Reference Temperature for AD595

[Sajjadkhan]

Hi guys, i have used AD595 in my home made soldering station, everything worked fine but when the solder cooled down for a long time then its temperature wasn't equal to the room temperature. my room temperature is 19 degree C, but AD595 was showing 30 degree C. then i figured out that problem is that it is enclosed in a box, there was one triac bt136 1 inch away from it and it was driving a 60W solder so it wasnt hot. transformer wasn't hot also as it was 3amps but driving a controller circuit only( big transformer just to make my box heavy). anyways when i opened the box and put some air through a fan, temperature dropped to 24 degree C. so i guess problem is with the reference temperature as its cold junction compensation doesn't wok in the box. so i figured that it has some extra pins and some of them are related to cold junction compensation but the datasheet didnt talk about it. so any idea how i can feed it with a reference temperature via lm35 or some other sensor?

 

[betwixt]

AD595s are very accurate as long as you follow the manufacturers instrucions very closely. Certainly, heating it up isn't a good idea but it's just as likely you have inadvertantly created a second thermocouple junction at the IC itself. Have you taken into consideration the type of solder and connection method you used to connect the thermocouple and how it is jointed to the IC itself? Remember that almost any connection between different metals could give as much voltage output as the thermocouple itself but not be at the point you want to measure.

Brian.

 

[FvM]

i guess problem is with the reference temperature as its cold junction compensation doesn't wok in the box. so i figured that it has some extra pins and some of them are related to cold junction compensation but the datasheet didnt talk about it. so any idea how i can feed it with a reference temperature via lm35 or some other sensor?

The AD595 operation principle requires to have the reference junction directly at the IC, or at least avoid a tempearure difference between IC package and refernce junction. Or use compensation cable between thermocouple and the IC. Otherwise AD595 is useless. You could replace the internal temperature reference with an external sensor, but then the AD595 doesn't pay at all.

 

[Sajjadkhan]

AD595s are very accurate as long as you follow the manufacturers instrucions very closely. Certainly, heating it up isn't a good idea but it's just as likely you have inadvertantly created a second thermocouple junction at the IC itself. Have you taken into consideration the type of solder and connection method you used to connect the thermocouple and how it is jointed to the IC itself? Remember that almost any connection between different metals could give as much voltage output as the thermocouple itself but not be at the point you want to measure.

Brian.

yep you have pointed out another problem. actually there are 2 extra junctions. i have attached the thermocouple to the element and cut it like 5 inches away to hook it up to the terminal block in the solder iron. then out of the solder iron,cable wire is gone to a 5 pin din connector and there has to be another junction for the pcb to solder(that junction can not be avoided),the ic is very close to this junction like 1/10th inches away. the junction inside the soldering iron can be avoided, i couldn't find the din connector separately so i used the 5 pin din cable and cut off its female part. like this one **broken link removed**, so i am wondering that junction in side the solder is putting extra positive offset.

- - - Updated - - -

The AD595 operation principle requires to have the reference junction directly at the IC, or at least avoid a tempearure difference between IC package and refernce junction. Or use compensation cable between thermocouple and the IC. Otherwise AD595 is useless. You could replace the internal temperature reference with an external sensor, but then the AD595 doesn't pay at all.

Hello FVM, i have seen 24V heating elements with 4 wires, to for power and 2 for built in thermocouple, if 2 wires are attached to the element for thermocouple then there would be another junction there, how would they compensate for that.

**broken link removed**

 

[FvM]

Extra junctions aren't a problem, if you extend the original thermocouple with compensation cable. If you extend it with copper cable, the effective reference junction will be at the the T.C. to copper interface, and the difference temperature between it and the AD595 is added as an error.