We will start off with the difference in the two features. They are often confused as the same but are not the same at all.
LVC is a circuit built into the 3 in 1 to prevent your batteries from being over discharged. The circuit continuously monitors the battery voltage. When the voltage gets below a certain threshold the 3 in 1 begins an automatic shutdown indicated by the flashing blue light. You should be able to perform a controlled decent when LVC activates. This circuit does not protect the 3 in 1. It protects your batteries.
The over current protection circuit protects your 3 in 1. This circuit constantly monitors the current consumed by the main and tail motors. When this current exceeds the limit, the circuit is broken, stopping all power to the motors. There is no automatic shutdown. Instead, your heli will just fall out of the sky. Your blue light will remain solid during an over current protection shutdown.
Now that you know the difference between LVC and over current protection, I am going to do my best to explain how the circuits work starting with the LVC.
LVCLike I mentioned earlier, with LVC the battery voltage is constantly monitored. This is done by the MCU through the use of a voltage divider. Just like the name says, a voltage divider divides applied voltage. You do this to give you a smaller voltage to be measured that represents the larger applied voltage. I suspect this is done on the mCPx because the MCU operates on 3VDC and the battery can supply up to 4.2VDC. This smaller voltage created by the divider is safe for the MCU inputs.
In a pure resistive voltage divider like the one used on the mCP X finding the output of the divider is easy. The formula is Vout=R2/(R1+R2)xVin. In this formula Vout is the dividers output to the MCU. Vin is the battery voltage. R1 is the resistor on the positive side of the divider and R2 is the resistor on the ground side of the divider.
In the figure below you can see a simple voltage divider. In this circuit the supplied voltage is 10V and each resistor is 10K. The formula would look like Vout=10k/(10k+10k)X10. This makes Vout = 5VDC.
Now, where can you find this voltage divider on the mCP X? It is directly under the tail FET and it even shares a trace with the tail FETs pull-up resistor. Looking at the picture below, the 80k resistor is your R1 and your 120K is your R2. Your battery is Vin and the purple line that goes through the board to the MCU is Vout.
Sometimes this circuit goes bad causing an early or late LVC. Personally I would not worry about a late LVC because I should be running a timer and paying attention to changes in head speed. However, an early LVC can be really annoying. If you need to repair an early LVC this can be easily done by adjusting the value of R1. The factory resistor is approx 80k. If this resistor has become faulty you can just change if for another 80k resistor. If the R2 has gone bad you can change it for another resistor of equal value.
Now, what do you do when both resistors are measuring good? Sometimes the resistors are good and it is the MCU that is faulty. To fix this you use the voltage divider formula to find a resistor that meets your needs. If the MCU is cutting off power when the divider is at 2.2VDC leaving you with 3.7VDC in your battery and you want to fly until your battery reaches 3.4V then you need to lower the value of R1 so that the divider creates 2.2VDC with a supplied battery voltage of 3.4V.
It would look something like this 2.2=120k/(R1+120k)x3.4
Solving for R1 should show that you need approx 65.5k resistor to make the LVC operate how you want it to.
I would not mess with the 3 in 1 circuit right away. It should be the last resort. There are several other causes of an early LVC that happen when the LVC circuit is functioning perfectly and doing its job. Any one of these or a combination of the conditions below can result in a early LVC.
The first is a bad motor. If you main or tail motor is worn out it could be consuming power that exceeds your batteries C rating. If the motor takes more power than your battery can provide, the voltage will drop and the LVC circuit will respond appropriately.
The second is a bad battery. If the battery is not able to meet the demands of the heli's electronics (servos, 3 in 1, and motors) then it will cause a voltage drop causing the LVC to activate and shut the heli down.
The final common cause of an early LVC is a bad battery connection. The battery connector and battery wires see a lot of abuse and get worn out. It is very common for the battery wires to become frayed at both the battery connector and the 3 in 1. This will cause a voltage drop under load and cause the LVC to activate. Inspect your wires carefully and pull back the shielding to check for fraying before attempting to modify the 3 in 1 LVC circuit.
That pretty much coveres the LVC circuit. I am sure there are more causes to an LVC. If I missed something, please post a comment and share your knowledge with others.
OVER CURRENT PROTECTIONNow we are going to cover the over current protection circuit. You can see this circuit in action if you do pitch pumps with a pitch curve that is to aggressive. This is why the mCP X manual tells you to adjust your pitch to 75%. If you go higher the motors will consume more power and activate the over current circuit.
The over current circuit operates by using 3 current resistors in parallel. You can see the resistors in the picture below.