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114 Bu il d Y o ur O w n Q u a d c o p t e r

Figure 5.26 BEC schematic.

output a maximum current of 1 A at 5 V. There is a maximum of 2 A available at the BEC
connector, since two regulators are paralleled. Two amperes is usually considered to be
plenty of current to power an R/C receiver with several servos connected to it. Normally,
there is not an issue when you connect a single BEC to an R/C receiver, which is the case
for most R/C aircraft. An issue arises when four BECs are connected simultaneously to
the same flight-control board. The flight-control board then uses the power provided by the
ESCs to power the R/C receiver connected to it as well as any auxiliary servos that may be
incorporated into the quadcopter. When you have the four BECs paralleled in a quadcopter,
then eight 7805s are in turn paralleled. Some quadcopter designers think that having
multiple BECs feed a common supply point can lead to trouble. This trouble could manifest
itself as uneven power production whereby one BEC circuit would eventually take the
whole load and overheat from the excessive current. It is my belief that there is no potential
problem provided that all the ESCs are the same model and all use 7805s as regulators. After
all, two 7805s are internally paralleled to increase the current capability, which leads me to
believe that externally paralleling BECs will easily be tolerated, with each set of regulators
providing a proportional amount of the load. The recommended solution for eliminating
this potential problem is to cut all but one of the red BEC leads that are connected to the
flight-control board.

Caution: Do not cut all the red BEC leads, or there will be no power flowing to the flight-control
board.

I have operated my quadcopters both ways and encountered no problems with either
one BEC powering the flight-control board or all four powering it. The only caveat arises if
you use different model ESCs because they can have regulator circuits that are different from
the very stable 7805-based units. If that is the case, I would recommend using only the single
lead configuration.

Propellers

Propellers, while seemingly simple, are in reality, quite complex devices. They are airfoils
that have been twisted to produce a thrust while rotating through an air mass. Propellers