Background: Pre-electric R/C airplanes used a receiver that was powered by a 3-pin, plug-in 5-volt NiCd battery, which also powered the 3-pin plug-in servos. Each servo uses a dc motor but the input electronics actuate the motor only if the servos are given a pulsed input, varying from 0 to 5 volts with a duration of about 0.001 second to about 0.002 seconds (1 to 2 milliseconds), at a nominal rate of about 50 times a second. When electric power arrived, it plugged into the throttle channel and therefore the ESCs (speed controllers) were designed to also control the motors, between off and maximum power, with this same 1 to 2 ms pulsed input.
And to let the motor battery also power the receiver and servos, the ESCs were designed with a BEC (battery eliminator circuit) that dropped the battery voltage down to the 5 volts required by the receiver and the servos. Finally, in our use of these ESCs, we use the BEC to power our timers (“flight managers”) that provide pre-programmed pulses to the ESC over the duration of a flight, emulating the output of the throttle channel on an R/C receiver.
Most ESCs use linear voltage regulators to convert the battery voltage to 5 volts for the BEC. These linear regulators are cheap and simple but they must dissipate power equal to the drop in voltage times the current drawn. Thus, for example, an ESC that is powered by a 5S LiPo battery (nominally 18.5 volts) and provides 0.1 ampere @5 volts to the receiver and servos must dissipate (18.5 – 5.0)(0.1A) = 1.3 watts as heat in the ESC. Because of this heat problem, ESC manufacturers limit the number and type of servos that may be powered by the BEC, based on the number of cells in the battery.
However, we don’t use receivers or servos (usually) and our timers require very little current (a few milliamperes, at most) and so (with the Phoenix ESCs, at least) I have never seen a heating problem with using the BEC to power my timers up to at least a 4S (14.8 volt) LiPo battery, even though the manufacturer says not to use the BEC to power any servos when using more than 3 cells. (For use with 5S batteries, I put a small linear regulator on the same circuit board with the timer, saving the expense and weight of an external BEC).
But R/C airplanes, especially large “3D” ones, require big and power-greedy servos to power their large surfaces, and they like voltages greater than 5 volts to increase the available torque. Some newer ESCs (including the Jeti Spin and the Hacker Pro, at present) make ESCs available that utilize switching voltage regulators that are much more efficient and can power the demanding servos even with high-voltage batteries—at a price and weight cost. We don’t really need this extra current capability—except perhaps for retract servos.
Potential problem: High BEC voltages. The timers made by Igor and me, at least, use a microcontroller chip that is specified for supply voltages between 3.0 and 5.5 volts (but with an absolute maximum voltage of 6.5 volts). I recently measured the in-circuit voltages provided by the BECs of fifteen different ESCs, by eight different manufacturers. All of the ones using linear voltage regulators provided voltages of 5.0 +/- 0.1 volt – very safe. However, a Jeti Spin 44 provided 5.554 volts, a Jeti Spin 66 provided 5.500 volts, a Hacker X-55-SB-Pro provided 5.632 volts, and a Hacker X-70-SB-PRO provided 5.644 volts. (Hacker told me that their BEC is designed to provide 5.5 volts when loaded down with the receiver and servos.) I really don’t think these higher Hacker voltages are a problem for my chips—but, just to be very conservative, my timer for the Hackers includes a power diode to drop the voltage into the specified range. We may need to watch this trend to higher BEC voltages.
The microcontroller chips we use are really pretty rugged; I’ve received only two “blown” chips out of the hundred or so that I’ve shipped in timers and throttle emulators. These chips can be damaged with (a) reversed power supply voltages [requiring a very clever offset of two pins into the three-pin connector], (b) excessive supply voltages [don’t use the 6-volt ESCs], (c) static electricity [rare once inside a circuit], or (d) a production defect [most likely in its early life—the so-called “infant mortality” syndrome]. The very good news is that a bad chip is most unlikely to even turn on a motor, because it won’t be able to provide the pulsed output that all ESCs require, and even if it should occur during a flight, I can’t imagine it doing anything worse than shutting down the motor.
Topic: BEC voltages -- a potential problem? (Read 1107 times)