in order of preference I will go for - in true miss world fashion EG: in reverse

6th place- the seperate 4 cell NiXX battery

5th place- the seperate 5 cell NiXX battery

4th place - the ESC BEC combo unit

3rd place - the UBEC powered by Nixx battery

2nd place - the UBEC powered by seperate LiPo

Ist place -  the UBEC powered by main flight pack

Quick...call me a cab !!

[Andy Harris]

Nick

  a) Timbo is right

  b) That is a rather big battery for an XT40 Pulse

I'd be a little concerned about weight.  My son flies a Graupner Saphira on 3700/4S and 3300/4S and this is a bigger model.  His Mini Pulse says up for 22 mins on 2000/3S.

I run 4200/6S for a 58" Extra, which is slightly heavier than the 3300/6S recommended.  I get a slight advantage on landings where it doesn't tend to float on too much.

Those Pulses are slippery devils, hope you enjoy it! ..... Andy

[Nick Rigg]

Thanks all for your responses

It would seem the consensus here is that Nigel may be a little over-cautious and that my set up will probably be perfectly safe with adequate cooling air throughput.

Andy - I'm interested in why you think the 5000 4S is a rather big battery - you also mention weight and if this is the reason for your concern, I would say the model's AUW is 5.5lbs which is within the range appropriate for the XT40

I think this weekend may be the one to give the Pulse its maiden. Let's see what happens.... 

[Tim Mackey]

On a small point of order regarding this article, I should just point out a couple of errors ( typos / whatever as I am sure Nigel knows anyway )

1) A caption accompanying a picture of a stand alone regulator suggested that this device steps down the current from battery to receiver. It doesnt - it steps down the voltage.

2) Perhaps more seriously - Within the sub heading FOR AND AGAINST -cutting the positive supply lead in an ESC BEC is discussed in order to disable it and use a seperate supply. It suggests cutting the the orange wire of a JR style lead. This orange wire is signal and the positive lead you need to cut / remove is RED ( as per the fuby style )

Cutting the orange wire will prevent any throttle commands being received and would cause much head scratching!

PS I also worry a little when AAA cells are recommended for anything other than small stuff - they have been proven to be pretty rubbish at delivering high currents - irrespective of their stated capacity.

[Pete Rieden]

On a further (perhaps pedantic) point, the statement that a regulator comprises "a transistor and a zener diode" is a little far from the truth. The internal circuit of a 7805 regulator looks something like this:

As you can see, it contains around 20 transistors, a zener and assorted resistances. This is, of course, a simplified version of what is actually on the IC...

PDR

[Former Member]

[This posting has been removed]

[John Bunting]

Years ago, in an electromagnetic flow-meter circuit, I used a resistor in parallel with a 7805, so the power dissipated in the regulator was less, and the current consisted of a regulated and an unregulated component. You just have to select the resistor so that the regulated voltage is maintained under all possible load conditions. Possibly this might help in some BEC applications. 

[Brian Parker]

Use a 5volt regulator for the receiver and a 6volt regulator for the servos. If more current is required, cascade them. Then mount onto an aluminium undercarriage to aid heat dissipation. Result….low component  count, low electrical noise level, low cost and a voltage regulation within 0.2%.

[Tim Mackey]

Please dont go straying too far into the depths of highly technical discussions guys - all very commendable and so on - but way above the heads of most members I suspect ... and not really true to the O/P as to decisions.

To BEC or not to BEC - that is the question dear scholars

PS we do have a section dedicated to DIY electronics and so on, please feel free to continue the debate there.

[Peter Ford]

I prefer a separate Ubec plus a separate small Lipo. Here's why:

1) Charging a small(ish) Lipo is no big deal. Its quick, and it is always ready for use

2) You can check the radio system without the motor being live

3) I lost my CAP 232 when a faulty connection on the motor battery WITHOUT separate UBEC battery caused everything to die.

4) I also have a special 2600 ma Lipo in my TX (Futaba 7C 2.4ghz)

5) I do not have any Nicad or NiMh batteries in use these days

[Nick Rigg]

Just a quick update. The Pulse maidened this weekend as originally built with just the Turnigy UBEC, no other batteries etc. The club's resident ace flew a full 10 minute no-holds-barred routine (thanks Steve) everything seemed nice and cool and the battery still had a good charge at the end (15.4 volts).I'll be interested to see how much longer it could have flown, maybe another 5 mins??

Anyway,  thanks for all your helpful comments here 

[Frank Skilbeck]

Nick if you look here http://www.modelflying.co.uk/forum/forummessages/mps/dt/4/UTN/2425/V/6/SP/ you'll see that you've used approx 70% of the battery charge and as general rule of thumb it's kinder to Lipo's to aim to use less than 80% of the capacity, you get more cycles out of them that way.  So I would say maybe 12 mins max but setting your Tx timer for 10-11 mins would seem to be a good option.

I

[Nick Rigg]

Thanks Frank, that's great.

The maiden was flown mostly at about three-quarters throttle so I reckon with more gentle use (which would be more up to my abilities) I could get some more time out of each charge. 

The prop used was a 11x5.5 so is it the case that if I used an  e.g. 11x7 I would be able to use less throttle for a given speed and maybe increase flight time that way?

[FLY ELECTRIC]

Hi Guys,

Just been given a link to this thread.

Firstly, re the sentence in the article saying that the UBEC unit switches the CURRENT between 5 and 6 volts, not guilty I'm afraid - whoever subbed the article can take the rep for that one!

As for the ORANGE wire on JR leads, guilty as charged m'lud! As I have no JR equipment I rang someone who uses it exclusively, and he assured me the orange wire was the positive: I should have researched this properly so I bend over to be spanked.

Finally, Nigel may well be being "over-cautious" in the article, but since a 3Kg aerobat went out of control at an Electric Fly-In due to a blown BEC (too many servos for the 3s Li-Po) and missed Nigel's 3-year-old son by less than a foot at more than 50 Mph before completely destroying itself in the ground less than 6 feet away from us, then yes, Nigel will continue to be "over-cautious".

If a model of mine ever injures a child, or worse, then at least it won't be due to a blown BEC.

Think about it.

Nigel Hawes.