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Old Jan 06, 2007, 03:19 PM
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BEC specs, especially for BECs which use linear regulators (which accounts for the majority of them), are a little confusing.

The 1.0~3.0A spec in this case, doesn't really mean that much, since what is important is how much power the regulator can dissipate. This is why in addition to the current spec, the number of servos is specified.

Take for example a 5v BEC, running from a 7.4V LiPo, and with an average current draw from the servos of 0.5 Amps (500mA). The regulator has to drop 2.4v (7.4 - 5), and with 500mA flowing through it, that results in a power dissipation within the regulator of 1.2W (P = VI).

I've attached the power dissapation graph for a common 5v regulator, with no heat sink (as is the case for many ESCs with integrated BECS), the maximum average power dissipation at 25 degrees C is about 2 Watts, so we are within its specs.

Now, imagine that the 7.4v Lipo is replaced with an 11.1v Lipo. The voltage that the BEC has to drop is now 6.1v, and assuming the same 500mA load, the power dissipation is now 3.05W, far above the max. rating of the regulator.

So, with linear regulators, you can't really read much into the '1-3A' spec.

Now, the thing that really counts is what happens when the regulator specs are exceeded. Unlike the ESC itself, which will just cut power to the motor when power specs are exceeded, the linear regulator just shuts down, cutting power to the receiver, and any servos that it is driving.

A common mistake when adding a pan/tilt mount to a plane is to overlook this spec!
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