Sensor Board

This board was designed to interface JP4 on the main board. It contains the current sensors, a resistive voltage measurement and an input for a temperature sensor. A J-Curve temperature probe is provided with the board.

Current sensors

The provided 400A current sensors need an unipolar supply voltage of 5V. They output 2.5V when idle, i.e. at 0A. At -400A the output is 1V and at 400A the output is 4V.

The board contains an offset/gain stage that converts this to 0-3.3V. As a side effect the signal is not inverted. Now the output is 1.67V when idle, 0V at 400A and 3.3V at -400A. These values are approximations. Therefor the software allows for another offset/gain correction for each current sensor.

Voltage sensing

The voltage sensing uses megaohm resistors to fullfill the isolation requirements from the ECE-100 being 500Ohms/V. As the typical operating voltage of through-hole resistors is 250V, 4 of them are used in series. The high impedance signal is converted to low impedance. This is cheaper than using real galvanic isolation. As a side effect isolation problems can be detected because any resistance in parallel to the sense resistors will affect the reading. The cutoff frequency is about 1Hz.

The formular for the maximum input voltage is

Temperature sensing

The temperature sense input has an input impedance of 1.2k. This matches the J-Curve temperature probe and is also the value assumed by the inverter software.