I've posted this thread in a different location in www.wattflyer.com
, but IMHO it's important enough to have its own thread.
This posting started when looking at another thread where the Mfg specified that the linear BEC in their ESC had a peak current rating of 0.7 Amps, or 700 milliamperes, that when used with a 3 cell LiPo with 11 volts DC output.
This max BEC current rating of 700 Ma is scary!!
Lets take a look. This ESC has a 3 cell LiPo on it, for a voltage of about 11 Volts DC. So we have two voltages to be concerned with in this linear BEC in the ESC. First is that 11 Volts DC into
the linear voltage regulator. Second is the 5 volts DC out of
the same linear voltage regulator. So simple subtraction of 11 VDC minus 5 VDC indicates we have to account for the difference of 6 Volts DC in the regulator itself.
Now, they have a published maximum current rating of 0.7 Amps on that BEC. So we have the watts lost inside the BEC itself as W=Volts times Amps, or 6 VDC times 0.7 Amps. That is 4.2 watts.
Anyone that has ever taken a 5 watt resistor, and run 4.2 watts through it will find out it's temperature will quickly increase to high enough to brand your fingertips if you ever touch it. (Running a typical wirewound resistor at its rated maximum wattage results in a resistor surface temperature on the order of 300 - 400 degrees F.)
So, if you run 4.2 Watts through a linear voltage regulator, you'd better have a pretty good sized aluminum finned heat sink on it, or it will heat up to several hundred degrees in only a few seconds.
Problem is, these linear voltage regulators in many ESC's do not have any sort of a heat sink to remove heat from the regulator itself. And, most linear voltage regulators have built in protection that shuts them off if they overheat, protecting themselves. (And crashing your model. By the time you get to the model wreckage, that regulator has cooled off and is working again.)
Bottom line, those "Peak current ratings" provided by the ESC manufacturer for their BEC's should IMHO be taken with a big grain of salt. Or perhaps the entire salt shaker. Those peak current ratings are only valid for a few seconds at best.
Out of curiosity, I took an LM7805 linear voltage regulator, hooked it up to 11.5 Volts DC on my variable power supply, and connected a 0.7 Ampere load to it. No heat sink was used. That regulator heated up to 175 degrees F within 25 seconds. That LM7805 regulator measures 0.37 by 0.60 inches. The regulator on one of my ESC's measures 0.25 by 0.25 inches. Since this ESC regulator has only 1/4 the area of the LM7805, that little regulator is going to get very hot, very fast when trying to radiate 4.2 watts.
Bottom line, if you think you might have a problem with the linear BEC in your ESC getting to hot and shutting down, you are probably right.
What's the solution? If you've got any question on whether your linear BEC can do the job, put in one of those switching power supply BEC's such as Castle Creations 10 amp uBEC.
How these uBEC's work is a little involved, but basically their internal parts are never in their "linear mode". Just think of taking a toggle switch, put it in series with a 20 Volt battery and a light bulb. Then switch it on and off with exactly a 20% on, 80% off cycle. At 100,000 times per second. (Difficult with a toggle switch, easy with electronic switching) So, with a little electronic smoothing circuitry, your output voltage would be that 20 VDC times 20/80 or 5 VDC. That switch will never get hot, since it is either a dead short (no heat) or open circuit (again no heat).
You can actually build up a simple switching power supply with only a half dozen cheap easily available electronic components.