testing a thermocouple

[evandude]

I am trying to fix my soldering station. the actual iron part connects with a 5-pin connector... 2 pins for the heating element, 2 for the sensor, and 1 for an earth ground for the tip. The sensor measures almost zero ohms, so I am assuming it is a thermocouple and not a thermistor.

I don't have any real experience with thermocouples so could anyone give me some info on how i might test this thermocouple to see if it is working? The soldering station isn't working so obviously i can't just turn the thing on and let it heat up and see if i get any voltage out of it.

Would dunking it alternately between boiling water and ice water, while watching the output on an oscilloscope work?

please note that i'm only asking about testing the thermocouple in this post. I just want to verify that it is working, separately of the rest of the circuit. Advice for testing other parts of the circuit can be reserved for a different thread, which i'll probably post up once i finish reverse-engineering the whole schematic.

thanks in advance for your advice.

 

[Oznog]

A thermocouple is two types of dissimilar metal wires touching at the hot end and insulated or separated until it gets to a colder room temp area where it's joined to copper wire. A very small voltage- like 50 mV- is generated based on the temp difference between the hot and cold sides of the wire times the Seebeck coefficient of the junction.

If there's low ohms continuity, I think that's all it takes. Some designs require the hot junction to be isolated from ground, and insulation breakdown is a common problem for those. But not all designs are like this so continuity to ground isn't necessarily a failure.

 

[Oznog]

Oh yeah, if I'm not mistaken don't some soldering irons work on Curie point instead? There's a temp for any type of magnet where it temporarily loses its magnetism when heated beyond a certain temp, I thought they used a tiny magnet and a tiny mag sensor for cheap on/off nonadjustable temp control. But I could be wrong, now that I think about it, steel component leads would stick to it. I don't recall seeing that happen.

 

[zevon8]

Oh yeah, if I'm not mistaken don't some soldering irons work on Curie point instead?

Yes, that is exactly how some work. The older blue Weller WTCP stations were designed that way. Each tip had a small metal end cap that was calibrated for a certain temperature. There is a switch inside the heater core, and when the temp is reached, the magnetism is lost, and the switch opens, cutting off the heater.

Steel leads did sometimes stick to them, but generally the leads stuck to the outside of the heater barrel, not the tip, since the magnet was down there.

You could hear them clicking on and off.

Maybe this is what yours does?

 

[evandude]

I don't think that's as likely as a thermocouple... but i really can't be sure. it doesn't click when operating, i know that (from the small amount of time i spent using it before it failed)

i won't get into it in this thread but the way the sensor portion is interfaced is analog (op-amp circuits, etc) so i doubt it is a "digital" type of magnetic temperature sensor as you mention.

also, the entire heating/sensor element is a separate unit from the rest of the iron. it is a narrow ceramic-coated tube with 4 wires coming out of it that is located inside the metal shaft of the iron. and finally, the iron is fully temperature-controlled.

**broken link removed**

so theoretically, if i hook the thermocouple up to an oscilloscope and dunk it alternately in very hot and very cold water, i should see a few dozen millivolts on the scope?

the station did come with an extra heating/sensor element, i was just hoping to confirm that the current one isn't busted (bunch of stuff blew on the main board, i just hope this wasn't one of the victims)

 

[zevon8]

Here is an article on thermocouple type and their output characteristics:
http://www.fact-index.com/t/th/thermocouple.html

Hope it helps

 

[Oznog]

so theoretically, if i hook the thermocouple up to an oscilloscope and dunk it alternately in very hot and very cold water, i should see a few dozen millivolts on the scope?

Unlikely. The value of the induced voltage is pretty low, maybe underneath the noise floor of your scope. You may do better with a low offset, low noise op amp, maybe with a lowpass filter on the input. The output impedance of a thermocouple's induced voltage is fortunately quite low.

 

[lavenatti]

Oznog was correct in his earlier post. If there is continuity through a thermocouple, it is good.

 

[evandude]

Okay, thanks for your responses. I checked the resistance of it and it's about 21 ohms, and the replacement one i have is about the same so I guess it must be alright.