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Thread: basic Adding light question

  1. #11
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    I think he's confusing a 30A ESC with the rating of the BEC, most of which are 2A to 5A. At a draw of just 50mA to 100mA, using the main receiver BEC as the power source is not unreasonable, whether you use a separate battery or the main battery. Powering the LEDs independent of the receiver would require a male-to-male cable, if I understand the connections of the device he referenced. Whether to use a separate BEC, or the BEC within the ESC, depends on a lot of factors, like how many servos and servo-like devices, like retracts, are powered through the receiver, and the rating and type of BEC. One could also use both BECs by combining them through two Schottkey diodes, assuming you can set the voltage above 5V (like 5.5 or 6) (perhaps too technical for the beginner, though).

  2. #12
    Yeah. I did make that mistake. Should all about BEC not ESC. So do you guys mean as long as I use a 5V external BEC, no matter how many 5V servos and other device in RC Plane still no Problem if we don't talk about the weight?

  3. #13
    Diamond Member Fred Huber's Avatar
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    Depends on the plane, size of servos, number of servos...

    A single "standard" servo can pull over 2 amps if loaded to wher it can't move the load.

    A high power servo such as the older HS-5955 can pull enough amps to melt down its own lead if you stall it.

    1/2 amp will fry some of the smaller micro servos.

    I try to supply enough power to allow for half the servos in the plane being stalled and the other half at 50% of stall current. You can get that much current demand in a 6 ch airplane in a high speed snap-roll.

    Then add the lights...

    You can either do a lot of testing to gather data on what current your system actually demands in flight... or you can install guaranteed overkill by expecting the servos to be loaded heavily.
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  4. #14
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    I'm with Mr. Huber on capacity. I have a 55" Cherokee with 6 full-size digital servos and retracts. I power the receiver and servos with one 5A switch-mode BEC, and the retracts with a separate BEC, since retracts are more likely to stall. It also has a separate, small 3S battery powering those BECs, leaving the main 4S battery to only power the motor. In my 50" T-28, I have a separate BEC for receiver, 6 9g servos, and retracts; motor and BEC powered by the same 4S battery. In slower, lighter foam aircraft, I let the BEC in the ESC provide all power, if it is a switch-mode type; otherwise, I use an external 3A BEC. You have to base your power source(s) on the size and kind of model it goes into.

  5. #15
    Oh so you can use plug 2 BEC into the receiver for different devices? Then for what purpose when add an external BEC we need to disconnect the BEC which integrated in ESC you have mention before?

  6. #16
    Diamond Member Fred Huber's Avatar
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    Some BEC circuits "do not play well with others"

    2 or more might try to "backfeed each other"... each sending current backward through the other(s) diring the portion of time they are off (switching mode) or trying to forec the low voltage supplying one(s) up to meet the high one (linear type). The result in either case is the same... ALL of the BEC circuits overheat... some will go into thermal shutdown (if they have that feature) the rest will catch on fire.

    You can add "Shottkey" diodes to prevent the backfeed issue completely. This does rob a little voltage and power from what is fed to the RX system.
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  7. #17
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    When I power something separately, like the retracts, I use a Y cable as an isolator; the connector that mates the receiver has the center wire pulled out of it, and the second BEC is plugged into one of the servo connectors, allowing it to power whatever is plugged into the other servo connector. I use a heavy duty 22ga Y cable for this, both for current capacity and strength. In this way, whatever is connected will still get the channel control signal from the receiver, and the grounds are common, which allows the signal to be 'read' correctly. In your case, if the LED controller does not need any signals from the receiver, you could just connect the BEC directly to the controller with a cable that has pins on both ends, AKA male-to-male.

    The Schottkey diode isolator trick is so that you have backup power, in case the battery to one source (BEC), or the source itself, fails. This is fairly common in large, expensive models that you don't want to crash just because a wire broke or a battery failed. It also allows for a current boost if a servo needs more than one BEC can provide.

    Schematically, my separate power method looks like this:
    Last edited by WintrSol; 11-19-2014 at 09:37 AM.

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