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Hacking Kawh 8586D Hotair Station to use A1321 Heater.

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firepower:
I modified a Kawh 8586D hotair soldering station to us the A1321 Thermistor heater (50W) instead of the original A1322 Thermocouple heater (30W).
I have reversed engineered the Soldering Iron op-amp circuit for measuring temperature using A1322 thermocouple going to micro controller.  Op-amp is LM358  going to ST8L152 micro controller, both using 3V3 suppy.

To modifiy it to use A1321 Thermisor heater add a 10K resistor in parallel with R17 (6M8) and a 4K7 or 10K resistor in parallel with R20 (27K).
Cal needs adjusting after doing mods. Set to 400C and adjust Cal until LED reads the same value as Tip.

Heats up now in 45 sec instead of 2 minutes and 45 sec.

I dont understand why the circuit is using a 22uF capacitor before it feeds the micro controller? Other similar circuits dont have this capacitor or if they do its only 0.1uF.  I also ended up changing the 22uF to 0.1uF.

thm_w:
22uF is a bit big, especially with the 10k.
Maybe they are trying to completely filter out the power switching on and off to the heater? If you don't sequence the ADC read at the right time it can give quite a large offset.

Surprised a thermocouple can handle those high temperatures.

firepower:
I ended up changing the 22uF capacitor to a 0.1uF capacitor. still works ok.
I looked at a lot of circuits on google for ST8L152 and none used capacitor in the ADC circuit.

Regarding Calibration, it work fine for high temps but is out of CAL at low temps below 200C.
but 200C is the lowest temp you can set. at room temp 35C it shows 135C but it normally shows --- when below 100C anyway.
This error is most likely the software being programed for thermocouple curve instead of thermistor curve.

All my soldering is above 350C and I much happier with faster heat up and more powerful heater.  50W vs 30W

thermocouple normally used is a K type and a Nichrome coil heater for A1322. 

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