Two BEC's in parallel ? - Page 7 - RC Groups
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Nov 05, 2010, 06:50 AM
Registered User
bob4432's Avatar
over in the multirotor section, the kk multicopter controller thread, the thread is nearly 500 pages and many, many people are running tris, quads and hexas w/ all the wires connected, so in those cases, 3, 4 and 6 escs running in parallel. most escs are the turnigy plush/hobbywing/pentium/rctimer level of escs, usually 30A or less rated and i haven't read a thread that clearly states that is an issue, in fact, not many threads at all concerning the matter.

i am personally running a quad w/ all esc wires attached w/ no ill effects.

my info comes from personal experience and the experience of those in that thread, among others in the multirotor section.
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Nov 05, 2010, 11:51 AM
Clyde, Ca. USA
john339's Avatar
I fly two different quads and one has the red leads connected and flies fine.
The other says to just use one red lead so I do.
It also flies fine.
May 02, 2011, 12:45 PM
Registered User
For those who say the BECs work like batteries: batteries work in parallel because they are rechargeable, so the one that has a slighter higher voltage charges the others and they all equalize at an average voltage. You cannot connect in parallel batteries that are not rechargeable!

You can't simply connect constant voltage power supplies in parallel since they will all do everything they can to impose their voltage over the others, no matter what... And they each will have slightly different output voltages.

That is what a true voltage source is: it locks in a predefined voltage and supplies whatever current is necessary to sustain that voltage. That means that the one with the higher voltage will act as a source and the others as sinks, and they all will eventually burn out.

From the Castle Creations manual:

While 5 amps peak should be more than enough power for
most aircraft applications, some heli applications are known
to draw extreme amounts of current. We strongly suggest
that you determine the max current draw of the servos you
plan to use in your application and add them up to determine
the load that the application will present. If that value
exceeds 5 amps, you must provide another source of power
for the system. We recommend the CC BEC PRO which can
provide up to 20 amps on inputs to 12S.

*Note 1: If you choose to use an RX battery pack or any
form of external BEC device, you MUST disconnect the RED
WIRE in the throttle cable from the Phoenix, Phoenix Ice,
Phoenix Ice Lite ESCs.

See http://www.castlecreations.com/suppo...User_Guide.pdf
Page 2.

Another one:
Follow the diagram above to disconnect the BEC circuit on Castle Thunderbird, Phoenix, Phoenix Ice, Mamba and Sidewinder series controllers. Phoenix HV and Phoenix Ice HV controllers do not have a BEC circuit so you must leave the red throttle wire intact when using an HV with the CC BEC Pro.

From here:
http://www.castlecreations.com/suppo...user_guide.pdf

And more, from Kontronic at http://www.kontronik.com/images/stor...2009_10_14.pdf

They have a specific harness for dealling with two BECs in a master slave configuration, which ARE NOT to be wired in parallel!

5.1 BEC-Kabel
Mit 5A Dauerstrom und 15A Maximalstrom
ermöglicht das JIVE-BEC auch die Verwendung
leistungsstarker Digitalservos. Um den vollen BECStrom
bereitstellen zu können, verfügen alle JIVE
Regler über zwei BEC-Buchsen, welche bei
Strömen über 2A auch benutzt werden müssen.
Die mit „Master“ gekennzeichnete Buchse muss
dabei an dem Empfängerkanal angeschlossen
werden der die Gasfunktion übernimmt. Die
„Slave“-Buchse kann mit jedem beliebigen freien
Empfängerkanal verbunden werden. Das
passende BEC-Kabel (Best-Nr.: 9250) ist als
Zubehör verfügbar.

1.5 BEC cable
5A and 15A continuous current with maximum current
JIVE enables the use-BEC
powerful digital servos. To take full BECStrom
provide to have all JIVE
Control over two BEC-liners which are in
Currents above 2A must also be used.
The "Master" jack must
case connected to the receiver channel
be the function takes the gas. The
"Slave" socket can be any free
Receiver channel to be connected. The
BEC appropriate cable (Order No: 9250) is as
Accessories available.

