I would like to build a circuit that switches a light when DC current is at a certain level (about 30 Amps). When the voltage across a shunt (50 mV can place either low or high side) rises above a set point, the relay should switch and turn on a light. I would like to use a 120VAC relay.

What's the best way to switch such a relay? I was thinking of using an opamp to amplify the voltage across the shunt. This will turn on an NPN transistor which should provide enough current to switch the relay.

Is there anything I should watch out for? Should I use a comparator instead of an opamp? Is there already something off the shelf that does what I need?

 

You can get integrated Hall-effect sensors that have a built-in amplifier that's isolated from the high-current line. Output is a voltage proportional to current. It's definitely the elegant way to do this job.

 

I second the Hall device approach. A 50mΩ shunt would drop 1.5V at 30A, so dissipate 45W of heat!