I'm learning op-amps at the moment, so try to bear with me because this text will most likely contain a lots of errors.
I have a chip resistor, 10mohm, that I intend to use as a shunt for measuring current. Vcc = 5V, I = 2A normally. I = 2.5A means things are going wrong, 3A = dead wrong. So, somehow I want to watch the voltage drop of the resistor, and when the current goes to high (I haven't set a limit yet, but let's say 2.5A) I want an NPN to conduct, just for the sake of testing.
So, I've got two measuring points on the resistor: Rs1 and Rs2. 2.5A with 10mohm is 25mV. That is, when the voltage drop over Rs1 and Rs2 > 25mV, trip the NPN. If voltage drop < 25mV, do nothing.
Rs1 and Rs2 goes to an opamp's in+ and in-. Other than that, I'm stuck. I am completely clueless what sort of feedback network I'm supposed to use to achieve what I want. Googling for "opamp differential amplifier" gives me http://www.electronics-tutorials.ws/opamp/opamp_5.html which seems to be basic enough for me to understand, but I've been reading this paragraph five times now and I just don't get it:
Then differential amplifiers amplify the difference between two voltages making this type of operational amplifier circuit a Subtractor unlike a summing amplifier which adds or sums together the input voltages. This type of operational amplifier circuit is commonly known as a Differential Amplifier configuration and is shown below
This sounds like it's what I'm trying to achieve, but what does it mean that the circuit is a "Substractor"? If I'm supposed to amplify the difference and letting the 25mV-level trip the NPN-base, what's substracting got to do with it?
I'm clueless, really.
Is differential amplifier the kind of feedback type I'm searching for here, for my testing?