The full technical explanation continue reading...

The reason that it's not wise to hook linear regulator outputs in parallel without designing for it, is that the control circuity inside the regulators that allow them to do their thing can begin to oscillate, that will lead to all sorts of nasty things, up to and including regulator failure.

For instance, hooking two 1-amp regulators in parallel to get a 2-amp source:
Due to variations in the devices and designs, the two devices will not properly current-share. One of the two will "win" and will source the majority of the current required. The other will supply little or no current or will even behave as a sink. In this case, the one that is doing all the work will run hot as it is the one dissipating all the power and heat. It can go into thermal shutdown to protect itself. Yes, the second one may then kick in, but as the first cools back down, it may once again begin doing all the work. Note that you don't end up with a 2-amp supply, you end up with a flaky 1-amp supply in that case.

From a National Semiconductors FAQ sheet:

[Quote]
Paralleling of 3-terminal regulators is generally not recommended because the devices will not share current equally. If, for instance, you try to make a 2 Amp regulator using two LM7812s or two LM7815s, the device with the higher output voltage could be carrying more load than the other. Or even worse, the second regulator may be totally off. The reliability of such a system is poor because of the combination of high temperature and high current in the first regulator.
A simple way to improve sharing is to insert a low value resistor (about 0.1 Ω ) in series with each output. The problem with this approach is that load regulation is poor, because the voltage drop across each resistor will vary as the load current varies, in this case 0.1V error for full load of 1A per regulator.
[End Quote]

With regard to switching regulators, that may not work well, either. Again, from another FAQ sheet at National Semiconductor:

[Quote]
When using two LM2599 switching regulators in parallel, one LM2599 turns off completely, while the second one, driving the same load came up but at a higher voltage. Is this normal?

This is perfectly normal. The regulator with the lower output voltage will drop to 0% duty cycle in trying to pull down the output voltage of the higher regulator. This is due to voltage feedback control.

The only way to parallel two voltage controlled switchers is with ballasting resistors (and possibly or''ing diodes) to allow their outputs to be of different voltages, just as you would with two linear regulators.
[End Quote]

Bad plan. Best to go with a single regulator, properly designed and rated for the job at hand. It has been working for you because so far the load hasn't been enough to burn the higher voltage regulator.

The consumption on the KK controller is negligible so the BECs may take longer to fail. As soon as you start adding servos or any other load, maybe for a FPV or any other application, the load will increase and the ESCs will heat up.
Last edited by jackheli; May 03, 2011 at 02:21 AM.
May 03, 2011, 07:19 AM
I don't want to "Switch Now"
pmackenzie's Avatar
Long post, shorter reply

Your first concern is moot. The three terminal devices we use can only source current, not sink it.
This is confirmed in the snippets from the data sheets you posted where the only concern is current sharing.
The servo leads and connections have enough resistance to produce current sharing between the two devices. Castle simply parallels the devices right on its boards.

As far as switching regulators, I would not suggest putting them in parallel because I have never tried it, although many have posted about success doing it.

Pat MacKenzie
Jun 11, 2011, 11:28 AM
low tech high tech
vtdiy's Avatar
Specific question:

I have a twin I'm building, using two Turnigy Plush 6A ESCs -- the spec sheet I got with the ESCs shows a BEC (linear) output of .8A. I was originally planning to use it with 2 Hextronics 9g servos, but have now read that these draw 500 ma, so those plus the receiver seem to be an overload for a single BEC.

I had already wired the harness for only one red wire to the receiver. Though now, reading this thread, seems less important to do that. Still not sure what to do, because:

1.) even if I run two wires to the receiver, I I read here that shouldn't count on double the BEC output capability
2.) On the other hand I also read that it might be possible to split up the BEC powered units, ie one BEC red lead to 1 servo and receiver, and one BEC red lead to the other servo.

