All About LVC and Over Current Protection

[Mega Stumbler]

LVC and Over Current protection



Today's article is going to cover 2 safety features built into the mCP X 3 in 1. The first one is the low voltage cutoff (LVC) and the second is over current protection. Just like in the FET write up, I am going to do my best to explain how these circuits work while not getting to specific or technical. My goal is to give you a basic working knowledge of the circuits.

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.

LVC

Like 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.

In the next circuit below the supplied voltage is still 10V but the value for R2 has been changed to 90K. This gives you the formula Vout=90K/(90k+10k)x10. This makes Vout = 9VDC.

The divider will respond to any change in voltage or resistance. In the mCP X the resistors are constant and it is the voltage that changes. Looking at the diagram above if we were to change the input voltage to 100VDC while leaving the same 10k and 90k resistors the circuit will respond by giving a higher output voltage. Vout=90k/(10k+90k)x100 Vout=90VDC. This reaction to changes in voltage is what drives the mCP X LVC circuit.

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 PROTECTION

Now 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 PTCs in parallel. A PTC is a positive temperature coefficient device. In simple terms, it is a self resetting fuse. You can see the PTCs in the picture below.

These PTCs are only hooked up to the main and tail motors. A surge of power in any other part of the heli will not cause an over current protection shutdown. The positive power for the motors is supplied directly by the FETs but the negative power is fed through the PTC to the motors. Both motors share the same 3 PTCs so that they monitor total current flow and not the individual current flow of a motor. This is why your tail FET can still burn. If the main motor is drawing little current but the tail is drawing to much current the PTCs will not respond because the total current flow (main+tail) is not great enough to trip the fuse.

The PTCs are made of a crystalline substance that is filled with carbon. The carbon conducts electricity through the crystals until the current becomes to high. Once the current gets to the PTCs tripping point the current flow heats the PTC and causes the crystalline substance to transform in to an amorphous state. Amorphous is just a fancy word for "it's not a crystal anymore". When it is no longer a crystal the carbon does not have a pathway to conduct electricity creating an open circuit. The PTC will stay in this state until power is removed and the PTC can cool and turn crystalline again. Both the tripping and resetting of the PTC happen in a fraction of a second when the conditions for each are met. The MCU is able to see this trip through a seperate circuit that I am not going to get into today. Just know that the MCU will not allow the motors to operate until you return to 0 throttle even if the PTC has reset.

PTCs reset fairly quickly letting you get back into the air. But, it can take hours for a PTC to fully reset to its original tripping point. This means that if you activate the over current protection during a flight session then the the circuit will trip at a lower current the next time during that same flying session until the 3 in 1 has had many hours to rest.

The mCP X has 3 PTCs all hooked up in parallel. The current of the motors is divided and shared equally among these three PTCs. If one of the PTCs trip then the other 2 carry the load of the tripped PTC. This will cause the other 2 to trip in rapid succession. This means that having one faulty PTC will cause all 3 to trip and make your heli fall from the sky under normal flying conditions. This is never fun but it can be avoided.

Before we get into 3 in 1 modification we need to look at the causes of a over current protection other than a faulty PTC. Unlike LVC a PTC can not trip because of bad batteries or frayed battery wires. However, a faulty motor can cause a PTC to trip. If the main or tail motor is bad, it will consume more power than normal. This extra power consumed will cause the PTC heat up and break the electrical connection. This saves your board from overheating and burning. Sometimes the over current protection is not activated from a single motor. If you have a old main and tail motor their individual power consumption may not be enough the trip the circuit but combined they can consume more current than the PTC can handle. The PTC will also trip if you have a shorted motor wire but never with a wire that is broken causing an open. When your pitch is set correctly, a faulty motor is the cause of most over current shut downs.

All causes of over current protection shutdown with the exception of a faulty PTC trace back to the motors. You can have bad main shaft bearings that cause additional stress on your motor that causes a shutdown. You can have a bad main gear or an improper gear mesh that stresses your main motor and shuts it down. You can also have blades that are to damaged to move through the air efficiently. This air drag means the motor will have to do more work and can result in a shutdown. Pretty much anything that can place stress on the motors can be cause for a over current shutdown. If you can't figure out why your heli is shutting down look very carefully at what work the motors are having to do to find the cause.

There are times that the motor is not the cause of an over current shutdown. Sometimes it is the 3 in 1 that is faulty. That is OK because there is a way to work around it.

In order for a PTC to work, the current has to flow through the device. So, if you have a faulty PTC you just make the current flow around it. This is very easily done by creating a jumper from one side of the PTC to the other. There are 2 ways to do this. The first way is to leave the PTC on the board and just solder a single wire to both ends of the device. The other method is to completely remove the PTC and replace it with a conductor like a piece of wire, a metal bar, or anything that will conduct the electricity freely. You only need to bypass one PTC because of how they are connected in parallel. By bypassing the one you bypassed all three.

