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To BEC or not to BEC?

R/C Submarine modelers

To BEC or not to BEC?

Postby subkraft » Thu Jul 16, 2009 2:06 am

I've been disabling the Bec's in my speed controllers. It seems to me that they only seem to work when the main battery is at maximum charge. Once it drops a little then the servo's seem to become unstable and weirdness ensues. I find that powering the radio directly or thru a voltage regulator seems to solve the problem.

I'm posting this in the hope that you guys might be able to tell me if I'm understanding what's going on correctly. It seems to have fixed the shaking servos in three separate instances now....and I'm interested to know if this is a common problem or if I'm missing something else.

Any comments appreciated......
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Postby Sub culture » Thu Jul 16, 2009 5:05 am

Are you running on 6 volts?
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Postby Davinci » Thu Jul 16, 2009 11:12 am

You are exceeding or close to the maximum current rating of BEC.
After it heats up it starts shutting down.
Many ESCs have a very Low BEC current capability.
A separate Voltage-regulator on a heat-sink is the best solution, unless you want to find another brand of ESC with higher current rating for BEC.
Enclosed in a WTC, like ours are, they overheat easier than they would in good ventilation.
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Postby Sub culture » Thu Jul 16, 2009 11:31 am

Or it could be that the voltage regulator can't reliably supply the 5 volts needed by the receiver.

If he's running a 6 volt system, then BEC's don't work well at all, especially if they're linear BEC's, which most are when built into ESC's.

Some receivers and associated equipment will work on as little as 3 volts (i.e. most Futaba) albeit at reduced performance. Others will get flaky.

Hence why I asked if he was running on 6 volts.

Most BEC's are adequate for the average sub model, assuming you have made efforts to ensure there isn't excessive binding in the linkages etc.
'Why are you staring at an empty pond?'

Want to dive your boat in crystal clear water? Then you had better Dive-in- http://www.diveintomodelsubmarines.co.uk
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Postby Davinci » Thu Jul 16, 2009 1:06 pm

Yes, the fully-charged battery voltage should be greater than 6.5 volts.
7.2 is good, 12. is better. :D

See, I can agree with you Sometime.
We don't have to argue All the time. :lol: LOL :lol:
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Postby raalst » Thu Jul 16, 2009 3:09 pm

sometimes the manufacturer specifies how many servo's his BEC can support. often, it's not a lot.
otherwise my rule of thumb is : you need 0,5 amps per servo.

and I agree with andy : your raw voltage should be a bit more than the
5 volts the servo's and receiver require. about 2 (minimal 1.5) should
do it.
Regards,

Ronald van Aalst

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Postby tsenecal » Thu Jul 16, 2009 6:24 pm

as raalst says, most car ESC BECs are designed for a SINGLE SERVO, the assumption being that cars only have a steering servo. this allows them to buy cheaper parts with lower amp ratings.


model airplane ESCs aren't, they are designed for 4 to 6 servos.

on my type II i use a seperate BEC and have the BECs on my CC mambas cut off.

I use this one, it has 5 amp output (10 servos by raalsts calculations)
http://www.hobby-lobby.com/ubec.htm?pSe ... ryId=86061

this one has a max of 10 amps, and is a switching BEC:
http://www.castlecreations.com/products/cc_bec.html
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Postby Skip Asay » Thu Jul 16, 2009 6:56 pm

"the fully-charged battery voltage should be greater than 6.5 volts. 7.2 is good"

"about 2 (minimal 1.5) should do it."

Au contraire. For a standard 5 volt linear regulator, a MINIMUM of 7.5 volts is required with more being better. The only exception to this is a "low drop out" regulator which can require as little as .2 volts over.

Generally speaking, the number of servos really doesn't make any difference since only 1 servo moves at a time.

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Postby subkraft » Thu Jul 16, 2009 7:28 pm

....Thanks to you all for the responses.

I've built 2 six volt systems and 1 12 volt. On the 12 I used a subtech voltage regulator and it seems fine. The 6 volters - one has a separate battery for the radio, the other a tap off the main battery for the receiver.
Been experimenting to see what works best.

What is the distinction between linear and switching bec's?

Regards

Ian
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Postby Davinci » Thu Jul 16, 2009 7:55 pm

I found good info about them at RC-Universe:

See Link:

http://www.rcuniverse.com/forum/m_7683693/tm.htm
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Postby Ramius-II » Fri Jul 17, 2009 1:13 pm

Hi All! :D
One additional consideration about voltage regulators is adding capacitors to the output of the BEC. Best is a combination of a .1mfd, a 1 mfd and a large value (1,000mfd or more). Voltage regualtors are know to oscillate (produce and AC component to the output) and if you look at the spec. sheets, a 1uf is typically recommended or required by the manufacturer to prevent oscillations.

Capacitors are like resistors for AC and the internal AC resistance varies by value and frequency. So for high frequency AC (such as the sparks produced by a motor) a .1mfd appears as almost a dead short.

The 1mfd stops the BEC oscillations and a large value capacitor will act as a voltage reservoir when a motor (such as in a servo) starts to move.

When motors start to move they will draw more current, thus lowering the voltage. When the voltage drops, the required current now increases.

Hope this is of some value.

Best, Ed
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