I want to control the 12V heating of an oxygen sensor with an ATmega328P. It draws about 6A when switched on when it is cold and about 1.5A at the operating temperature of 350°C. It is powered with a 12V/10A switching power supply.
My circuit is measuring weak signals from different sensors (thermocouple and resistance thermometer besides the oxygen sensor) and is a little sensitive to noise, so I am not sure if it is a good idea to connect GND of that big switching power supply with GND of my circuit which I think I have to do if I would use a MOSFET? The insulation between the heating and the connectors of the oxygen sensor is > 30MOhm so at the time being, there is a separation. The circuit is powered by a small 9V linear wall-wart.
To keep both GNDs separated I thought about using a 5V relay that can switch up to 10A but its 225 mW are too much for the AVR, but I guess I could use a relatively simple transistor to drive the relay.
Would that be a reasonable solution or do I worry too much about noise from the switching power supply?