Once the PTC is bypassed you will no longer have over current protection at all. If you are careless this can be a bad thing. If you pay attention to your heli, maintain your motors, and remember to hit throttle hold, bypassing the PTC is not a problem. When horizon hobby made these circuits, they designed them for everyone all the way down to the most novice pilot. These novices might need the circuit but you don't right? I don't think so, but this is a decision you will have to make for yourself.

Hopefully this information gave you a better understanding of your 3 in 1 board. It can be difficult to present so much information in a logical order that everyone can follow and understand. If I goofed somewhere please call me out on it. Or if you have any questions then ask away and I will do my best to help.

My next topic will be how to repair a damaged power trace by installing a via on the 3 in 1. You will be able to find it HERE first.

Mega

[CyclicPete]

Another Excellent write up Mega

[Nytflyer]

Very informative thank you!

[MiataJohn]

Nice job Kevin!

[Elmosaurus]

Awesome!

E.

[automat]

great write up!

[Haket]

Great info thanks !, I had one question on the OCP function, in my experience when I get a hard tail motor shutoff the main motor is still under-power, but from your write-up it suggests that both motors shutoff. Did I misunderstand something ?.

[kurmet]

if we are running a brushless set up are we using the the lvc in the esc or the 3-1 and also the overcurrent protection ?

[CalRotor]

Haket,

I think what's happening is your tail rotor, with virtually no momentum, seems to dead stop, while the main blades, with much higher relative momentum, keep spinning and give you what is effectively a tail blow out.

[Mega Stumbler]

Calroter, you hit it perfectly.

When using a double BL setup you are using LVC on both the 3 in 1 and the ESC but you are not using the over current protection. When using just a BL main your tail is still going through the over current circuit but out probably will not offer any protection since the circuit is designed for the main and tail together.

This is why tbo on a BL setup is rare.

Sent from my DROIDX using Tapatalk 2

[Haket]

Hmm, I've seen two error cases:

1.The bird goes limp and falls from the sky like when you hit TH mid flight
2.TBO occurs and the main rotor seems to be driven (more throttle causes faster rotation) causing the heli to rotate violently (like it would if the tail motor plug fell out). Are you saying that #2 can't happen due to the OCP circuit ?

[4712]

@Kevin: I like the work and Your engagement. So thanks a lot for your writeup.

But let me write two remarks here:

1. How did You get the values of the resistors? I measured here 91K/82K appx. for V1 and V2 board (I never had a V2 1.5 btw.).
2. What made You think there are PTC use in the over current protection?
Did You measure it, or is it a assumption? Well if You measure it, You will find, there are three 50 milliOhm resistors (R050 size 0603) in parallel which results in 50/3 = 16.6 milliOhm. I don't know the manufacturer but this here might be one possible replacement. Because they are rated at 100mW, three of them are needed in parallel, So together they are rated for 4.3Amps.
If You measure the resistance with high current (5Amps) You will see what happens.

HTH

[kurmet]

but doesn't the main esc have overcurrent protection ? i think it dose

[4712]

Originally Posted by kurmet

but doesn't the main esc have overcurrent protection ? i think it dose

Yes it does, but it is managed bei the MCU.

[dahli.llama]

Very interesting.

One question on the OCP. Is this what causes the motors to shutdown during a crash if the blades are stalled? Or is there a different circuit that monitors that kind of thing?

[4712]

Originally Posted by dahli.llama

One question on the OCP. Is this what causes the motors to shutdown during a crash if the blades are stalled?

Yes. It is responsible for both Main and Tail. So, if going brushless one can remove 2 of the 3 resistors to make it more responsive if tail is stalled.

[itsmillertime0]

What are the benefits of removing the resistors? I'm planning a double brushless build.

[4712]

Originally Posted by itsmillertime0

What are the benefits of removing the resistors? I'm planning a double brushless build.

None. Sorry, forgotten to mention: what I wrote is only valid for Main BL.
With double BL the change has no effect, the OCP will be disabled anyway.

[4712]

One short update to the OCP
Both, the main and the tail motor are connected with their (+)-pole to the driving MosFets.
With the (-)-pole they are connected to a common point (not GND).
The current of both motors now finds its way to GND (= Battery minus) through the 3 resistors, each 50 milliOhms, which build one shunt with 16.66 milliOhms. As soon as the motors run, there will be a voltage at this shunt, which directly corresponds to the actual current flowing through the motors.
The voltage is now connected to one ADC Input of the MCU (over a RC filter).
As soon as this voltage reaches 100mV, the MCU activates the OCP.
So easy to calculate: I=U/R -> I=0,1/0,01666 = 6.0024 Amps.
To test the OCP I take a R47 resistor at the main motor plug.
I'm thinking about collecting all Infos about the 3in1 (including a schema)... but it's a bunch of work :-(

[MeatRocket]

Great information, but is the v2 board more prone to OC shutdown? I just replaced my v1 board with a v2 board and it cuts off in mid-flight under any sort of load. I've replaced both motors & the battery, still does it.