Standalone BEC and servo spliter

Like many of you, I am getting an FT Cruiser ready to go. For me, the FT will be my first twin motor build. As David had explained in the build video of the FT Cruiser, you can't run two ESC's power feed to a receiver, or you may burn out the ESC's BEC capability due to different outputs of the units. This got me thinking. Just how much power do the 4-6 servos need?

So I got out my digital Volt/OHM meter, set it to test amperage load, grabbed an extra receiver, 4 servos and an ESC, rigged them up and starting throw controls around on my radio. Now keep in mind the servos where not tied to anything, just hanging in the air or on the ground as I did this test. I got a 1.3, 1.4 amp spike! Keep in mind, I was moving all the controls, trying to push the system to its limits to see what kind of pull the system had. If I was to fly a plane this way, it would crash very quickly.

Now I am planning on using two Turnigy 18amp BASIC esc's, each having a 2 amp BEC, but I can only use one BEC, and with no load I got a big spike in power usage. I would like to have more than just 2 amps after my test, since in the air, the servos will have a load on them. Good chance for pushing that 1.4 amp limit. Should be ok, right?

My first though was to use a couple of Diodes to prevent feedback of power from one ESC to another. For those who do not know about electronics, a Diode is an electronic component used to allow power to flow in one direction. Its kind of like a check valve in plumbing. This lead me to my first, very simple BEC coupler set up using Diodes.

Add on a capacitor to help filter power flow and some wire and done!

Now for testing. Power from one ESC did not reach the other ESC, and prevented any potentially damaging feedback. Only problem is that power going into this circuit was 5v, but after the Diodes, only 4.3v reached the receiver. Not good since servos are rated for at least 4.8v.

Now I know a thing or two about electronics. After messing with a blown up 18amp ESC, I quickly found the BEC circuit of these ESC's. They use two 78D05F 5v voltage regulaotrs made by KIA. After some research I found that each one of these regulators can handle 1 amp of power each. Now, you can go out to almost any electronics shop and find a very simple voltage regulator for sale, called a LM7805. The LM7805 is a +5v voltage regulator. The 78D05F is a version of the LM7805, only in a different package and amperage load capability. While 78D05F can handle 1amp of power, the LM7805 can handle 1.5 amps of power.

Since the BEC of my receivers use two 78D05F's, I am going to make a stand alone BEC using two LM7805's, and get a good 3amp BEC. Since this is going to be used on the FT Cruiser, I also need a servo splitter. Also, since I also don't have a single 2200mah pack, but do have two 1050mah packs, a good cross connection set up would be good too.

After a bit of work on my CAD program I had the basic design done.

Time to scuff up some blank PCB board and get this transferred over.

I used the hot toner transfer method, where you print out the circuit diagram onto cheap laser printer gloss paper, then iron it onto the PCB.

The method is not 100%, but a bit of black Sharpie dose wonders to touch up any bad spots. Now time to etch the board.

 Once etching was done a good bath in some tinning solution and I got this.

A nice, double sided custom made BEC board. Time to populate it with components.

Here we have the two LM7805 voltage regulators, a .1uf Capacitor (Capacitor will say 104 on it), a .33uf Capacitor (will say 334 on it. Capacitor shown says 221, since that was all I had when I took the picture) and some heat-sink compound. Time to fire up the soldering iron and get to work.

A small layer of heat sink compound is placed on the LM7805's backs. One thing to note about this tab, It is tied to the Negative leg of the regulator, so it can not touch any positive power source, or bad things will happen. The odd way I bent the legs was to go through the holes I had laid out on the PCB.

Nice and soldered to the PCB with a screw securing them down. The PCB itself will also be acting as a heat sink for the LM7805, which is why the board under neath and around the regulators is so large.

Now we add on the capacitors. The .33uf Capacitor goes to the input side of the power feeding the LM7805's and the .1uf Capacitor goes to the output side.

Finish adding the servo wires and some nice 12g OFC wire and we are good to go. Once again I hooked up the receiver and servos and began my mad servo throw testing. Using my inferred thermometer I monitored the heat of the BEC board. After several minutes of my table top rodeo and pulling 1.4amps of power, the PCB reached 95°, 20° above what the room was at. No fan or air flow over the BEC, and only a 20° increase is great.

Pros and Cons of this set up.

Cons:

Weight, I do have more to go into the air, but not a huge amount.

Complexity, It dose take a bit of skill to make this but it should be worth it in the end.

Pros:

Shorter power run to the receiver. DC power dissipates quickly. There is a reason we have AC power running into our homes. Less chance of power loss due to the back and forth power runs.

Higher voltage. If I wanted, I could easily replace the LM7805 regulators with a higher voltage IC. The LM7806 is a 6v regulator. The servos I plan to run can handle the 6v of power.

Now I simply need to get this into the FT Cruiser and enjoy the fun of flying a twin. If you guys want, I can upload a PDF of the PCB for this BEC.