So my question is: if 1.) is correct, and 2.) is possible should I:

Cut the red lead on one servo cable and run the spare red ESC line to that?

And:

Will that positive source backfeed down the signal line somehow to the receiver and cause problems?

Sorry, to have to ask, but the servos and receiver are black boxes to me, and I don't want to just smoke test it without asking those who know more about it first.

Thanks for your help.
Jun 11, 2011, 04:45 PM
Registered User
I suggest cutting all the red wires, and getting ONE 5V switching reg with plenty of current, 5 or 10A, then you are done.
Jun 11, 2011, 05:23 PM
low tech high tech
vtdiy's Avatar
Thanks, but I only need to power two small servos and an OrangeRX receiver.

If possible. and I'd like to fly tomorrow. and I'd like to be able to accomplish that with what I have.
Jun 11, 2011, 05:37 PM
Registered User
Then #2 should work. Nothing can blow up, as long as you don't have any other
connections.
Jun 11, 2011, 09:02 PM
low tech high tech
vtdiy's Avatar
Thanks jglenn, I'll hook it up tonight on the bench and try it out.
Jan 22, 2012, 06:21 PM
You are a "go" for reentry
Maxthrottle's Avatar
Quote:
Originally Posted by PeterVRC
hmmm.... technically current will flow from ahigher potential to a lower potential.
That is irrespective of how it was made, or its source,. eg battery, linear BEC, Switching BEC.
So when they generate "+5.0V" it will take ANY, and al,) paths it can to get to a lower potential (voltage point) that forms a circuit path.
Two BECs in a system do form a circuit as they have a common ground.

So unless the BEC's have identical outputs - which they never will - you will have formed a somewhat complicated circuit of where current will flow. BEC1 to RC, BEC2 to RC, and possibly a bit of BEC1 (higher V output one) into BEC2.
This doesn't matter if they are linear or switching.

I suspect that BEC output circuits, whether by design, or just the common components used in outputs (transistor? FET?) will prevent reverse current flow into them. Or even if it doesn't prevent it, the difference in the two BEC outputs should be so low that the current flow is minimal.

As long a any point has a higher potential than another (eg BEC1 being higher than BEC2) then current will not not come from the lower potential point. BEC2. So BEC1 has made BEC2 redundant..... at least in the static no load situation.
But if BEC1's load (heading towards max current it can do) causes its potential (voltage) to drop, then at some load point the two BEC's will become equal voltage and then they will both be supplying current.

I would say that:
1) Parallel BEC's should not be a problem.
2) One will operate as the 'master" - the higher voltage output one. Until its load causes its voltage to falter and thus the other BEC becomes and "equal" and begins to supply current too.

eg Two 5A BEC's
Until one is loaded (not necessarily needing to reach 5A to drop its voltage) a fair amount, the second BEC will be more or less dormant, in current terms. By about 4A needs (just a guesstimated figure), or higher, the second BEC will have joined in to suipply current.
So in total you wil have 10A capability. But until that crossover point, one BEC does the large part of the work.

And I don't see that linear or switching should make any difference. At the output pins, after whatever process they use to form the 5V, that is what it is....5V DC at the output. Switching are just far more efficient as they don't burn off the excess voltage as heat, but apart from that the 5V output is the same (well as good as - technically not quite).
https://www.rcgroups.com/forums/show...postcount=1010

Is this explanation valid?
Jan 22, 2012, 07:04 PM
Registered User
No. It all sounds sort of logical in a basic electrical theory sense but demonstrates no understanding of how the various different BEC circuits actually work.

OTOH this has now been explained so often that it's really boring. Basically anyone is welcome to believe whatever they like and do as they wish. Paralleling regulators is the sort of thing that you quite often get away with...until suddenly you don't. Not my problem, it won't be my equipment going up in smoke or my plane crashing to the ground with no working radio .

