Large model yachts with large sail servos often employ the ‘Y’ lead to save putting large current draw through the voltage bus of the receiver. I suspect that is why they have been ignored by footydom (aparently).
There is a reason for us to use the ‘Y’ lead though which just dawned on me… the 6v torque figures printed on your little servo box. I will be using a ‘Y’ on an HS-81 on ‘Harpy’s’ sail control. You do need to use 1.5v cells though as rechargeable will only give you 4.8v anyway.
Graham
I don’t understand what you’re gaining with the Y on an HS-81. Receivers work fine on 6V.
Unless you think that you’ll get a significant voltage drop to the servo by going through the receiver (which I highly doubt).
–Doug
http://www.rcsailing.net/forum1/showthread.php?t=3860&highlight=BEC&page=2
see post 18. So my understanding is that the BEC adjusted voltage is that then supplied by the receiver to the servos. If that information is wrong and the 6v Rx input is ‘bussed’ to the Rx servo outputs then I am wrong and there is no need for a ‘Y’ lead.
Graham
I just read post 18 and I have never heard of a BEC in receiver before.
I’ve been flying RC electrics for the past ten or so years, and the only concern we have with BEC’s is that the ones built into ESC’s (motor controllers) are inadequate when you run a battery pack of higher voltage than 3 lipo’s in series. That’s around 11.1 volts. If you feed the typical BEC more voltage than that it gets HOT and can burn out. So we buy a separate BEC. Often one that uses a switching voltage drop circuit. Now why would we do that if there was already a BEC in our receiver?
BECAUSE THERE IS NO BEC IN THE RECEIVER!
We use the same RC gear as boats except for the frequencies, Berg, Futaba, Hitec, J&R, Airtronics and whatever. It’s all the same stuff. As mentioned, I’ve been flying electrics for about ten years, but my history in RC goes back to the dawn of proportional, and my first RC gear was kit built single channel stuff with an escapement and a button.
Finally, the issue with servos and very small receivers. The buss that feeds power to the servos in some very small receivers is just a thin trace that can’t handle much current. Sometimes when someone sticks four or six large servos on one of those receivers and moves a few controls at once, as in doing a snap roll, they get some glitching, because the resistance of that trace prevents it from supplying all the current needed. The servo sees that as a voltage drop. That is the only problem that I know of with voltage on small receivers that involves a buss, and it doesn’t apply to a footy with two servos.
I have heard some minor concerns about certain servos being run at six volts and the general advice is to limit those servos to about 5.5 v. max. That said, I will add that I have ignored that advice and run at six volts. (five nimh cells in an engine powered plane) I had no problems, but maybe there was some risk and reasons like that are why some people use a BEC with five cell nimh packs. These days I fly with lipo cells and mostly rely on the BEC in the ESC, unless I am flying four cells or more, then I buy a BEC to handle the excess voltage. There are ESC’s with switching BECS that can handle any reasonable number of cells, but they are high current, expensive things, and I only have one for a large powered glider that is waiting for some bench time.
In other words, based on my own experience, I see no reason to put a BEC in a footy and I have never before heard of a BEC being built into a receiver. If it were built in why would anyone need a BEC in the ESC and why would anyone make and sell BEC’s and likewise, why would anyone buy a BEC?
Pete
I run a Futaba FP-R112JE receiver in my Soling 1M that states it contains a BEC. It is only noticeable on heavy wind days when I have been sailing for more than 4 hours. I find then that I have no steering response while hauling in the sails to closehauled. Also no winch response while steering, but that is not so noticeable. The BEC is normally of use only in electrically driven boats where the drive power is taken from the same set of batteries as the radio receiver and the servos. It is intended that steering control should be retained when attempting to use full drive power. The function of the Y-connection harness is to allow such full power and still retain steering control. This is seldom of any importance in sailing boats.
Interesting. My RC experience is exclusively in airplanes where the BEC is a separate item and only of use in electric powered airplanes or, in a limited way when someone uses lipo in a giant scale airplane. The BEC is never in the receiver, but either in the ESC or an independent device.
I really can’t see the point in a footy and I don’t imagine that most of the receivers used in footys would have one, but it’s interesting to learn there is such a thing.
Pete
Well having googled a few things I am probably going to find that a Y lead is not necessary but at least I know why now so to maybe explain a few things as I see them.
This is the Futaba link to one of the most commonly used receivers, the one which comes wth the $39 Futaba 2DR set, I use four of these at the moment. http://www.gpdealera.com/cgi-bin/wgainf100p.pgm?I=FUTL0104
The receiver has BEC included as stated on the case. (and yes this originally came as a suprise to me too Pete after 18 years in electric flight and 36 years in RC generally). Input voltage to these type of Rx’s is 4.8 - 8.4 v. And considering the following I guess that what the BEC is doing is supplying the Receiver itself with 4.8v (or thereabouts) and feeding the full input voltage down the servo socket buss. How heavy that track is can be questioned but I now am sure that is no issue with the servos we use. Also this would mean that I am already getting 6v (from dry cells) to the servos and therefore already have the 6v torque without resorting to a ‘Y’ lead.
From the Futaba FAQ…
What is the voltage of the output signal?
Voltage out is based on Voltage in. Receivers typically operate on a 4.8V DC input voltage. This can be increased to 6.0V, where more power and speed can be derived from attached servos, but a decrease in lifespan for said items might result as well due to increased stress. An ABSOLUTE minimum operating voltage is very near 4.0V. Rx’s typically use a 3.3V regulator which requires an additional 0.7V to power itself, resulting in the 4.0V minimum. In application, however, it is typical for performance to become severely degraded as power falls under 4.3V DC.
So I am tossing the ‘Y’ lead I just made :rolleyes:
Graham
Most 75 and 27 MHz AM 2 channel receivers have a BEC because they are made for R/C cars and fast electric boats which run a six (7.2 volts) or seven (8.4 volts) cell battery for the motor. The BEC steps this down to 4.8 for the receiver and steering servo so a second battery pack isn’t needed. Receivers and servos can’t take that kind of voltage. These applications only have one standard servo and a speed controller which do not draw much current.
4 channel AM receivers designed for airplanes don’t have BEC. These receivers have been pretty much replaced by FM for airplanes.
The problem with BEC’s is that they are usually limited to 1 to 1.5 amps. This probably isn’t a problem with a footy and a HS-81, but in a meter boat with a quarter scale size winch servo it can be a problem. Even with just a 4 cell battery pack, it is a good idea to use the Y connector so that you can get the full current from the battery directly to the winch servo. Voltage gives you speed, current gives you power.
I’ve never seen an FM receiver with BEC, and you usually see BEC’s in speed controls for airplanes and boats. Most BEC’s in speed controllers (and winches like the RMG) warn that if you use more than 2 or 3 servos, don’t use the BEC because they can’t support the current load. For the RMG winch, the red power wire is cut between the winch and receiver, and the receiver and servos are powered by a separate battery.
Even without a BEC it is not a bad idea to use a Y connector with small FM receivers if they have small power bus traces on the circuit board. It is possible if you stall a big winch you can burn out these traces. They are not really design for high current loads.
Bert
When the factory (Hitec) includes them with their big servos (like the 815BB) and when Tower Hobby goes to the expense and effort to recommend the “Y” harness with the big servos, I’m not sure why we would think anything else than to use them?
