Hey,
I thought I'd post this here in case someone wanted to build a really easy BEC.

All you need to make the BEC is . . .
- 7805 voltage regulator
- two 47uF capacitors
- servo lead
- battery connector

To build one yourself you can just follow the schematic below.

Here is a video I made about the BEC. . . .

DIY BEC - Very Easy (1 min 32 sec)

Make sure the 7805 is well cooled. Linear regulators like the 78xx's are very inefficient and it's going to shed a lot of heat. I like to use a few more caps for cleaner power on mine.

-K

(Note: I don't use this for RC applications, I like switching BECs for RC use.)

it weak and very hot.
you should use power switching mc34063,mc 3485 ,lm 2596, lm2576......

I would not use a 7805 with a 3S battery pack in a plane, unless the regulator was well heatsinked.

Without a heatsink the circuit is really designed to fail.

You can use such a simple circuit (although I would use 1uF ceramics as the capacitors) with 2S if you swap the 7805 for a LDO type. But then only for small planes with very modest current requirements.

My plan is to use only a 2s lipo and there will only be two servos in use on the plane. It's for a small plane powered by just a little cox engine.

Originally Posted by srnet

I would not use a 7805 with a 3S battery pack in a plane, unless the regulator was well heatsinked.

Without a heatsink the circuit is really designed to fail.

You can use such a simple circuit (although I would use 1uF ceramics as the capacitors) with 2S if you swap the 7805 for a LDO type. But then only for small planes with very modest current requirements.

What is a "LDO" type? I'm brand new to small electronics and building my own electronic stuff so I don't know too much of the jargon. . .

Originally Posted by PyroMan

What is a "LDO" type? I'm brand new to small electronics and building my own electronic stuff so I don't know too much of the jargon. . .

LDO = Low Drop Out.

A standard regulator such as an 7805, needs an input voltage of around 2 - 2.5v higher than the regulated output voltage, so about 7.5v input to provide a 5V regulated output.

I use LM2940s to provide the 5v supply in my small gliders that run off 2S, the LM2940 is an LDO regulator, and only needs about 5.5v input to provide a regulated 5V output.

If the 2S were to go as low as 6V a 7805 would only be providing maybe 3.5v output, and the radio might stop working. A LM2940 would keep the radio supply at the full 5v value down to 5.5v. Clearly running a 2S Lipo down to 5.5v or lower is not at all good for it, but at least the plane does not fall out of the sky.

Thanks for the explanation. I'll look into that.

Pyro,

I'd advise you to use an LM2940 since it doesn't actually drop out at all which is much safer than the type that do. When the input voltage reaches the point at which the device no longer regulates, the output follows the input down a fraction of a volt lower.

A.

Even though they're crap, most smaller escs under 40amps use 7805

Si.

I thought most BECs and ESC/UBEC type devices used a switching supply topology...

Are they really just using a regulator and burning up all that extra voltage as heat?

Almost ESCs with internal BECs used to use linear regulators for the BEC. Quite a few of the smaller ESCs still do. Switched regs are much more common now but linears have not completely gone away.

Steve

Even worse, they often "double" the current output by soldering two v-regs together without series current limiting resistors. In some cases they even solder the packages directly on top of each other!

This is a Turnigy Plush (HobbyWing Platinum) 12A. The reg is a 78D05F which, according to Google, is rated at 0.5A.

The 7805 datasheet shows dropout begins at 7.2v
As others have said, if you must use a linear, much better to use a low-dropout regulator.
This is a good one - 3 amps, Low dropout, convenient TO220, and only 78 pence each.
Note that the capacitors are essential with most LDO's as they form part of the control loop.
Cheers
Phil

Originally Posted by SimonChambers

Even worse, they often "double" the current output by soldering two v-regs together without series current limiting resistors.

true they dont "double" but other than that theres nothing really wrong with parallel regulators. most here these days acknowledge parallel lipos is ok but the regulator myth lives on. they dont "fight" each other nor does one "carry the load" (unless seriously mismatched, ie 5v&6v). bipolar junction resistance which normally works against you in many applications saves the day here. also note that series resistors and diodes make matters much worse. specially with ldo types.

it is true that two do not exactly double capacity but if they are kept out of each others thermal environment (unfortunately not the case in many escs) its close. soldering them right next to each other dont exactly multiply amps but is definitely a successful method to boost current.