Arduino Forum

Using Arduino => Sensors => Topic started by: matinzk on Jul 28, 2011, 08:11 pm

Title: AD595AQ for Car Exhaust Gas Temperature
Post by: matinzk on Jul 28, 2011, 08:11 pm
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.
Title: Re: AD595AQ for Car Exhaust Gas Temperature
Post by: frank26080115 on Jul 28, 2011, 08:48 pm
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.
Title: Re: AD595AQ for Car Exhaust Gas Temperature
Post by: matinzk on Jul 28, 2011, 08:49 pm
I can't do that, I need it to be mounted properly as it is.

M.
Title: Re: AD595AQ for Car Exhaust Gas Temperature
Post by: EmilyJane on Jul 28, 2011, 09:19 pm
Are you using a grounded junction thermocouple or an isolated junction one?
Title: Re: AD595AQ for Car Exhaust Gas Temperature
Post by: matinzk on Jul 28, 2011, 09:24 pm
A grounded one.

M.
Title: Re: AD595AQ for Car Exhaust Gas Temperature
Post by: EmilyJane on Jul 28, 2011, 10:01 pm
Did you ground either sensor lead to pin 4 of the chip?
Title: Re: AD595AQ for Car Exhaust Gas Temperature
Post by: matinzk on Jul 28, 2011, 10:02 pm
Yes,

Pin1 to Pin4.

M.
Title: Re: AD595AQ for Car Exhaust Gas Temperature
Post by: EmilyJane on Jul 28, 2011, 10:05 pm
That is not recommended for a grounded sensor. Try it without.
Title: Re: AD595AQ for Car Exhaust Gas Temperature
Post by: matinzk on Jul 28, 2011, 10:11 pm
The same. Goes up even more!

M.
Title: Re: AD595AQ for Car Exhaust Gas Temperature
Post by: EmilyJane on Jul 28, 2011, 10:15 pm
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?
Title: Re: AD595AQ for Car Exhaust Gas Temperature
Post by: matinzk on Jul 28, 2011, 10:22 pm
My circuit is connected to a L7809CV. L7809CV is connected to car's ignition.

The circuit's GNDs are connected to Arduino GND.
Title: Re: AD595AQ for Car Exhaust Gas Temperature
Post by: EmilyJane on Jul 28, 2011, 10:30 pm
Is the Arduino ground connected to car chassis ground?
Title: Re: AD595AQ for Car Exhaust Gas Temperature
Post by: EmilyJane on Jul 28, 2011, 10:33 pm
I just reread your original post. Is the thermocouple that works at home the same as the one in your exhaust pipe?
Title: Re: AD595AQ for Car Exhaust Gas Temperature
Post by: matinzk on Jul 28, 2011, 10:34 pm
Yes, from the same place and grounded type.

M.
Title: Re: AD595AQ for Car Exhaust Gas Temperature
Post by: EmilyJane on Jul 28, 2011, 10:41 pm
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?
Title: Re: AD595AQ for Car Exhaust Gas Temperature
Post by: matinzk on Jul 28, 2011, 10:45 pm
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.
Title: Re: AD595AQ for Car Exhaust Gas Temperature
Post by: EmilyJane on Jul 28, 2011, 10:55 pm
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.
Title: Re: AD595AQ for Car Exhaust Gas Temperature
Post by: matinzk on Jul 28, 2011, 11:03 pm
They both read wrong.
Title: Re: AD595AQ for Car Exhaust Gas Temperature
Post by: EmilyJane on Jul 28, 2011, 11:07 pm
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.
Title: Re: AD595AQ for Car Exhaust Gas Temperature
Post by: matinzk on Jul 28, 2011, 11:14 pm
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?

Thanks.
Title: Re: AD595AQ for Car Exhaust Gas Temperature
Post by: EmilyJane on Jul 28, 2011, 11:29 pm
You could try that. Otherwise it's a mystery to me.
Title: Re: AD595AQ for Car Exhaust Gas Temperature
Post by: EmilyJane on Jul 28, 2011, 11:34 pm
By the way, why did you choose to run it off 9V? That part works on 5V.
Title: Re: AD595AQ for Car Exhaust Gas Temperature
Post by: matinzk on Jul 29, 2011, 12:22 am
I needed temperature of more than 300'C so I increased the input voltage.

Title: Re: AD595AQ for Car Exhaust Gas Temperature
Post by: EmilyJane on Jul 29, 2011, 01:08 am
Ah, of course. Did you consider this part? http://www.sparkfun.com/products/307 (http://www.sparkfun.com/products/307)
Title: Re: AD595AQ for Car Exhaust Gas Temperature
Post by: matinzk on Jul 29, 2011, 08:10 am
No I haven't, but does that solve my grounding problem?

Thanks.
Title: Re: AD595AQ for Car Exhaust Gas Temperature
Post by: EmilyJane on Jul 29, 2011, 06:31 pm
Quote
No I haven't, but does that solve my grounding problem?


No, only in the sense that it might correct the problem that we haven't been able to sleuth out yet. Plus I just like the fact that it has a digital interface so it leaves out some steps in processing the data.

Without looking at your project, I'm at a standstill. Can you post the EXACT schematic diagram and some closeup photos of all sides of your physical circuit?
Title: Re: AD595AQ for Car Exhaust Gas Temperature
Post by: matinzk on Aug 01, 2011, 06:53 pm
I got an ungrounded thermocouple and everything is working normally now.

Thanks for all the help.
Title: Re: AD595AQ for Car Exhaust Gas Temperature
Post by: EmilyJane on Aug 01, 2011, 11:07 pm
Good deal!