another "real world" scenario where the iron misbehaves:
- removing SMD capacitors using two irons, one being a traditional GROUNDED one. On some components the issue happens and the tip reaches 500C.
Still no answer from Aixun.
If I owned a Aixun T3A, I would do a test if it's HF noise upsetting the temp readings. Capacitance from the tip to PE (such as when soldering a board with scenarios OP mentions), not necessarily a direct ground loop. Prob. it's own SMPS noise gets into the op-amp.
Just try it - take a 0.1uF cap and touching the tip to one lead, the other lead is grounded - should have no effect at all - unless the TC op-amp flips out, and then the software propaganda campaign cover up fails, which is what I think is going on. It ignores TC temperatures wildly out of range - causing the usual head in the sand (ostrich) problem... tip temp is skyrocketing... that crappy algo is a total fail and needs a rewrite.
Like I've said a few times, adding a Schottky or fast recovery diode across the heater (backside of GX connector) stops all that. Try a 1N5819, SS24 or ES1B etc. what you have lying around in the junkbox.
Can you give more insights on how one would select the "right" diode for that job? What values are important, besides recovery time? Max. Voltage, current, peak reverse voltage?
The back-EMF diode should be fast-recovery and take peak current of what the heater gets (for usec though) and PIV of well over 24V.
1N5819 40V 1A 25A peak 1/2 sine is plenty overkill. For SMT consider B140 or SS14 (SMA package) or bigger SMB B360, SS24 etc.
It does two things: clamps -ve spikes from ESD and the heater/cable inductance from damaging things like MCU inputs, as well as blowing the power mosfet. As well it will help with the op-amp input recovery time but it's a mystery part they are using with no noise filtering either.
Another T12 clone controller just put a 0.1uF? capacitor across the heater, as a cheap sort of snubber. We need to save micropennies here.
I say it's at least two problems - noisy hardware and the Eversolder software algo.