Homemade BEC

Article by Kenneth Macrae:

I’m trying to get back into flying and while going through “pre-preflight” checks, I realized that my nicad batteries are not to be trusted, perhaps as a paper weight or to play dodge ball……

I recently bought a 8 amp linear BEC for my ¼ scale cub ( which I still haven’t started building……) and thought, it can’t be complicated to make my own. And it wasn’t.

I did a quick search and one gets two types of “normal” voltage regulators. The trusty 78L05 or similar which requires the supply voltage to be a least 2 volts higher than the regulated voltage i.e. you would need a 7 volt battery pack (at least) to supply a constant 5 volts. The “other” type of regulator is a LDO type, “Low Drop Out”.

The drop out voltage is the lowest permissible supply voltage for the regulator to maintain the output voltage.

You get different types of LDO’s which can have an impressive drop out of 0.3 volts at full load right up to 1.5 volts at full load for that regulator. The regulators which have a low drop out obviously cost more….

Using the 0.3 volt LDO, at full load, the supply voltage then needs to be at least 5.3 volts to ensure 5 volt output.

The BEC I built makes use of a maximum 2S lipo (7.4 volts) and can work right down to 6 volts (the lowest limit for a 2S pack without damaging any of the cells). The advantage of using LIPO’s for a receiver pack are numerous and could probably occupy a number of pages, but for me it was:

• More trustworthy than my old nicads

• Easy to charge (if you have the right equipment) with in about an hour

• Predictable and no memory effect

I’ve added additional circuitry to give an indication of the pack via 2 LEDS (light emitting diodes).

Above 6.4 volts, the green LED is on

6.15 to 6.4 volts, the green and red LED are on

6.15 volts and below, the red LED is on

Once the red LED is on, it’s time to stop flying and charge the battery.

The specs for my homebuilt BEC are:

• Constant 4.81 volts at 1 amp

• Can comfortably handle up to 3 amps peaks

• Has a maximum practical limit of 2S lipo (I don’t see the point in using more LIPO’s as the extra “volts” are converted to heat and it’s a real waste…..)

Below are some graphs showing the performance of the BEC:

Figure 1 Vout at 1 amp - fully charged lipo was not used

Depending on the capacity of your LIPO will dictate the length of your total flights. With the built in “battery checker”, most of the guess work would be removed.

Figure 2 Vout at various loads