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Do I need a BEC?

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  • #21
    BTW, it you have to power a lot of servos, one of these may help: https://www.servocity.com/servos/ser...g/power-boards

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    • #22
      What is the best voltage setting for the Castle10? It is default for 5.1volt, but I have some planes with digital servos. Would 5.5volt be better?

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      • #23
        You need to search for the Voltage limits of the servos. Some are rated 6V (five NiCd cells), so can go up to about 7.5V, the full charge of a NiCd pack, and others say 4.8V, so maximum 6V. Newer servos can be powered directly by 2S LiPos.
        I generally use 5.5V, just to make the servos a little more responsive, at the expense of a bit more current to the analog servos.

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        • #24
          Expert opinions please I'm working on a plane , and this is my first where i'm going to have a seperate BEC.
          I have all my parts now for my Hanger 9 Spitfire. Ready to do my soldering/wiring of my ESC and BEC. And start assembly of this Spitfire
          Question on wiring. Should i NOT deviate from the provided wiring diagram? OR....as in the pictures i took, wire my ESC & BEC in the way i've laid it out in the pics? Pic 2 & 3 are the same configuration. As you can see, going 12s.

          Thanks,
          Lon

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          • #25
            Originally posted by Lon View Post
            Expert opinions please I'm working on a plane , and this is my first where i'm going to have a seperate BEC.
            I have all my parts now for my Hanger 9 Spitfire. Ready to do my soldering/wiring of my ESC and BEC. And start assembly of this Spitfire
            Question on wiring. Should i NOT deviate from the provided wiring diagram? OR....as in the pictures i took, wire my ESC & BEC in the way i've laid it out in the pics? Pic 2 & 3 are the same configuration. As you can see, going 12s.

            Thanks,
            Lon
            Lon........Don't deviate from the drawing. The way you have it laid out on the table is a serial setup for 12S which is fine so far until you jam that down a 6S BEC.
            In the drawing the BEC is only getting fed by one (1S-6S) battery.

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            • #26
              Originally posted by OV10 View Post

              Lon........Don't deviate from the drawing. The way you have it laid out on the table is a serial setup for 12S which is fine so far until you jam that down a 6S BEC.
              In the drawing the BEC is only getting fed by one (1S-6S) battery.
              Thanks OV10!!

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              • #27
                Originally posted by Lon View Post
                Expert opinions please I'm working on a plane , and this is my first where i'm going to have a seperate BEC.
                I have all my parts now for my Hanger 9 Spitfire. Ready to do my soldering/wiring of my ESC and BEC. And start assembly of this Spitfire
                Question on wiring. Should i NOT deviate from the provided wiring diagram? OR....as in the pictures i took, wire my ESC & BEC in the way i've laid it out in the pics? Pic 2 & 3 are the same configuration. As you can see, going 12s.

                Thanks,
                Lon
                Two issues: (1) Depending on the BEC, it might not be spec'd to tolerate 50v. It might immediately blow up if you put 50v across it. (2) Even if it is spec'd to tolerate 50v, the performance will be worse at 50v than at 25v. A switcher works by drawing from the battery a fraction of the time - if you switch 50v down to 5v, then it will draw current from the battery 10% of the switching cycle, and the other 90% of the switching cycle the current will be supplied by the inductance & capacitance of the passive components in the BEC. If you are switching from 25v to 5v, then 20% of the time the battery will supply current, and 80% of the time it will come from these passive components. So they can supply more current in the latter case because they have to do so for a shorter time. Somewhere in small print in the spec sheet for your BEC you might see this mentioned. Or you might not. They might call it a "10A BEC" on the box for example, but in fact the current decreases as headroom increases. The current number highlighted on the box is usually towards the higher end of the range. The current you will get with a headroom of 500% or 1000% is a lot lower than what the box says.

                Also, they emphasize this in the instructions, but make sure 0v is the same in your BEC and your ESC - 0v side of the BEC MUST be the black wire going into the ESC. If you connect your BEC across the other battery, effectively 25v-50v, then it will start to operate as normal, because it still sees 25v across it. But its ground goes to the receiver. This also has a ground from the ESC, which is connected to the black wire of the battery. So now you just shorted the two terminals of the low-side battery though your ESC and receiver. Kaboom! Most likely the receiver blows up first, but it can damage both. I have seen this done before, and fortunately the receiver became a flaming open-circuit quickly enough that the ESC survived.


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                • #28
                  on my 12S aircraft I use a separate 2 cell LiFe
                  (actually 2X 2S LiFe and a Spektrum PowerSafe RX)

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                  • #29
                    Originally posted by fhhuber View Post
                    on my 12S aircraft I use a separate 2 cell LiFe
                    (actually 2X 2S LiFe and a Spektrum PowerSafe RX)
                    Me too, exactly the same. It is a bit of extra work to take these in and out and charge them, but I'm sure they can supply much more current than any BEC. And the redundancy is good. And I don't need to charge them every flight, just once for the day. Also you have to make sure all elements in your system can withstand 7.2v peak, whereas a BEC can be set for whatever output voltage you want.

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                    • #30
                      Originally posted by Gringotuerto View Post

                      Me too, exactly the same. It is a bit of extra work to take these in and out and charge them, but I'm sure they can supply much more current than any BEC. And the redundancy is good. And I don't need to charge them every flight, just once for the day. Also you have to make sure all elements in your system can withstand 7.2v peak, whereas a BEC can be set for whatever output voltage you want.
                      It is best to check that all servos are rated the same voltage regardless of how you are powering the RX system. Then supply the correct voltage.

                      A local club member was trying to make an airplane work with a mix of 4.8v, 6.0V and 7.4v nominal servos. The 4.8 v servos could not withstand more than 6.0v. The 7.4 v nominal (rated for 2S LiPo direct) servos would not operate properly below 6.0v. he was supplying the system with 5.0v regulated. The result was constant issues with ALL of the servos, preventing it from ever passing a pre-flight control system test.

                      He sold me the aircraft, before I discovered the cause of the problems. Its getting ALL 6.0v nominal rated ( rated to handle 7.4 v maximum) digital servos installed. Even the 9 gram servo for the choke. (strange location for the choke servo required the small servo)

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