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explaination of BEC

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explaination of BEC

Old 07-01-2008, 08:34 PM
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Default explaination of BEC

hey guys. i have the align 35x esc with the align 430xl motor at 3700kv. this setup has worked fine until yesterday. my esc has never gotten really hot until yesterday. seems like i would get a surge of power then it would just start losing power. my tail servo would go crazy along with the gyro. ***?
I'm using this setup with 3 hs65mg and a futuba 9650 digital servo with the dx6. when i went over to investiagate why the sudden lose of control and power. i touch my esc and it was extremely hot along with the motor.

after doing alot of research of potential causes i think i may have found one.

This explaination is by the author "GERD." helifreak member with the handle PSILO so the credit go to him. if you own a spectrum radio and receiver you better read this.

BEC - What is it and why buy another?

I thought I would just post this to try and clear up some of the confusion around what a BEC is, what it does and the differences between them as it seems to confuse a lot of people and is very relevant to those of us using Spektrum 2.4Ghz

What is a BEC?
BEC stands for Battery Elimination Circuit and that is exactly what it is.

What does a BEC do?
Before lipos and advances in battery technology, the normal method of powering a receiver was through a seperate NiCd battery pack providing 4.8v to the receiver and servos. As battery technology progressed the BEC was introduced to replace the 4.8V receiver pack (making the model lighter) by drawing power from the higher voltage main pack and dropping it down to 4.8V for the rx and servos. So quite simply the BEC eliminates the need for a seperate power supply for your receiver and servos.

What is the difference between built in and external BEC's?

Generally the BEC found in an ESC is a linear voltage regulator (or linear mode bec), quite simply the way these work is to take the input voltage (11.1V on a 3S lipo) and outputs 5V to the rx. The problem is that it has to dispose of the excess 6.1V and to do this it converts it to heat. Generally these linear mode BEC's are rated at 2-3 Amps, however what the manufacturers do not tell you is that this rating is only true with a 6V input. Using a 3S (11.1V) input the BEC will only deliver around 0.5A before it starts to overheat. Linear BEC's rarely reach more than 50% efficiency and can run as low as 10-15% efficiency.

Generally external BEC's are switching voltage regulators (or switched mode bec), these do not care about input voltage and can run up to around 30V+ input. A switching regulator works by taking small chunks of energy, bit by bit, from the input voltage source, and moving them to the output. This is accomplished with the help of an electrical switch and a controller which regulates the rate at which energy is transferred to the output (hence the term “switching regulator”).

The energy losses involved in moving chunks of energy around in this way are relatively small, and the result is that a switching regulator can typically have 85% efficiency. Since their efficiency is less dependent on input voltage, they can power useful loads from higher voltage sources.

A quick Comparison

Lets take our beloved belt as an example. It uses an 11.1V input and the Esky ESC has a 2A linear BEC. In order for the BEC to output 5V/1A the current flow into the BEC has to be at least 1A so the power on the BEC is 11.1V * 1A = 11.1W however the useful output power is just 5V * 1A = 5W so the BEC has to convert 6.1W of power to heat causing it to get very hot. This gives it an efficiency of just 5W/11.1W = 45.%

Now lets say we disable the onboard BEC and replace it with an external switched mode BEC like the Hobbywings 3A UBEC. For the switched mode BEC to output 5V/1A the current flow in is just 0.50A (actual test data), so in this case the power on the BEC is 11.1V * 0.5A = 5.6W with a useful output of 5V * 1A = 5W meaning the BEC only has to convert 0.6W of power to heat and runs at an efficiency of 5W/5.6W = 89%

Do I need an external (switched mode) BEC

If you are using digital servos or more than 3S Lipos then yes definitely as the linear mode bec's will overheat trying to provide any useful output.

If you are running spektrum 2.4Ghz receivers then it is highly advisable as they draw significantly more current than a normal receiver and are very susceptible to voltage drops. If your linear bec is trying to provide a lot of current it will overheat and fail to provide the correct voltage causing the spektrum receiver to reboot.

I hope this clears up some of the questions about BEC's and is useful.



Old 01-23-2012, 09:35 PM
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Default RE: explaination of BEC

I am a newb and that was just the type of information I was looking for. I'm just starting my first ARF build and wanted to use the spectrum rc that came with my tx. Thanks for possibly saving me hours of frustration!
Old 03-12-2012, 12:26 PM
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Default RE: explaination of BEC

Awesome! So helpful and just what I needed to know. I hear it is unsafe to run two switching power sources in parallel (or into one RX) can you explain this?
Old 03-12-2012, 01:12 PM
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Default RE: explaination of BEC

ORIGINAL: Justin Lloyd

Awesome! So helpful and just what I needed to know. I hear it is unsafe to run two switching power sources in parallel (or into one RX) can you explain this?
The reason is that a BEChas a voltage regulation.
If it did not had any then it would not matter if you connected two or more in parallel.

As there is a regulation circuit (actually a chip, which chip is a circuit by itself) and the fact there is no chip the same, so regulation differs, then one BEC sees a load and regulates the voltage and the other BEC sees an expected desired volatge and does not do anything, something like it shuts down its rest system (power supply).
Now if the first BEC runs out of power due to a high load which means high current demand, more than the BEC can supply then the other BECstill will not power but only after some time. So there is a voltage (and power) drop anyway.
The biggest problem is the above can happen all the time. As it has to regulate the voltage to a point when there is a slight difference then the second BEC tries to take over. This confuses the first one and goes to sleep and vice versa and all this time this happens there is power hickups.

I hope you got the idea.

So... the only thing you can do to make your power supply better is to have a suitable BEC.
One that can handle (output) a little more current than the actual max needs, so it never reaches the limit and it powers all the time no matter what.

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