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Nov 24, 2015, 10:43 PM
Balsa addiction since age 3
ScottSails's Avatar
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Discussion

ESC with SBEC (3A) PLUS using a separate SBEC (3A)


I have not found this here so thought I would post what I am trying to do and get some constructive criticism.

I have an electric powered glider with 4 small digital servos (EMax ES08MDII) that draw 660ma at stall at 5V to be used for 2 ailerons and 2 flaps. I also will have 2 similar servos in the fuse for rudder/elev.

The only servos that I expect to possibly get near stall currents are the two flap servos at higher speeds in a controlled dive at "high" speeds. The ailerons are limited in throw, the flaps will go 90 degrees.

So, my plan is to use the 3A switching BEC from the ESC to power the servos and receiver in the fuse and a separate, standalone, 3A switching BEC for the wing servos and SBUS Decoder (FrSky, allows a single connection to the wing for all 4 servos).

Only grounds would be tied together, not 5V. This seems like it would be a common way to get multiple power sources without going to say a separate standalone 10A BEC?

I am looking for others that have tried this and either had success or failure.

Thanks,
Scott
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Nov 24, 2015, 10:56 PM
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Wintr's Avatar
I use a similar setup in aircraft that have flaps and retracts; receiver and main flight servos powered from one BEC, flaps and retract motors powered from another. As long as the 5V outputs are isolated, I don't see how you can have a problem.

Another way would be to connect both BEC outputs to a common supply through a pair of Schottkey diodes, but there will be some voltage loss; if the BECs can be adjusted to about 5.5V, that wouldn't be an issue, either. Assuming both BECs have the same output voltage, the diodes would ensure fairly equal current sharing.
Nov 24, 2015, 11:02 PM
Balsa addiction since age 3
ScottSails's Avatar
Thread OP
Quote:
Originally Posted by Wintr
I use a similar setup in aircraft that have flaps and retracts; receiver and main flight servos powered from one BEC, flaps and retract motors powered from another. As long as the 5V outputs are isolated, I don't see how you can have a problem.

Another way would be to connect both BEC outputs to a common supply through a pair of Schottkey diodes, but there will be some voltage loss; if the BECs can be adjusted to about 5.5V, that wouldn't be an issue, either. Assuming both BECs have the same output voltage, the diodes would ensure fairly equal current sharing.
Ah thanks. I had looked at using diodes to provide some redundancy, but the current sharing would be good. The BECs I have aren't adjustable so I didn't want to have them "fighting" each other or generating noise.
Nov 25, 2015, 02:23 AM
Registered User
Quote:
Originally Posted by Wintr
......Another way would be to connect both BEC outputs to a common supply through a pair of Schottkey diodes, but there will be some voltage loss; if the BECs can be adjusted to about 5.5V, that wouldn't be an issue, either. Assuming both BECs have the same output voltage, the diodes would ensure fairly equal current sharing.
Diodes are unlikely to result in load sharing. The highest voltage BEC will always supply the whole of the amps load but, if that BEC fails, the other one will then kick in.

The original suggestion of one BEC supplying some of the servos is the only way to divide the load. So one BEC goes to receiver as normal, and a couple of servos go to the receiver as normal. But the servos that are going to be fed by the other BEC have only their ground and signal wires connected to the receiver, and then have their ground (you'll have to splice another ground wire onto the existing) and power wires connected to the other BEC.

I split the power that way when I have retracts, so they can't jeopardise the radio's supply if they stall.
Nov 25, 2015, 03:50 AM
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scirocco's Avatar
IIRC I did a similar thing also using a FrSky decoder to isolate retract and flap servos on a 17cc gas P-47. As I recall, I plugged a Y-lead into the receiver S-Bus port with the centre red wire removed from the plug, the decoder into one arm of the Y and an SBEC into the other arm of the Y. And then the decoder happily fed dedicated power to the 3 servos involved.

In the end though I looked at how much I was drawing from the packs and keeping multiple packs charged, decided it was overkill and reverted back to a single power source - an original Koolflite Ultimate BEC, which is rated for 3A continuous 5A peak and runs 8 servos.

Personally, I think 3A is plenty for the setup you describe. Even when your flap servos are working hard, the 4 others will be experiencing very low loads at glider speeds and control deflections. Another option is to look at the Castle Talon 35 if your motor power suits - it has a 7A peak / 5A continuous BEC, where peak is defined a long enough for typical servo transients.
Nov 25, 2015, 06:25 AM
Balsa addiction since age 3
ScottSails's Avatar
Thread OP
I was/am on the fence on whether 3A is enough, hence I purchased some "5A" BEC's. Turns out they are really 3A (LM 2596 based) but they do pull continuous 3A and hold voltage (and also heat up to 140F in about 2 min).

The Castle Talon would solve the problem as well, thanks for that info. (I typically have used $30 hobbywing ESC's and don't know much about Castle). I did buy a YEP 40A with 5A rated BEC (but I have not measured or tried it). I should have figured the Helo guys and/or big scale guys had dealt with this problem.

I did get a little concerned with length of wires and I can run a longer group of wires from the battery to the Wing BEC and worry less - it will probably be 12-18" Long and likely 22ga wire.

Thanks all for the input!

Scott
Nov 25, 2015, 01:53 PM
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scirocco's Avatar
I have a YEP in a 6 servo Pulsar 3.6. Does the job just fine.
Nov 25, 2015, 02:29 PM
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Wintr's Avatar
Quote:
Originally Posted by abenn
Diodes are unlikely to result in load sharing. The highest voltage BEC will always supply the whole of the amps load but, if that BEC fails, the other one will then kick in.
Common diodes, yes, but Shottkey diodes will change conduction Voltage more linearly with current, so, unless the sources are more than a couple of tenths different, the diode feeding the most current will increase its Voltage drop until the other diode begins conducting more. Sharing isn't perfect, but when one source reaches the current limit, its Voltage will drop just a bit more, allowing the other source to catch up.
Nov 25, 2015, 10:03 PM
Registered User
There's a thread about using a dual Schottky to provide backup power (sorry I can't give the link because I can't figure how to do it on my tablet), and the possibility of the load being shared is discussed in some of the posts. My recollection is that the conclusion was that sharing, rather than just switching when one source fails, was unlikely to happen in practice. Maybe with identical voltage sources, but how likely is that, given manufacturing tolerances?
Nov 25, 2015, 10:14 PM
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Wintr's Avatar
As I said, sharing is not perfect, but, unless the sources are different by more than a few tenths of a Volt, it is still relatively even; e.g.: you may get 3A from one BEC and 2A from the other, closer in some cases. This is due to the semi-linear forward Voltage change with current of the Schottky diodes; it doesn't take much increase in current to shift that Voltage by 0.1V. At low currents, all the power may come from one BEC, but, as the current rises, the other will begin to contribute.
Nov 26, 2015, 05:12 PM
Balsa addiction since age 3
ScottSails's Avatar
Thread OP
I think this is the thread that was referenced on the Schottky diode..

https://www.rcgroups.com/forums/show...60&postcount=1
Nov 26, 2015, 09:44 PM
Registered User
Thanks ScottSails, that's the one I was thinking of. The discussion about the possibility of sharing the load starts around post #272.

(Must learn how to select text, copy, and paste on my Android tablet!)


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