Hakko FX-888 soldering iron thermocouple type

[npelov]

Hi,

I ordered a (probably fake) hakko handle from ebay. I want to make 12V soldering iron.

I thought it's sensing temperature by thermocouple and I tried to measure it using Brymen BM869 DMM. Since I didn't sense any voltage on any contact I tried to measure the voltage of a normal k-type probe using mV range - I got 0.39 mV for body temperature (couldn't find other stable thermal source) over room temperature of 26 deg. C. So that's 390 uV/41 = 9.5 above ambient or 35.5 deg.C. So the multimeter is fine.

I heated the heater to 320 deg.C and it still doesn't show any voltage.

I tried between the thermocouple and the heater - 80-90V p-p, but with iron disconnected it's high resistance between these pins. It's coupled in some way - I don't care how, but it's not what I'm looking for.

The thermocouple's resistance is 3 ohms at room temperature and about 5 ohms at 300 deg C. The heating element is 50 ohms at room temperature, 100 ohms at 300 deg C. That's according to the pinout I found on the net. Of course it doesn't make sense the heating element to be 50 ohms. It should be 10.6 ohms which at 26V is 2.5A -> 65W.

I'm attaching photo of what I thought is the heater and I got no voltage while the heater is heating up. If there is a heater and it's heating up I should be able to see what's driving it right? Is 50Hz, not RF. Shouldn't be that hard.

Obviously I'm too frustrated to think and I'm making a stupid mistake, so will someone let me know what I'm doing wrong.

Update. I misread the pinout. The heater is the other two pins and the voltage measured is ~ 22Vrms or 65Vp-p. I didn't measure it before probably because of loose connections or something. But I'm still not getting any voltage out of the thermocouple. Not a millivolt. And it should be about 12mV at 300 deg. C above ambient. It's probably normal that heater is 3 ohms when cold. But the thermocouple shouldn't be as high as 50-100 Ohm (after all it's a short circuit) and it should have at least some voltage when hot.

P.S. I also used my genuine hakko (both the station and the iron) to make the measurements - same result. The fake iron works fine with the genuine station.

[DimitriP]

The thermocouple's resistance is 3 ohms at room temperature and about 5 ohms at 300 deg C.

According to the 888D instuction manual  http://cdn.sparkfun.com/datasheets/Tools/fx888d.pdf , accoss the heater you should measure 2.5 to 3.5 Ohms at room temperature, and accross the sensor 43-58 Ohms.

Here is the pinout for good measure

[npelov]

Well, it really helps when you calm down a bit. I searched the net and ... I'm not getting any voltage because it's not a thermocouple. It's a thermistor:
http://www.instructables.com/id/DIY-Soldering-Station-1/

It does work fine as a thermistor - the resistance jumps to over 100 ohms when I heat it. The question now is how do I get the temperature from the resistance? Is resistance-to-temperature relation linear? Most thermistors I tried are not linear.

[KL27x]

You doin it analog or digital?

Analog, it doesn't matter. Just label the dial at 200C, 250C, 300C, etc, wherever the ball lands.

Digital, just take some readings at the aforementioned temps and plot them on a graph. Connect the dots with a smooth curve, and you have as many data points as you could possibly want. If you're a mathematician, you can come up with an equation. If you're lazy, just plug in as much of that data as you want into memory.

[npelov]

Digital. That sounds simple enough. I don't need too high accuracy. I'll probably use it at only one temperature.