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As some of you may know, I'm building an RC Revell 1/72 Gato conversion. The drive motors will be running @ 6V (five NiMH in series). Under these circumstances, would it be safe to run the Rx off the main battery? Big Dave advises relying on the ESC BEC while Dave Merriman advises setting up a separate BEC because most ESC BECs do not have sufficient current capacity. I have seen several ads indicating that the featured receivers could be run on 6V or possibly up to 7.2V. Any advice would be much appreciated.
As Andy suggests, compatability with 5-cells veries from manufacturer to manufacturer. I can speak to having run Futaba receivers with a mix of Futaba and Hitec servos for many years in R/C aircraft without failure.
Another interesting detail to be aware of is that given a 4-cell and a 5-cell battery pack of identical capacity, 4 cells will give you a longer run then 5...
http://www.subcommittee.com/forum/icon_biggrin.gif Thanks guys. Sounds that
Thanks guys. Sounds that I might be able to do without the BEC after all! There are a number of reasons that I would prefer to forgo the BEC. 1) I like to adhere to the KISS principle wherever possible. 2) An ESC without BEC is less expensive. 3) As far as I Know, all voltage regulator circuits operate at some level of inefficiency. This is why they usually require some form of heat sinking. I'd rather save that battery power to operate the model.
You can get some that are 97% efficient (might have to DIY though) and as Kevin said if you run at a higher voltage you decrease the run time. So I wouldn't necessary use increased run time as an excuse
Great to know that some voltage regulator circuits can reach 97% efficiency. I have no idea how efficient most ESC BEC circuits are though. I doubt if most get as high as 97% I have been considering using the Mtroniks 10A Viper Micro Marine. It has a 5V 1.2A BEC. I'm running 6V because I want reasonable drive motor performance. I had considered 7.2V but space considerations make it impractical in my case. Given the same capacity but higher voltage, one would expect some loss of efficiency. At higher voltages, the whole drive train would be operating at higher rpm so more waste heat would be generated. The motor specs are important as well. Motors are designed to operate most efficiently at a certain voltage and amperage.
From what I can see there is no correlation to the price of an ESC being cheaper without a BEC. There are expensive and inexpensive ESC's with and without BEC's. Actually most of the high end ESC's tend to come without and are more expensive. Just my 2.5 cents. BD.
sigpic"Eat your pudding Mr Land"
"I ain't sure it's pudden" 20K
Thanks Dave. Guess you're right. I like the Mtroniks micro ESC because of it's extremely small size and features. It's only 26mmx25.6mmx9mm, weighs 15g and is 100% waterproof! The BEC provides 1.2A @ 5V (6W) which should be more than enough to run the Rx and the required micro servos. Of course, this begs the question as to why Dave Merriman insists on disabling the ESC BEC and setting up a stand alone BEC. Unless one is using large servos with very stiff linkages, 6 watts should be plenty for a sub of this size.
Well, hobby servos draw about 300ma MAX so worse case scenario is that you have enough juice for only four servos. Add to that any extra functions you want and of course the RX and you could reach your limit....
Most all of us 8-Balls use the Mtroniks ESC'S in one sub or another. I use the Viper-15 in my gato. Dave's way of bypassing the ESC is actually the best way to it. Most of my larger subs have stand-alone R/C power. Sometimes we lack the room or decide to do it the easy way. Not necessarily the safest way!! BD.
sigpic"Eat your pudding Mr Land"
"I ain't sure it's pudden" 20K
Thanks again folks. You have given me a lot to think about. On the surface, only the rudder servo will be working. Submerged, the rudder and fwd plane servo will be working most of the time with occasional help from the aft planes for pitch control. I doubt that with micro servos, I would exceed the BEC 6W max power rating. However, I'll measure the max current draw on the bench before I hit the water.
Something to keep in mind.....the servos DO NOT ALL MOVE AT THE SAME TIME. The pulse train consists of a start pulse followed by ch1, ch2, ch3, etc. and ending with a stop pulse, and then it starts all over again. Now this all happens at a frame rate of about 15 - 20 milliseconds which gives the impression that everything's happening at the same time but it isn't. So what that means is that that 1 amp BEC should work just fine in our applications.
This has been a very interesting string of posts, and the responses to Mermaid have been very useful, but Skip Asay has provided some information that is priceless. I've been building R/C boats since long before digital equipment was available, but never knew that the pulse train to the servos was serial in nature. It takes the uncertainty out of powering up a multi servo R/C system with a less robust BEC than casual obsevation may suggest. Skip, thank you very much. You is DA MAN.
Actually, I am not all too sure about the servos not moving at the same time. I agree that the pulses do not arrive at the servos at the same time. However, a servo compares the requested position with it's actual position and moves it's arm accordingly. Now, how long that takes depends I would say. It depends upon the desired and actual position, the servo's max. speed of movement, possibly the load, and whatever else there might be. And that time can definitely be longer than the 20 milliseconds to the next servo-pulse.
So if you now have several servos going towards their new positions they will draw current at the same time, thus loading the BEC.
Also, maintaining the current position might draw current as the load on the servo is high: for example keeping the rudder steady in a side-current.
Of course, most of the time only one or two servos will move at the same time so, most of the time, the current draw from the battery will be moderate.
Thing is that you want to be certain (in my view, that is) that the entire system of battery, BEC and servos can deal with the extremes of the patrol: emergency surfacing, fighting the seaweed, octopi, ducks and what else might be threatening. Especially at those moments you want to know that your setup can handle it all, not just the average no-problem patrol.
I am currently designing my own electronics for my sub (a Nautilus) and have designed a power supply of 2 amps to the servos and dive-electronics and stuff.
Guestimating some 5 servos or so at 300mA each and a bit for the rest, I know that everything will be okay.
Another thing with regards to the number of cells; be aware that freshly charged batteries can have quite a higher voltage than the 1.2V. 5 cells might then deliver perhaps up to 7 volts which can damage electronics. The specs of the receiver and the servos wil tell you about that.
In my experience these faults may not show immediately, yet may cause a degrading over time, or certain electrical characteristics to be substantially outside their normal parameters. This need not be a problem, in fact, most of the time it wil be perfectly okay. Nevertheless, a BEC to keep the voltage okay is preferred I'd say.
Mermaid, your approach of measuring the max current an go from there is most certainly a very good way to go.
Great stuff everyone! This info is indeed priceless not only for me but for anyone who is setting up an RC sub control system. As I said, I'm going to measure the max control current on my model and act accordingly. I don't think that going on faith is an option on this important topic. I would hate to misplace my faith and loose the boat as a result.
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As some of you may know, I'm building an RC Revell 1/72 Gato conversion. The drive motors will be running @ 6V (five NiMH in series). Under these circumstances, would it be safe to run the Rx off the main battery? Big Dave advises relying on the ESC BEC while Dave Merriman advises setting up a separate BEC because most ESC BECs do not have sufficient current capacity. I have seen several ads indicating that the featured receivers could be run on 6V or possibly up to 7.2V. Any advice would be much appreciated. 
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BD.
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