Current Comparator Circuit Design

Hello all,

I wanna make a current comparator circuit which compares the two incoming currents.

Please help me to design this... Details are as given below.

One of the current which is gonna be compared will be of fixed value of
12mA (works as threshold). And other one will be dynamic.

If the second incoming current exceed the threshold level (12mA) then decision

making has to be done and relay at the output end must be switched off...

Please Help ASAP....

DC or AC current?

Do you need to measure the high-side line or can you measure the low-side line?

Is this a mains voltage or mains-isolated low voltage system? Supply volts?

@crutschow--
Its DC current. and i just need to sense the variation in the incoming second current if its not 12 mA then further system should be shut off. so whether its high side or low side if its not 12mA then i need decision making.....

@alec_t --

It's isolated current not the mains current... I just need to compare two isolate entity of currents....
Is Op-amps are good enough to do this job satisfactorily..????

If you can measure the low side current (in the common lead) then you can use a single-supply or rail-rail type op amp. You just put a small (shunt) resistor in the common lead and then amplify the voltage drop across the lead with the op amp.