Precharge circuit

A motor controller acts a capacitive load. If we connect the load to the power directly there will be a huge surge in the inrush current which can destroy the motor controller and the control electronics thereafter. However, if we simply connect it with a series resistor in series with the load, there will be heavy power wastage at the resistor and the load will never be able to reach the voltage and hence power that it is supposed to operate at. So, our solution to this is a pre-charge circuit where we initially connect the load with a series resistor and once the load voltage has risen to 95%, we bypass the resistor with a MOS and a relay by directly connecting the supply to the load. We also use a diode across the relay to prevent the emf discharge from destroying electronics. Pull down resistors are used at the mosfet to prevent false triggers. 

We reference the capacitor voltage against the battery voltage, both voltages are reduced by a resistive divider and then fed in to the octo coupler. The output from the octo coupler is fed into an operational amplifier that acts as a comparator. The output of the op-amp is used to trigger a MOS which then triggers a relay to which our capacitive load is connected.