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Topic: AD595AQ for Car Exhaust Gas Temperature (Read 3868 times) previous topic - next topic

matinzk

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.

frank26080115

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.
Freelance engineer, consultant, contractor. Graduated from UW in 2013.

matinzk

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

M.

EmilyJane

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

matinzk


EmilyJane

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

matinzk


EmilyJane

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

matinzk


EmilyJane

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?

matinzk

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

The circuit's GNDs are connected to Arduino GND.

EmilyJane

Is the Arduino ground connected to car chassis ground?

EmilyJane

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

matinzk

Yes, from the same place and grounded type.

M.

EmilyJane

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?

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