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I just want to post this what I read in RC groups.
Lucien is the Scorpion guru at Innov8tive designs and I love his posts, specially about BEC's and their limitations:
The thread is called Scorpion outrunners in the power system section. great reading.
Now, just think, if you have a servo that isn't up to snuff and is pulling alot more current than you might think, specially high torque servos these days, digital is even more current for some.
I only use built in bec on really small boats 29" or less with stock motors, if that. Mostly Ubec or an rx pack.
Cheers!
Lucien is the Scorpion guru at Innov8tive designs and I love his posts, specially about BEC's and their limitations:
In the last email you just posted, you stated that you are running a 55 amp ESC with an HK-2221-8 motor on a 4-cell Li-Po pack. When you run an ESC with a linear type BEC circuit on 4 cells, you MUST disable the BEC circuit and run a seperate switching type BEC. If you look on our website in the description of this part, you will find the following notice.
The current rating on the BEC in this speed controller is based on using a 2-cell Li-Po battery input (7.4 volts). If you run the ESC on 3 Li-Po cells (11.1 volts), the current rating needs to be de-rated by 50%. If the ESC is run on 4 Li-Po cells (14.8 volts), the BEC circuit needs to be disabled and a seperate switching type BEC circuit should be used.
You can disable the output of the internal BEC by pulling the red wire out of the control lead connector and taping it up so that it does not short out against anything. You can also use a short 22ga servo extension that has the red wire cut and plug it onto the control lead so you do not have to modify the ESC. Failure to disable the BEC circuit when running on a 4-cell Li-Po battery will result in over-heating of the BEC circuit and can lead to failure of the BEC circuit and the Speed Controller.
When you run the speed controller on 4 cells, the BEC circuit has to drop 16 volts from the battery down to 6 volts for the servos, and the remaining 10 volts has to be converted to heat to get rid of it. If you have a twitchy tail servo or a gyro that has too much gain, the tail servo can draw as much as 2 to 3 amps of current, and this equates to 20 to 30 watts of power that gets converted to heat energy in the BEC circuit. The average small electronic soldering iron is rated at 25 watts of power, so you can see how the BEC would get rather hot operating under these conditions!
The use of a seperate switching BEC will eliminate the heat problem that you are having. Instead of turning excess voltage to heat, switching BEC's turn on and off quickly and only take as much energy as needed from the motor battery to make the lower voltage. For example, if you try to step 16 volts down to 6 volts in a switching type BEC, the circuitry in the BEC will turn on and off about 100,000 to 150,000 times per second. To create 6 volts from 16 volts, during each pulse the transistor in the BEC will turn on for 6/16 or 37.5% of the time and turn off for 62.5% of the time. The short pulses of 16 volts get run through an inductor and capacitor to filter out the pulses, and the end result is a constant 6 volts DC output. By doing it this way, the switching BEC circuit only uses the energy it needs, and virtually no waste heat is given off. This alows the switching type BEC circuits to have an efficiency rating of greater than 90% when a linear type BEC would only have an efficiency rating of 30 to 40% under similar conditions. The only time a switching type BEC gets hot is when you begin to to push the max current draw beyond the max rating for the BEC.
Just so those of you out there understand what goes on "Under the hood" of a Switching type BEC circuit, here is a simplified explanation of how they work. Below is a photo of a generic speed controller that has an internal Switching type BEC circuit built in.
The big square part in the middle with the number 220 stamped on it is the output filter inductor for the BEC circuit. The two yellow squares in the upper left corner are the input filter capacitors, the 8-legged IC chip in the bottom left corner is the switching controller chip for the BEC circuit and the black rectangular part between the IC chip and the inductor is the output filter capacitor. On a side note, the little square part with 20 legs is the microprocessor that controlls the Speed Controller itself.
The Inductor with the number 220 stamped on it is the main output of the BEC circuit. All of the current that leaves the BEC goes through this part. You can see a circle in the middle of the inductor in the photo above. Inside the circle there is a coil of wire wrapped around the middle of the part, and the ends of the wire are soldered to the silver tabs at the top and bottom of the inductor as seen above.
