First of all, sorry for the alphabet soup of acronyms in the title of this post. Before we get stared discussing the differences between opto ESCs and BEC ESCs (sorry again), let’s go over a glossary of the acronyms used throughout the rest of this post:
- → Electronic Speed Controller: (see below for definition)
- → Optoisolator: An optoisolator is an electronic circuit that allows an electronic signal to be transmitted without an electrical connection. Inside an optoisolator there is a small gap with an LED on one side and a photodetector on the other side. (Oversimplification notice) An incoming electronic signal (for example from a flight controller) is translated into a series of flashes. Those flashes are detected by the photodetector on the other side. It is like communicating with somebody using a flashlight which you turn on and off to form Morse code. So a signal can be transmitted using light instead of electricity.
- → Battery Eliminator Circuit/Universal Battery Eliminator Circuit: The BEC/UBEC (which are basically the same component) is basically a beefy voltage regulator. Its purpose is to transform the high voltage from the multirotor battery (typically about 11.1V) to a lower voltage (usually 5V) needed to power lower voltage devices like flight controllers and servos. If we were to simply plug a battery directly into a flight control board, we would just fry the flight controller. So, we put the BEC/UBEC between the battery and flight controller to lower the battery’s voltage to an appropriate level.
What is an ESC?
ESC stands for “Electronic Speed Controller.” In the Selecting Motors tutorial, we discussed that one of the differences between brushed motors and brushless motors is that brushless motors require control circuitry. ESCs are controllers for brushless motors. They control the flow of electricity into the electromagnetic coils inside the motor in such a way that the motor shaft is made to turn. ESCs are used to control the rotational speed of brushless motors.
There are two types of ESCs: opto ESCs and BEC ESCs. The type of ESC you use for a multirotor project is very important because you will need to use a completely different power system for each type. Unfortunately, the two types of ESCs look almost exactly the same, except for the information on their labels.
This is a BEC ESC.
So when you are shopping for ESCs, make sure you pay close attention to which type of ESC you are getting.
What is a BEC ESC?
A BEC ESC, which is sometimes called an UBEC ESC, is basically two electronic circuits in one package. First you have the ESC circuitry which takes signals from the flight controller and, based on those signals, adjusts the speed of a connected motor. And the second part of the package is a BEC which takes the high voltage from the battery and reduces the voltage down to a level that can be fed into the flight controller.
Combining an ESC and BEC into one package is convenient because, just like we did in the SK450 Dead Cat build, the ESCs can be plugged straight into the flight controller and they serve the dual purpose of controlling the motors and supplying power to the flight controller. BEC ESCs allow for a simpler power system setup, just connect the battery to the ESC, and the ESC to the flight controller and you’re done.
The downside to using BEC ESCs is that in order to step the battery voltage down to a level appropriate for the flight controller (for example 11.1V down to 5V) the BEC ESC dumps the excess energy as heat. After you fly a multirotor equipped with BEC ESCs for a while, you will notice that the BEC ESCs can be quite hot. Too much heat can damage an ESC or cause it to shut down, which is obviously not good if your multirotor is a few hundred feet off the ground.
What is an Opto ESC?
An Opto ESC has only the ESC circuitry in the package and lacks a battery eliminator circuit. So, like all ESCs, Opto ESCs take commands from the flight controller and control the speed of the motors, but they cannot be used to supply power to the flight controller. Let me say that again because people get really annoyed when they order a bunch of Opto ESCs only to discover later that they need to order another part to provide power to their flight controller (I made this mistake myself). Opto ESCs cannot be used to power the flight controller.
Therefore, if you are using Opto ESCs in your multirotor build, you will need to buy a UBEC. The UBEC is placed between the battery and flight controller to provide power to the flight controller at an appropriate level.
This is a UBEC. It steps the battery voltage down to a level that can be safely used by the flight controller.
The obvious disadvantage to Opto ESCs is that you need to purchase an additional component for the power system, a UBEC. This makes your multirotor’s power setup a bit more complex and adds more wires to your craft.
Opto ESCs do have a couple advantages over BEC ESCs though. Because they lack BEC circuitry, Opto ESCs are about 20% cheaper than BEC ESCs. For example, the BEC ESC pictured above costs $10.50 from HobbyKing whereas the Opto ESC costs $7.99. So with these particular ESCs, multiplying the savings by four in the case of a quadcopter, you save a total of $10 on ESCs. Even after you purchase a UBEC, you will still save a few bucks. This is also useful if you need to replace ESCs in case of failure.
Speaking of failure, using if you build a multirotor to use BEC ESCs and an ESC fails somehow, for example from overheating, you will lose all power to your flight controller and your multirotor will crash. By using Opto ESCs, if an ESC fails you will lose control over one motor, but your flight controller will still be powered so fail-safe features can be used to safely land the multirotor on however many motors are left.
Using Opto ESCs and a UBEC
As mentioned above, if you are using Opto ESCs on your multirotor, you will also require an external UBEC. Whereas if you are using BEC ESCs you do not need any additional components.
The power system for a multirotor using Opto ESCs requires an external UBEC whereas BEC ESCs do not require additional components for the power system.
Setting up the power system for a multirotor using Opto ESCs goes like this:
- Connect the Opto ESCs to the motors
- Connect the Opto ESCs to the flight controller
- Connect the Opto ESCs to the battery (will not supply power to the flight controller)
- Connect the external UBEC to the battery
- Connect the external UBEC to the flight controller (now the flight controller will turn on)
Connecting Opto ESCs with UBEC to KK2.1 Flight Controller
(video coming soon, estimated Monday December 15, 2014)
Source: Black Tie Aerial
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