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Old Sep 25, 2012, 10:18 PM
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Canada, ON, Windsor
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That's what I meant in my previous posts regarding BEC's. Most are "Power Limited" meaning as the input voltage goes up the output current will drop. AND, as I stated, those ratings are usually in a minimum 5MPH cooling airflow. Without airflow that rating is a lot lower. It may take a bit to heat up and hit thermal shutdown, but it will eventually.

NOTE 1: It does not matter what is creating the heat load. Either a high current draw in the BEC circuitry itself or the ESC shunting a lot of power to the motor can run the temp in the unit up to shutdown. That is why I asked if your ESC has a tag stating the max BEC output current.

In any case, many of the onboard BEC circuits are only rated for 1-3A feed to the flight control system. Borderline in my opinion, especially if you are running a few extra servos, (or in your case the lighting). You may have started out right at the max limit and the extra lighting you added pushed it over.

If you have digital servos the problem is exacerbated as they are current hogs by design. A normal analog servo updates position 50-60 times/second. This is the source of the "Buzz" you hear/feel if the servo arm is loaded one side or the other. In comparison digital servos update 250-300 times/second and frequently "Squeal" under load. They are far more precise, but any motor has the highest current draw on startup and when that happens within the servo up to 6 times as often you can do the math.

In fact, on larger digital servos the current draw under load or stalled can exceed 1.5A EACH! Due to that I opted for 2S LiFE packs, (A123) running through 20A Castle PRO BEC setups in my larger scale planes. Life is simpler now since a lot of the newer servos are what is called HV rated and can run directly on a 2S LiPo, (7.2V) with no longer the need for a BEC at all, but I have yet to invest in changing all mine over as it would have to be a complete systems upgrade. Unless every servo is upgraded I would still need a BEC for any of the standard voltage servos which are basically limited to 6V.

NOTE 2: By adding a stand-alone BEC you have several advantages. First is the option of getting one with a higher current output right out of the box. Second many of the "Switching" type BEC's have the option of setting the output voltage to 5 or 6V. That higher voltage is still fine and gives slightly faster servo speeds with higher torque. Switching type BEC's are also more efficient as they do not drop the battery voltage down to what your system can tolerate by dumping the excess as heat. They do just as the name suggests by switching the output ON and OFF rapidly and smooth that output with some internal buffering. From what I have found the switching circuitry is relatively easy to design. Quieting the noise on the output is where most of the work goes and where I suspect a lot of the lower end units will skimp. However, having said that most of the new units are a lot better.

Finally, by physically separating the ESC and BEC you can avoid any heat sourced from the ESC itself getting to the BEC and adding to any thermal load. All the stated advantages make the system far more reliable for little extra cost or added weight.

I forgot to mention previously that if you run a separate BEC, along with pulling the stock RED wire you MUST ensure all the BLACK wires are still connected. This will happen anyway if you simply leave them within the plugs. This gives a common ground to help eliminate electrical noise and sets a system wide common voltage reference.
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Last edited by Cougar429; Sep 25, 2012 at 10:32 PM.
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