I have an Aoyue 936A soldering iron rated at 24V/70 watts.
I came across a cheapo soldering iron rated at 24V/75W and rewired it according to the pinouts of the original one. The thermocouple readings are identical between the two irons, but the heater's resistance is different by a few ohms.
When I first plugged in the iron, I got a rising temperature reading, but the temperature ran away and the iron started to glow pink. The station displayed "err." I heard a faint transformer hum while it was warming up. I unplugged the iron and turned off the station. Luckily, no damage was done to the station, but the experience leaves me with some questions.
I suspect the 75W iron drew too much current which spazzed out the temperature controller. Could I still get this iron to work? Would it be possible to add a resistor in series with the heater? Since I don't need a higher temperature than 300C, could I get away with a diode in series as an alternative?
PLOT TWIST: @BruceAbbot boldly claimed it has nothing to do with the wattage. So I did an experiment by taking apart the soldering station and tracing all the wires on the soldering iron to rule out any wiring issues. I hooked up another thermocouple to the tip of the iron to read the temperature on another meter. I attached a pair of grabbers on the iron's thermocouple wires to read the voltages.
I experienced the same issue as before with the overheating. After probing around, I discovered the '-' had continuity with the station's case ground. I moved around some wires making sure there was no shorts and tried it again, it mysteriously worked! After scratching my head for a while, I realized I left my multimeter in resistance mode. It's really cool how the resistance corresponds to the temperature in Celsius so closely! (see photo 1)
I had a hard time believing it made a difference so I tried it again, but this time switching off the multimeter while it was reading. To my surprise, it failed. Notice the temperature discrepancy (see photo 2).
Bruce Abbot is indeed correct! But what is going on here? Why does the issue go away only while reading the resistance? Do I have to keep my multimeter hooked up to solder? Electricity is weird!