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HelicopterMain Discussion › Running BECs in parallel
05-01-2011 07:26 PM  6 years agoPost 1
jackheli

rrProfessor

Vancouver - Canada

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Found this from an FAQ on multicopters. The poster insists it is okay.

What do you think?
Do I plug all my ESCs onto the KKmulticontroller? PDF Print E-mail

The KKmulticontroller gets its power from the ESCs. You can plug all your 3 pin plugs onto the control board as it will connect the power in parallel providing 5V to the board.
My post:
I am sorry but this is very wrong, unless there are diodes or some other type of protection for the BECs. If not one BEC will be fighting the other until they pop.

his reply:
No.
ESC don't "fight" with others. ESC just work as paralel source.
Exactly like battery.

my reply back:
No Sir. The 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 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.

and back
I saw many equipment powered with paralel battery

I'm flying over year many diferent multicopters -
tri, quad, six with a few diferent controllers:
KKUk (few version), GU-344, FY90Q, MultiWii, Arducopter.

ALL powered from paralel ESC ZERO problems

Thousands of pilots are flying the same way.
But you know better

First link from google:

http://www.rcgroups.com/forums/showthread.php?t=1126572

And the thread clearly says not to do it! go figure...

It's easy to find an excuse to do wrong. Hard is not to find an excuse to do right.

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05-01-2011 08:29 PM  6 years agoPost 2
Heli 770

rrProfessor

USA.

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Before we had any 10A. BECs, I had asked Western Robotics for a 10A. BEC. Their reply was to run two WRL-HBECHC(5A) in parallel and I may want to add a heat sink to them.

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05-02-2011 04:58 AM  6 years agoPost 3
GetToDaChopper

rrElite Veteran

Las Vegas , NV

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Hummmm well i'll be..........

    ▲
  ▲ ▲
▲ ▲ ▲ One of a Kind !!!

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05-02-2011 05:17 AM  6 years agoPost 4
dkshema

rrMaster

Cedar Rapids, IA

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The answer depends on how the regulators inside the ESCs have been designed. If they are nothing more than linear regulators, it's a bad idea to simply connect the outputs together, in parallel.

A small series resistance in line with each regulator output would help with the parallel operation hookup, but that also means your output voltage will not be constant, it will vary with the load.

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.

Additionally, due to variances in the components and even the design of the regulator itself, simply hooking them in parallel may not give you the desired effect.

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. 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.

The "small series resistor" I noted earlier is added to help with the current sharing, but as noted, you no longer really have a well-regulated voltage, as the output voltage will vary with load.

From a National Semiconductors FAQ sheet:
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.
With regard to switching regulators, that may not work well, either. Again, from another FAQ sheet at National Semiconductor:
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.
Bad plan. Best to go with a single regulator, properly designed and rated for the job at hand.

-----
Dave

* Making the World Better -- One Helicopter at a time! *

Team Heliproz

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05-02-2011 08:26 AM  6 years agoPost 5
jackheli

rrProfessor

Vancouver - Canada

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thanks dkshema. Exactly what a learned about voltage sources.

Try hooking up two high voltage high current power supplies in parallel and see what happens...

It's easy to find an excuse to do wrong. Hard is not to find an excuse to do right.

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