Answered: PIC current overload

VEX V5 Technical Support OFFICIAL Tech Support
#1

Hello, I have a custom vex robot with 4 wheels tank drive using 6 269 motors. I have mounted custom wheels with high traction so they almost never slip. Whenever the wheels stall, the led on the PIC microcontroller will turn red and it will shut down and turn on again. The problem is when it turns on, all the motors receive a full pwm signal for a few hundred ms (which causes the robot to turn since the left motors are reversed from the right motors). When this occurs, the motors draw enough current to overload the 4 amps limit on the microcontroller, so it shuts off and turns on again. And this cycle goes on in few hundred ms intervals. Once I lift the robot off the floor, the robot will go back to normal since the wheels are now free spinning. So my question is, how do I fix this problem and why do the motors receive a full pwm signal when powered on?

Here’s some additional information for the diagnosis. The microcontroller I am testing with is new and I’ve also tested against an old PIC microcontroller which gave the same results. I am using 6 NiMH 2000 mAh duracell batteries as the robot battery. My gear ratio is 5:3 and everything mechanical is securely fixed. The code being tested against is a basic easy C project with just code for the motors.

Thanks so much.

#2

Your batteries can not supply the current needed by your motor configuration. When this issue happens, the voltage output from your batteries drops which causing the unit to reset. If you overload the 4 Amp Internal Breaker and cause it to trip, your Motors Outputs would stop working temporarily but unit would not flash RED; therefore, you are not tripping the Breaker. You will need to change your batteries or change the load on your motors.

#3

Do you suggest to get the vex 7.2V battery instead? And also, is there any way to prevent the motors from receiving a signal when powered on?

#4

Yes, I would try the VEX 7.2V 3000 mAh battery (http://www.vexrobotics.com/products/accessories/power/276-1491.html). Also, if you do start tripping breakers you may also want to consider a Power Expander (http://www.vexrobotics.com/products/accessories/power/276-2271.html). More info on the Power Expander (https://vexforum.com/wiki/index.php/Power_Expander). The brief period of the 3 wire motors turning ON during power-up can not be corrected – they is how they react when power is applied. . The brief period of the 3 wire motors turning ON during power-up can not be corrected – they is how they react when power is applied. )

#5