The wire used in these inductors is rather small, usually something like 28ga or 26ga size, and that is what limits the maximum output current of the BEC circuit. If you pull too much current through a switching BEC, the output inductor will start to get hot, and if you run enough current through it, you will burn up the wire in the inductor and it will fail. Once this happens the BEC will cease functioning and will not produce any more output voltage.
Lucien
The current rating on the BEC in this speed controller is based on using a 2-cell Li-Po battery input (7.4 volts). If you run the ESC on 3 Li-Po cells (11.1 volts), the current rating needs to be de-rated by 50%. If the ESC is run on 4 Li-Po cells (14.8 volts), the BEC circuit needs to be disabled and a seperate switching type BEC circuit should be used.
You can disable the output of the internal BEC by pulling the red wire out of the control lead connector and taping it up so that it does not short out against anything. You can also use a short 22ga servo extension that has the red wire cut and plug it onto the control lead so you do not have to modify the ESC. Failure to disable the BEC circuit when running on a 4-cell Li-Po battery will result in over-heating of the BEC circuit and can lead to failure of the BEC circuit and the Speed Controller.
When you run the speed controller on 4 cells, the BEC circuit has to drop 16 volts from the battery down to 6 volts for the servos, and the remaining 10 volts has to be converted to heat to get rid of it. If you have a twitchy tail servo or a gyro that has too much gain, the tail servo can draw as much as 2 to 3 amps of current, and this equates to 20 to 30 watts of power that gets converted to heat energy in the BEC circuit. The average small electronic soldering iron is rated at 25 watts of power, so you can see how the BEC would get rather hot operating under these conditions!
The use of a seperate switching BEC will eliminate the heat problem that you are having. Instead of turning excess voltage to heat, switching BEC's turn on and off quickly and only take as much energy as needed from the motor battery to make the lower voltage. For example, if you try to step 16 volts down to 6 volts in a switching type BEC, the circuitry in the BEC will turn on and off about 100,000 to 150,000 times per second. To create 6 volts from 16 volts, during each pulse the transistor in the BEC will turn on for 6/16 or 37.5% of the time and turn off for 62.5% of the time. The short pulses of 16 volts get run through an inductor and capacitor to filter out the pulses, and the end result is a constant 6 volts DC output. By doing it this way, the switching BEC circuit only uses the energy it needs, and virtually no waste heat is given off. This alows the switching type BEC circuits to have an efficiency rating of greater than 90% when a linear type BEC would only have an efficiency rating of 30 to 40% under similar conditions. The only time a switching type BEC gets hot is when you begin to to push the max current draw beyond the max rating for the BEC.
Just so those of you out there understand what goes on "Under the hood" of a Switching type BEC circuit, here is a simplified explanation of how they work. Below is a photo of a generic speed controller that has an internal Switching type BEC circuit built in.
The big square part in the middle with the number 220 stamped on it is the output filter inductor for the BEC circuit. The two yellow squares in the upper left corner are the input filter capacitors, the 8-legged IC chip in the bottom left corner is the switching controller chip for the BEC circuit and the black rectangular part between the IC chip and the inductor is the output filter capacitor. On a side note, the little square part with 20 legs is the microprocessor that controlls the Speed Controller itself.
The Inductor with the number 220 stamped on it is the main output of the BEC circuit. All of the current that leaves the BEC goes through this part. You can see a circle in the middle of the inductor in the photo above. Inside the circle there is a coil of wire wrapped around the middle of the part, and the ends of the wire are soldered to the silver tabs at the top and bottom of the inductor as seen above.
The wire used in these inductors is rather small, usually something like 28ga or 26ga size, and that is what limits the maximum output current of the BEC circuit. If you pull too much current through a switching BEC, the output inductor will start to get hot, and if you run enough current through it, you will burn up the wire in the inductor and it will fail. Once this happens the BEC will cease functioning and will not produce any more output voltage.
Lucien
Now, just think, if you have a servo that isn't up to snuff and is pulling alot more current than you might think, specially high torque servos these days, digital is even more current for some.
I only use built in bec on really small boats 29" or less with stock motors, if that. Mostly Ubec or an rx pack.
Cheers!
Nortavlag Bulc 
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