Datasheets for MOSFET transistors indicate such a parameter as RDS. This is one of the main parameters of the transistor and it shows the resistance of a fully open drain-source channel of the transistor. At the same time, the gate-source voltage is indicated at which the transistor will be fully open. RDS of the IRF730 transistor is 1 Ohm at a gate-source voltage of 10 Volts. How to find out this resistance and what will happen if the gate voltage is less?
To do this, assemble a simple circuit. A 10 kOhm resistor between the gate and source serves to quickly discharge the gate-source capacitance. If you do not apply voltage to the gate, the drain-source resistance will be very large, on the order of tens of megaohms. Now apply a voltage of 10 Volts to the gate-source and the drain-source resistance becomes equal to 1 Ohm.
Now experiment. Start slowly lowering the gate voltage and you will see, how the resistance of the drain-source channel will increase. At a gate voltage of 6.3 Volts, the resistance will be 16 Ohms.
What happens if the drain-source resistance is high? In pulsed devices, this will lead to rapid heating of the transistor and it may fail. Never apply a gate-source voltage lower than specified in the datasheet!