Steve
Oct 14, 2014, 02:54 AM
Registered User
Just reading this topic and it seem so go in circles.

I like many have flown for years with parallel linear BEC,s with no problem.

Main reason is redundancy, if one BEC fails you still have the others to provide power for radio, servos etc.
Oct 14, 2014, 03:42 AM
Registered User
Quote:
Originally Posted by john1196
Just reading this topic and it seem so go in circles.

I like many have flown for years with parallel linear BEC,s with no problem.

Main reason is redundancy, if one BEC fails you still have the others to provide power for radio, servos etc.
Unless one fails short circuit (less likely but far from impossible) in which case it kills the other(s) and you have a crash. If only life was as simple as "More is always better" .

Steve
Oct 14, 2014, 10:28 AM
Registered User
I believe here is some confusion about the use of the BECs.
Switching BECs should not be connected parallel, as long as the manufacturer doesn't tell it's okay to do so.
Switching circuits may or may not react to the switching of the other circuit, hence the term "fighting each other". It's used because most RC modelers just get the "red is plus" right and do not have more knowledge of electronics.
So it is better to make them be on the safe side and disconnect all but one switcher.

For analog BECs, it is usually no problem to parallel them, as long as they are the same voltage and type.
The relatively long and thin cable will provide some equalizing resistance, so the most loaded BEC (the one with the highest voltage) will sag a little, and the others will take over some of the load.
But i won't say it is possible to load a let's say 3 times 3amps analog BEC with 9 amps.
It seems here it better to leave them alone, too.
So rule of thumb: Do not connect BECs in parallel, if you're trying to load more than the smallest can handle or if the types/voltages/rating are different.

In a car/heli/plane, you usually have some Servos that draw a lot of power.
Paralleling some may or may not be problematic. As long as the max. current draw for all Servos and other loads is equal to or lower than the limits of one BEC, you'll be fine as long as they're analog. Paralleling won't help here increasing load, so it's useless, you'll need several BECs being capable of carrying full load.

In a multicopter, there is almost no load on the BEC line, so paralleling them also does nothing, but even the smallest ESC will suffice. Simply plugging them in is just simpler and less work, as they are usually of the same type.
The one with the highest voltage carries all the load, but as it is quite small, there is no problem.

Regarding the redundancy = security:
it is wrong to think paralleling some BECs will increase the chance of a safe landing.
There are three types of error possible with a BEC:
a) it just doesn't supply anymore, no current flow, like disconnected
b) it doesn't regulate, output voltage rises to battery voltage
c) it shorts

a) There is a chance the broken voltage regulator will just "disappear" electrically, and the other BECs can supply enough power to supply the ESC controller. In this case you'll be able to fly and land as if nothing happened. Too bad this is quite rare. If you have common plus, the chances are small it'll work, if you have separated plus you're collecting debris either way: at least this motor/ESC will stop.
b) bam. at least this ESC will blow, on common plus maybe the others and the controller will, too.
c) also bam. This ESC will stop, maybe the others too. Also chance of burning the lipo.

So, the chances of surviving an ESC/BEC failure are greater on connected BECs, but don't count on it.
Aug 11, 2015, 01:49 AM
Registered User
Hey guys,

I am running two 120a ESC's with switching BEC's on an electric skateboard. I only use throttle and both are powered by a single battery. It seems when I have the red wires both plugged in I can turn on the pair of ESC's with only one of the two power switches. When one red is disconnected, both need to be switched on manually. It is more convenient to leave the red wire plugged in and only have to turn on one switch. I know this is a controversial topic, but does anyone forsee damage to my system with both reds plugged in? I am not concerned if one wheel rotates slightly faster than the other and again, no servos attached. I just don't want to burn anything out.

http://www.rcjuice.com/fvt-120a-brus...-car-esc-2-6s/

Thanks


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