Ultra sensitve current sensor

[Dragon Tamer]

I have been working on what should be a fairly simple circuit for several weeks now and I haven't had any luck getting it to work. What I am trying to do is to find the current through a coil and detect if the current goes slightly higher than it's nominal current draw.

The circuit is for a stepper motor controller that I am designing and the circuit needs to be sensitive enough to detect a current ratio of a 1µA change for every 1mA of current draw (roughly). It is to work off of the principal of when the motor is running, and it gets stuck, the current through the coil increases slightly.

I have a circuit that I made that works to some extent, the problem is that it's not quite sensitive enough, but if I modify any of the values, it stops working properly. As of right now it has a ideal voltage detector, a coil current sensor, and a diff amp combining the two. I can post the schematic later, it's not finished yet.

Thanks for your help.

 

[Reloadron]

Posting the circuit you are using may help but overall good luck resolving a difference in current draw of 1 uA, even 1 uA per mA. You are really getting down in the dirt. What you need to look at is the motor's free running current best defined as the motor just sitting there running without any load. Then look at the motor's running current under various loads working towards stall current when the load on the motor brings the rotor to a stop which should be the rated torque for the motor. (Stuck Current is Stall Current). However, trying to resolve differences of uA is not going to be easy with a handful of off the shelf components. What is the free running current? What is the stall current and at what current do you want the motor to trip? How exactly is the motor current being monitored?

Ron