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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?

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  • \$\begingroup\$ Is the heater just a resistive element? Where do you expect the noise to come from? \$\endgroup\$ Jun 4, 2015 at 6:29
  • \$\begingroup\$ He wants to prevent noise from outside (the AVR and/or it's supply) to enter his circuit containing the sensors. I would also use a relay assuming the heater element has to switched on/off only a couple of times a minute (or less frequently). Yes you can just use a transistor to switch the relay from the AVR, but I would use a MOSFET as that would make it even more easy, look here: activewireinc.com/docs/Aw_TechNote.htm under "2 Interface with MOSFET output". That is what you need. There is already a flyback diode in the 2n7000 so no need to worry about that. \$\endgroup\$ Jun 4, 2015 at 7:12
  • \$\begingroup\$ @Rimpelbekkie Indeed the heater will be switched on and off only once or twice a day. Thanks for the link, it is what I was looking for. You answered my question so you could just turn your comment into an answer :-) \$\endgroup\$ Jun 4, 2015 at 10:48

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I would also use a relay assuming the heater element has to switched on/off only a couple of times a minute (or less frequently). Yes you can just use a transistor to switch the relay from the AVR, but I would use a MOSFET as that would make it even more easy, look here under "2 Interface with MOSFET output". That is what you need. There is already a flyback diode in the 2n7000 so no need to worry about that.

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