I was reading the instructions for a Pixie 14 speed control and I was wondering why it is that with the more cells it runs on the less number of servos it can power. From 6-7 cell it can power 3 micro servos, from 8-10 cells it can power only 2 micro servos. From10-18 cells no BEC can be used. I've seen this on other speed controls so I was wondering what the principle behind this was.
Because the higher cell count makes the regulator work harder to regulate the voltage so to cut back they tell you to use less servos, you may need to get a different regulator if you want to use more servos.
The principle is P=IE ( power = current * voltage ). The regulator acts like a resistor to drop the extra voltage across itself. The higher the voltage, the higher the power IF the current is kept steady. So, as the voltage is raised on the regulator, the current must be reduced to keep the power at an acceptable level.
David
To reduce the voltage from pack voltage to 5 Volts for the receiver and servos, the BEC regulator has to dissipate the energy as heat. The larger the difference between pack voltage and 5 Volts, the more energy it has to dissipate to reduce the voltage, and the more heat it generates. Regulator chips are very small, and can only dissipate so much heat without getting too hot and burning out.
Don't forget to mention that all of these explainations apply to LINEAR regulators. There are switch mode regulators available that don't derate as the voltage increases, BUT I don't know of any ESC using anything other than a linear BEC.
Jim
I realized I forgot this after I hit post. Thanks for the update.
There's a product called the "Ultimate BEC" that I believe is a switching regulator.
I also don't know of any ESCs that have a switch mode BEC but AIUI The Ultimate BEC
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