Explain to logic behind this circuit

 

power source is connected on the left side.
load is connected on the right side (after Q1).

in normal operation current flows through R1 and Q1 to load.
Q1 works as a switch, R1 works as current sensing shunt - the higher the current, the higher voltage drop across R1.
Q1 is normally always on because of R8. but can be turned off by Q2. this happens when comparator output is low (lower side of R6 is close to zero).

comparator compares voltages across R1 and R4.
Voltage across R1 changes with load current. Voltage across R4 is fixed.

 

power source is connected on the left side.
load is connected on the right side (after Q1).

in normal operation current flows through R1 and Q1 to load.
Q1 works as a switch, R1 works as current sensing shunt - the higher the current, the higher voltage drop across R1.
Q1 is normally always on because of R8. but can be turned off by Q2. this happens when comparator output is low (lower side of R6 is close to zero).

comparator compares voltages across R1 and R4.
Voltage across R1 changes with load current. Voltage across R4 is fixed.

R1 and R4 are not in parrallel, neither in series, how can comparator compare the voltage if input current increases/decreases?
How do i calculate the voltage comparison value i.e. +ve side of comparator?
How do i calculate current cut off point?

 

The voltage across R1 varies with the output current. This is connected to the to the non inverting input of the comparator via R2.
The voltage on this input is the input voltage minus the voltage across R1
The voltage on the inverting input of the comparator is preset by the referance voltage and R4 and R7 . So the voltage on this input is the input voltage minus the voltage across R4. When the votage across R1 is greater than the voltage across R4 the output of the comparator changes state.

Les.

 

R1 and R4 are not in parrallel, neither in series, how can comparator compare the voltage if input current increases/decreases?
How do i calculate the voltage comparison value i.e. +ve side of comparator?
How do i calculate current cut off point?

you need to apply theory that you were learning in your courses. one of the topics that you absolutely have to know is about an ideal operational amplifier. it tells you that in ideal world, input current for the OpAmp (or comparator) is 0 A. so the voltage at point between R1 and R4 does not change if you disconnect comparator from the circuit. and in that case it should be obvious that R1 and R4 are in series...