Present sense resistor is typically located in the switch mode power supply section. Latest version of monitor-the b+ circuit also have this present sense resistor. If you repair electronic equipment particularly the switch mode power supplies, the present sensing resistor is connected to the source pin of power FET. The value of present sense resistor is in the range of 0.1 ohm to about 1 ohm. The values are depends on the design of each power supply.

In monitor power supply where pulse width modulation (pwm) ic UC3842 are used. Pin 3 of this power ic is called I-sense which means current sense and this pin is connected to source pin of POWER FET. The purpose of this current sense circuit is for over current protection. This circuit generally samples the voltage drop across a resistor in series with the switching FET transistor. If the present rises abnormally due to some short circuit in the secondary side such as a shorted secondary diode or horizontal output transistor, the voltage will go beyond a reference level and shutdown the pulse generator which is the PWM ic. This circuit provides protection to components in the event a trouble occurs. In some cases overload happen-the power section will blow and most of the time the components changed are fuse, power FET, pwm ic, zener diode and present sense resistor.