|May 13, 2009, 06:48 PM|
A Possible Explanation for Some Spectrum Problems at SEFF 2009
Iím not sure if you remember but I had a problem at SEFF with my foamy that I originally thought was due to a Spectrum ďlock outĒ. The incident confused me because until that point I had had no problems (at SEFF or before). I began to look further into the problem and I also kept your problem with the Slick in mind too. After a sizable amount of reading and analyzing I believe I may know what happened to both my foamy and your Slick.
I believe the problem is the speed controllers internal BEC (Battery Eliminator Circuit) Most ESCís have internal BECís that use a linear regulator. The linear regulator converts the input voltage (say 3s at 11.1V or 4s at 14.8V) to a usable receiver voltage (say 4.8-6.0V). It does this by converting the excess voltage (5-9V) into heat. The linear regulator uses resistance as a simple and cheap way to step down the voltage. The Airboss 80 has a Linear BEC. The Elite series uses a switch mode BEC which I will talk about later. Also, these BECís are rated assuming a 6 volt input source. As the input voltage goes up the BEC cannot convert all of the excess voltage into heat thus the output is reduced. This said it is possible that the internal BEC that is rated for 3A could put out 2A or less on 3s or 4s. The 4 HS-81ís that you were running could easily draw 2A under normal flight conditions (.5A each). All of this being said it is easy to see the problem.
The question still remains: why did the problem not occur earlier? Why was there only a problem at SEFF and not before? To get the answer we must return to the way in which the linear regulator works. The regulator converts excess voltage to heat. The one factor that everyone in the RC Groups thread on spectrum problems seemed to neglect was the temperature. The week of SEFF was the hottest it has been all year, with temperatures in the 80ís and even 90ís during the day. If the BEC is trying to dissipate heat an increase in ambient temperature impedes its ability to do so. When most BECís are unable to provide adequate amperage or overheat they will shut down and reset.
In my case I was running 4 HS-65ís and a light system pulling upwards of 1A through a BEC rated for .9A. It worked fine through the winter when temperatures were cool but on the first flight in hot weather it quit. (I put the lights and big servos on the foamy in October when the weather got cool. Before that I had smaller servos and no lights and everything was ok.)
In your case, we built the Slick in the fall (October or so). It performed great all winter. When the hot weather hit you had a problem. Also the twitching Buddy described after the crash would make sense it were trying to reset. A cycling on and off as the BEC was cooling down to an operational temperature would make sense. Once the BEC had cooled sufficiently it would supply voltage as usual and everything would work fine (as was observed).
In order to keep this from happening again I suggest we start using external BECís on all ESCís with internal linear regulators that are using 3s or more. The external BEC is a Switch mode BEC. To the best of my understanding this works by switching voltage on and off to maintain a constant 5 or 6V. in this set up there is not really any heat to dissipate. Some ESCís like the Eflite 60 and the Airboss Elite have a Switch mode BEC already integrated. These do not need an external BEC if you are using less than the recommended cell count.
In my opinion the external BEC is an inexpensive insurance policy for a $200 airframe. I have one Dimension Engineering BEC and really like it. After figuring out what happened I think my foamy and my Precision Aerobatics 260 will both get external BECís. Iím not too concerned about crashing the foamy but I would hate to lose my 260 (but I guess Iím preaching to the choir here). My Eratix has an external BEC (that I that I now know is too small too small). It really doesnít need it since the Eflite 60 has the switch mode BEC, so I am going to move it to the 260.
To the best of my knowledge this is what happened to us at SEFF. I hate that this learning experience cost you a great airplane. I think if we make some changes and pay more attention to our BECís that this will not happen again. I have included the links to most of the sources I used if you want to read up on your own. Also there is a link to Dimension Engineeringís Sport BEC which I think will solve your problem (Mac also has them). There is also a link to Dimensions very helpful article about BECís. As far as I can tell Spectrum technology had nothing to do with any of our problems at SEFF, so I still like my JR/Spectrum stuff.
I hope it helps. It was very wordy but I wanted to be precise and correct amidst all of the speculation that I was seeing on RC Groups.
Please leave your thoughts and comments.
HS-81 Specifications http://www.rctoys.com/pdf/hitec-servos/HIT-HS81.pdf
Airboss ESC http://www.extremeflightrc.com/html/airboss.html
Sport BEC http://www.dimensionengineering.com/SportBEC.htm
BEC Q/A page http://www.dimensionengineering.com/BECFAQ.htm
Phoenix 25 ESC Instructions http://www.rctoys.com/pdf/Phoenix-25...-Auto-Lipo.pdf
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