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Hooking up 2 BEC without going thru RX
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rcs2022
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Hooking up 2 BEC without going thru RX - 09.12.2012, 05:15 PM

Need some Help Please!

Was trying to hookup my Servos to an External BEC without going thru the Rx, as i am planning on running the Steering servo on 7.2v and the throttle on 5.5v or 6v and both from external BEC's.

This is the Layout i have in mind, which I actually tried but it did not work?!?! any thing wrong i am doing?



If i opt to go thru the esc as per the layout on Brian Page, Script Asylum; What is the maximum amperage / current which the rx will handle. and how do i connect both servos?





Apologies if i am double posting again in a separate thread.
   
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BrianG
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09.12.2012, 06:03 PM

Your top diagram looks almost OK - just move the black wire of the BEC to the battery pack negative (you have the BEC ground floating at 1/2 battery voltage - which is a big no-no).

Receiver's current capability is pretty much only limited by the size of the traces connecting the + and - pins, so around 3-4A before voltage drop starts becoming a concern. But since you are wiring the power to the servos separately, you don't have to worry about this.
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rcs2022
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09.13.2012, 01:09 AM

Thanks a lot for your reply BrianG, just let me comprehend this;

The black wire of the BEC you are referring to is the incomming to the BEC or the outgoing.
If it were the incoming i can just hookup my BEC to battery 1.

or should i take a black wire lead from the BEC out (-) to the Battery (-)
   
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rcs2022
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09.13.2012, 01:34 AM

Should this work: ?
The Changes:

1. Added the second BEC.

2. Rearranged the (+) and (-) of the batteries, ESC and BEC so they are all hooked up to the same (-)

3. Added a (-) wire lead from the BEC out to the Battery (-)



I'll try to try it soon and post my findings.

And BrainG, Thanks again!
   
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09.13.2012, 02:05 AM

Nope, still wrong, you are tapping off of one battery, you need to power the bec(s) off of BOTH BATTERIES.

In your second diagram you have the bec's positive power source hooked up to the positive of pack 2, leaving out pack one and only giving you half voltage.

Leave the negatives where they are on your second diagram, but move the positive power source for the bec(s) to the positive wire on pack one (not pack 2 like you have it.

To clarify- you want the full 6s powering your bec(s), and you are trying to tap off one battery. You should use the same positive and negative battery cable you plug into the esc to power the bec(s)

Your new way solved the floating ground issue, but you always want to power off of both packs, otherwise one pack will be depleted before the other, and that's no good in a series set up.


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rcs2022
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09.13.2012, 02:32 AM

Thanks for your input Dr. _paralyzed_, notwithstanding the issue of draining the batts, would it work. Lets say i put in another small 1000mah 3S lipo or nimh receiver pack meaning another source to power the BEC, the grounding to get the proper signal has to do with the negative (-) of the BEC "out" or "in"?
   
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Arct1k
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09.13.2012, 08:36 AM

I've had issues in the past with this as no common ground.

You just need 1 bec for both servos. The only time I've seen the need for more was on a super rock crawler with multiple 7980s.

Just wire bec to esc input leads and everything will be fine.
   
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suicideneil
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09.13.2012, 11:34 AM

You have the BEC connected to the wrong battery- it must only be connected to the main ground, and the positive wire coming from the same pack:



( ignore the rx and servo wiring as it doesn't apply to you )

This is how how Castle says to wire up the BEC when running a couple batteries in series. You could also wire it to the main ground, and main positive on the other pack so that both lipos power the BEC, but it isn't required as such ( a larger step-down in voltage results in the BEC running warmer and supplying less current, so running it from a single battery makes more sense ).
   
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rcs2022
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09.13.2012, 02:58 PM

First of all apologies again for having the same question in 2 separate threads... my fault and it wasn't a good idea.

Anyway,

Thanks suicideneil, from what you posted I think I'm understanding the issue a bit more, I surmise if the rx is powered from the same source powering the servos, things should workout.

Brainstormed the information gathered here (I'm no electrical Engineer); got some time and did some bench test trials; first diagram:

Have the external BEC get the same power source as the ESC.
RX is powered by the internal Bec of the ESC
Servos are powered by the Exernal BEC

Tested on bench and it worked.



The second trial, what i understood is that the if the RX is powered from the same source powering the servos, things should workout. Well also tried it on bench and it also worked;

Got a separate third 3S battery as source for the external BEC.
Disconnected the red wire between ESC and RX.
Powered the RX from the external BEC out.
Servos obviously get power from the external BEC out.
the only drawback is if i have the external BEC set to 7.2v it would fry the RX i think.
The following diagram should clarify this:



Yet to try, but i think should work;

Same scenario as in the second diagram;
Add a another external BEC 7.2v to power the steering servo.
The 6v external BEC would power the throttle servo and the RX.
Both external BEC's get same power source, the third battery.

AND AGAIN THANKS TO ALL FOR THEIR INPUT; IT WORKS
   
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BrianG
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09.13.2012, 04:31 PM

I personally wouldn't use the 2nd diagram.

First, the ESC's BEC can power the receiver by itself with ease. Also, putting the receiver on a different BEC circuit than the servos might provide cleaner power to the receiver (and less chance of odd "reboots" if using Spektrum).

Second, there is really no benefit to using a separate battery for the BEC. It's just one more thing to charge and more weight. Any gains in runtime using a seperate battery for the BEC would be canceled out by the weight of that battery. If you really want to put the servos on their own battery, just get a 2s LiFe pack and hook it directly to the servos without a BEC at all.
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rcs2022
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09.13.2012, 05:13 PM

The second diagram was a test if i needed to hookup 2 BEC's; not for power but to have 6v and 7.2 v respectively for the throttle and Steering servo. I can run (which just finished hooking up on one 6v bec) both servos on 6V which would be sufficient and ok. The servo i have switched to is the HK15298B; 18kg-cm on 6v and 20kg-cm on 7.4v; a 18kg-cm is already a substantial increase over the 12kg-cm of the SF-50 stock Savage Flux servo. Or i might have to find myself a decent and cheap 7.4 v for the throttle.

Got the HK to see if 120 or 150 usd for a higher torque servo like one of these have my eye on:
Hitec HS-7955TG or Hitec HS-7950TH

Probably then would stick to the 7955 as it is max rated at 6v.
   
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Arct1k
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09.14.2012, 10:04 AM

2 will work as you are creating a reference ground between the batteries.

I dont really like it though as its going from battery -> esc -> rx -> bec -> battery.

Also by only having the BEC across a single battery you will be always drawing extra power from a single pack and puting the packs out of balance which just means more time on the balancer or worst case you will mess up your voltage cut off.
   
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