Power Monitoring PCB

May 2021

Abstract: 

After dealing with several power issues on the odometry board provided by our vendor, I decided to build board to display the power status of our critical subsystems (an overview of the project this board was built for can be found here). The goal was to use the voltage monitoring test points on the odometry board as input to the power monitoring PCB. In addition, I added test points in series with the input lines so we could probe input voltages without having to remove the chassis cover of the robot. If the input voltage to a subsystem falls below its operating threshold, the respective status LED will be turned on.

Process: 

The main component of this circuit is the LMN339N IC. This IC consists of four comparator circuits which are the driving component behind the low voltage cutoff concept. Each input voltage is stepped down via a voltage divider circuit and fed in as positive input to the IC (pins 5, 7, 9 & 11 as seen in figure to the left. The voltage divider resistors are currently spec'd such that at cutoff voltage, the input voltage will be 3.1V. This way when the the cutoff voltage is reached, the comparator will output 0V as the input has fallen below the threshold voltage. The Vcc and Vref will be supplied to the IC via a Jetson AGX. I decided to the Jetson AGX as this micro-controller has a built in voltage regulator. 

 

The LMN339N has an open collector output which behaves like a switch which is either connected to ground or an open circuit depending on the input. When the input voltage for an input-output pair falls below the reference voltage, the output acts like an open circuit. An LED is connected in series between Vcc and this output such that when the output is connected to ground, the LED will shine. We have also placed current limiting resistors in series with these LEDs to reduce the current to 20mA.

Result: 

The manufactured PCB can be seen in the image to the right. This board is currently mounted on the robot and used as a visual indication of the status of each our subsystems.