Is ESC without BEC an OPTO ESC – some quick explanation
I will try to keep it super simple (KISS) and explain why an ESC without BEC is not necessary an OPTO ESC. BEC stays for battery eliminator circuit. It is an electronic circuit, which delivers power to other electronics, like your flight controller or receiver. That is why we connect the red wire of the servo cable to the FC, it simply outputs (hopefully stable) 5V.
Some ESCs don’t have an integrated BEC and can not supply power for your controller or receiver (for example: KISS ESCs, Afro Race Mini 20A or XRotor 15A). In this case you need a separate BEC for powering them. There are also power distribution boards with integrated BEC. I personally use the ACSP3 by Nick Arsov.
So far so good. The problem is that many ESCs retailers and manufactures (mostly Chinese) don’t know, that an OPTO ESC is just not equal to ESC without BEC. OPTO means that the ESC have an opto-isolator. If you Google it, you will find that, it is a optical component that transfers the signals between two isolated circuits by using light. The main purpose is to prevent rapidly changing voltages or high voltages on one side of a circuit from distorting transmissions or damaging components, like the receiver, on the other side of the circuit.
I have never experienced any problems with my non-OPTO isolated ESCs, but thought it is good to know what an OPTO ESC really means.