engr-sajjad
Newbie level 1
Make me Understand....
i have searched a circuit for current sense or current monitoring and i have come across a circuit which i have attached.
i tried to figure out that how this circuit works and it goes as follows.
1st consider that load is not present. then Va = Vb and and transistor is in cutt off state where Vc (collector voltage) =
Va. so at this stage Va=Vb=Vc=V+=V-=Vcc=5v. the transistor will stay off and thus Ve ( emitter voltage) will be 0v.
Now lets add a little load, thus Va will be slightly greater that Vb. this in turns will make the op-amp go in on state i.e.
saturation which will then force the transistor to go in saturation. now when transistor will be going in saturation then
Vc starts decreasing and as it becomes a little less than vb the transistor will be again switched on by the op-amp and
again when Vc becomes slightly higher then again transistor will go in saturation and this process will keep on going.
so i thing that this will shows us the voltage in terms of current but with very little oscillations. how little? i m using
LM358 so it can detect difference of minimum 2mv...so 2mv amplitude of oscillation at the threshold level. which i think
can be ignored or filtered out by installing a capacitor in parallel.
in this whole scenario i have supposed that op-amp is acting as comparator. obviously there is no question that its is a negative feed back but there is a possibility that it is some how acting in positive feed back with some finite gain but positive feed back introduces hysteresis doesn't it?
so plz tell me if im right or wrong and do explain this if i m wrong. ill be thankful. op-amps really confuses me. by the way i have tried this circuit and it works with out any hysteresis.
i have searched a circuit for current sense or current monitoring and i have come across a circuit which i have attached.
i tried to figure out that how this circuit works and it goes as follows.
1st consider that load is not present. then Va = Vb and and transistor is in cutt off state where Vc (collector voltage) =
Va. so at this stage Va=Vb=Vc=V+=V-=Vcc=5v. the transistor will stay off and thus Ve ( emitter voltage) will be 0v.
Now lets add a little load, thus Va will be slightly greater that Vb. this in turns will make the op-amp go in on state i.e.
saturation which will then force the transistor to go in saturation. now when transistor will be going in saturation then
Vc starts decreasing and as it becomes a little less than vb the transistor will be again switched on by the op-amp and
again when Vc becomes slightly higher then again transistor will go in saturation and this process will keep on going.
so i thing that this will shows us the voltage in terms of current but with very little oscillations. how little? i m using
LM358 so it can detect difference of minimum 2mv...so 2mv amplitude of oscillation at the threshold level. which i think
can be ignored or filtered out by installing a capacitor in parallel.
in this whole scenario i have supposed that op-amp is acting as comparator. obviously there is no question that its is a negative feed back but there is a possibility that it is some how acting in positive feed back with some finite gain but positive feed back introduces hysteresis doesn't it?
so plz tell me if im right or wrong and do explain this if i m wrong. ill be thankful. op-amps really confuses me. by the way i have tried this circuit and it works with out any hysteresis.