Thermocouples shouldn't be difficult right?

[Jonny]

Hey all. Let me tell you a tale of woe...

So my lovely jarcar supplied solder station is now obsolete after 7yrs and my iron failed so I thought I'd gamble on an iron of same spec, looks physically the same, but from a different station and swap the plug over. Well it was a 50/50 chance. The element is same rating so all good there. The thermocouple I wouldn't have thought would be anything special but it doesn't give an ouput. My old irons thermocouple has an increasing voltage and resistance with temp and puts out around 10mV @ approx 130degC. The new one puts out 0mV and has a fixed resistance of 54ohm. Of course the controller can't work with that.

What kinda thermocouple behaves like this? Or did I happen to purchase a factory dud iron that I can't return as it's now modified.

Yes I wired the plug correctly

[Kleinstein]

For a thermocouple 10 mV at 130 C is allready quite a lot of voltage. It might be a type E thermocouple.

https://en.wikipedia.org/wiki/Thermocouple#Nickel-alloy_thermocouples

[jeroen79]

an iron of same spec, looks physically the same

Assumptions.
Post the brand and model of both stations so we may look things up.

The thermocouple I wouldn't have thought would be anything special but it doesn't give an output.

How do you know it is a thermocouple? It might be a thermistor.

My old irons thermocouple has an increasing voltage and resistance with temp and puts out around 10mV @ approx 130degC. The new one puts out 0mV and has a fixed resistance of 54ohm.

If you could do some more extensive measurements of resistance and voltage over temperature we could say more about it.
What does it do at the operating temperatures? (350C)

[mikerj]

A low resistance with zero voltage suggests it could be a platinum resistance thermometer.  If so it's not compatible with a thermocouple interface unfortunately.

[Jonny]

I let it get stinking hot and resistance went to 150ohm, guess it's a thermister then. Was only a $30 gamble. Maybe I can modify the input circuit so this forms part of a resistor divider on the opamp input or something.....