AD595AQ for Car Exhaust Gas Temperature

Hi everyone.

I am making an EGT meter to be able to read the temperature from my exhaust pipe in the car.

I have used the typical circuit in the datasheet for the AD595AQ chip.

I am using a thermocouple that is designed for temperatures between 0-1000'C.

I am also supplying 9v to the chip, and using a voltage divider to make the voltage half, to be able to read it with Arduino without frying the chip if the temps get too high therefore the voltage.

Everything seems to be working fine when I connect the thermocouple at home! However when I connect the circuit to the thermocouple on the exhaust pipe on the car, the temperature and therefore the voltage out of the chip rise as much as 10 times!

I've read somewhere as the probe on the exhaust pipe is grounded to the car it causes this. But how can we solve the readings to be corrected?

Any help is appreciated.

Thanks, M.

Maybe you can mount the sensor to the car without it being grounded? I know there are some pads that are used to stick heat sinks to computer chips, try that.

I can't do that, I need it to be mounted properly as it is.


Are you using a grounded junction thermocouple or an isolated junction one?

A grounded one.


Did you ground either sensor lead to pin 4 of the chip?


Pin1 to Pin4.


That is not recommended for a grounded sensor. Try it without.

The same. Goes up even more!


That's the correct way to use a grounded thermocouple with that chip. Let's check other things. Have you grounded your circuit to the vehicle chassis? Is your circuit powered from a nine volt battery or from some derivation of the vehicle battery?

My circuit is connected to a L7809CV. L7809CV is connected to car’s ignition.

The circuit’s GNDs are connected to Arduino GND.

Is the Arduino ground connected to car chassis ground?

I just reread your original post. Is the thermocouple that works at home the same as the one in your exhaust pipe?

Yes, from the same place and grounded type.


Okay, at the car, disconnect the exhaust TC and substitute the one that worked at home. If that works okay then touch it to the car chassis and see what happens. What you're describing sounds like a ground loop problem. Are you sure that the Arduino AND its power supply are grounded directly to the car chassis?

I have substituted them with one another the same thing.

Arduino uses the same L7809CV chip as the AD595AQ, and the L7809CV is connected to the chassis.

By the "same thing" do you mean they both read wrong now or that just the one on the car reads wrong. What happens when you touch the "home" one to the car.

They both read wrong.

Oh, okay. Something happened to your circuit in transit apparently. You need to thoroughly check it for loose, broken wires, short circuits, etc. Anything that is different from when it last worked correctly.

It works fine when I don't connect it to the car, so the probe that's on the car, when removed and pin 1 connected to pin 4 it works fine!

So the chip is fine, isn't it?

Is it better to go and get an ungrounded probe then? And leave the circuit with pin 1 connected to pin 4?