MOSFET current sensing

  
The circuit shows the method of `mirror` current sensing a MOSFET. A fully conducting MOSFET is resistive and behaves exactly as a resistor. It therefore you limit the voltage across the MOSFET when it is conducting you automatically limit the current flowing through it. In addition the MOSFET``s on resistance has a positive temperature coefficient so as it warms up, keeping the voltage across it limited will automatically reduce the limited current flowing through it, keeping it within its safe region over a wide range of junction temperatures.
MOSFET current sensing - schematic




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