Redundant, Scalable and Cheap BEC

by Bert Van Kets

The power to the RX is one of the most important things in a reception system. BEC (Battery Elimenator Circuit) eliminates the use of a heavy RX battery and uses power from the main battery to run the RX. It needn't be said that this system needs to be very reliable.

An Ikarus BBS guru, Suzanne, has posted a BEC system that is very cheap, redundant and yet scalable. The proposed circuit provides 1A max per voltage regulator (7806CT). If more than 3A are needed, just add another block.

Here's the post from Suzanne:

Parts list for a n Amp home-built BEC: (n = 1 to 5 or even more

n Voltage regulators 7806CT, (50 cents each)
n 1A Schottky Diodes, (50 cents each)
2n capacitors 10 uF 25V, (20 cents each)
1 piece of aluminium profile

If you use a different type of regulator, make sure that it has either input or ground connected to case, otherwise you're in trouble.

Mount all regulators on the aluminium profile, solder one capacitor between each input pin and ground and another one between each output pin and ground. Then solder the + end of a Schottky diode to each 7806 output terminal and solder all the - ends together to form the 5V output.

Advantages:

- good cooling
without going into any detail or doing the math, three regulators on a common heatsink can provide much higher current than a single regulator on the same heatsink. This has to do with thermal resistance from chip to case and with limited thermal conductivity of the heatsink.

- extremely cheap.
5 pc. of the 7806 cost less than $3, a fraction of the cost of a monolithic 5A regulator

- built-in redundancy
Even if one regulator fails or goes into thermal shutdown, you still have n-1 to supply power to your receiver and servos. The schottky diodes make sure that a failed regulator cannot pull the +5V output to ground.

- scaleable
Just use as many regulators as you need - 2 for 2Amps max, 3 for 3Amps max An Eco 8 with all digital servos will pull approx. 2.5A peak.

If you want to experiment, just add a 220 Ohms resistor and a green LED between each regulator output pin and ground to monitor the regulator outputs independently. Then, put a load on the BEC until it gets hot. You will find that after a while the first regulator will go into thermal shutdown (usually the one in the middle), but the others will still supply current to the load. After a few more seconds, if you don't remove the load, the others will shut down, too. This is a safe way to test the limits. If you move the sticks for a while, and one of the LEDs goes off after some time, you need a bigger heat sink. Don't try this with a regular BEC without redundancy!!!

The output voltage is actually 5.6 Volts, which makes the servos go faster and produce more torque. If you want 5.3 Volts, use normal diodes instead of the Schottky diodes.

The Schottky diodes actually serve two purposes: redundancy and current distribution. Do not try to simply parallel the regulators!

Here's a pic to help you visualize and build it:

a pic of a BEC created by the designer

image provided by alanwsg

If you create a PCB layout, please send it to me, then I will put it on this page.