KSGER T12 temperature reading issue — display rises on its own - Page 1
Forum section: electronics / repair

author: Lordandreus208
author_rank: Newbie
timestamp: July 01, 2025, 10:20:04 pm
content: Hi everyone,I have a KSGER T12 soldering station that seems to have a weird issue with temperature reading and regulation.What happensWhen I turn it on, the tip heats up correctly and reaches the set temperature.However, once it gets there, the displayed temperature keeps rising slowly, even though the power output shows 0% and the heater is clearly off (I see the drop of tin starting to harden)After 20–30 seconds (sometimes more, sometimes less), the station seems to "realize" that the actual tip temperature has dropped significantly — like 150–200°C below the target — and starts heating again.This loop keeps repeating: false rising temp, no power output, then sudden correction and reheat.What I've checked so far1. The tip doesn't actually overheat — the rising temp on screen is fake. The iron starts to cool while the display increases.2. I checked the NTC sensor inside the handle:I measured resistance in my GX12-5 connector between pin 2 and pin 3, and got 7.7 kΩ at room temperature, which should be reasonable.3. I opened the station — no visual damage: no burnt components, no bulged capacitors, all solder joints look good.4. I also tested the MOSFET that drives the heater.I checked it for shorts using a multimeter in continuity mode — it’s not shorted (reads ~867), and the station seems to correctly stop supplying power when the display shows 0%, or else it would probably overheat.So I don't think it's stuck "on" or leaking.My questionHas anyone experienced this behavior before?Do you know what component on the board could cause the temperature to drift upward on screen while the tip is cooling down in reality?Any advice on what to test next (and how to do it pls) — maybe the op-amp, pull-up resistors, or voltage regulators — would be greatly appreciated.Thanks in advance!

author: amyk
author_rank: Super Contributor
timestamp: July 02, 2025, 03:31:13 am
content: Seems like defective firmware. Check to make sure the temperature input to the MCU is behaving as expected.

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 02, 2025, 01:02:07 pm
content: Quote from: Lordandreus208 on July 01, 2025, 10:20:04 pmI haven't had such exact behavior but a bit different: a weird jumping +-50 % temperature readings when there is EMI-noisy powerful AC-DC power supply is working (with reduced or almost absent EMI-filters inside), It was close to my T12 station, less then half a meter. In my case the problem was absent Y-capacitor between primary and secondary side. There was a place for it on the PCB, so I soldered typical 2.2 nF Y2 capacitor, and it became almost fine (not clean but at least much better).

author: Lordandreus208
author_rank: Newbie
timestamp: July 02, 2025, 04:27:12 pm
content: Thanks for the suggestion! I followed up on the firmware theory and did several tests to isolate the issue. Here's what I found:New observationsThe station powers on in sleep mode, as expected.However, immediately upon startup (without touching the handle), the displayed temperature jumps straight to ~243°C — even though the tip is completely cold to the touch.As soon as I set the target temperature to 320 °C, the tip begins to heat for a few seconds, but then the display keeps climbing above the setpoint (e.g. ~360/380 °C) even though the power drops to 0% at 320°C.Then the read begins to slowly fall (1/2°C per second) and if it reaches the 320°C mark in less than 1 minute it starts to heat again (and the temperature skyrockets again in a few seconds), if it takes more then all of a sudden the temperature reading drops a lot (200°C or so) to the probable real temperature, and it stars heating again, repeating the process one more time.What I measuredI took voltage readings on the pins of the resistor labeled R12 on the PCB, with the handle both disconnected and connected.When the handle was disconnected, I measured:~3.33 V on pin 2 of the GX12-5 connector (the NTC line), which is expected in open-circuit conditions.When the handle was connected and the station was in sleep mode, I measured the four visible pins of R12 (on the side closest to the GX12-5 connector). The voltages were, from left to right:0.00 V3.34 V3.34 V2.14–2.15 VWhen the station was active, setpoint at 320°C and heating (at least for a few seconds I guess), those same pins read:0.00 V3.34 V3.34 V2.14 V (still stable)I also observed that the LED labeled D1 would get slightly brighter when I probed certain R12 pins — though this may just be loading from the multimeter.What I thinkCould it be that the ADC pin on the STM32 is faulty or is it more likely to be a firmware issue?Do you have any ideas or suggestions?Thanks again for helping!

author: Lordandreus208
author_rank: Newbie
timestamp: July 02, 2025, 04:33:15 pm
content: Thanks for the reply.I'm happy you were able to fix the problem!It doesn't appear to be my case, unfortunately (or not haha).

author: floobydust
author_rank: Super Contributor
timestamp: July 02, 2025, 07:52:21 pm
content: You don't say what version of controller this is, a real KSGER or copy, new or old problem - so it's too hard to help you. Thank you for the information.Change CJC source in the menu from wand to MCU temperature sensor, that can rule out the CJC sensor as the problem.Common was for the copycat controller boards to use a shitty op-amp that drifted and had problems. It's SOT23-5 on the board, what number is it?*edit: I said "On some controller board builds, there are problems with the thermocouple amplifier. A cheap junk op-amp is used and temperature readings have pretty bad accuracy. Crappy ones are "621K" compared to better "8551" {marking code}.I have drawn schematics for some KSGER T12 controller boards:https://www.eevblog.com/forum/reviews/t12-stm32-v2-1s-soldering-station-controller-schematic-etc/The other thing I found is an open thermocouple does not always read that way (depends on the op-amp used) so I add a 1MEG pullup to guarantee it reads "OPEN".

author: Swake
author_rank: Frequent Contributor
timestamp: July 03, 2025, 09:27:51 am
content: My opinion also goes into the direction of the MCU not capable to read the NTC correctly for some reason. Possibilities:- Failed ADC in the MCU- Intermittent or failing contact related to the soldering iron. (think NTC or ground contact)- Something else failing in the circuit that connect the NTC to the ADC pin (I have no idea what the schematic looks like, floobydust is mentioning an opamp).- Power supply unstable. Either one powering the MCU or the one for the soldering iron.- Software fault, on the ADC side temperature read-out side.You could check the ADC pin of the MCU by connecting a simulated input instead of the NTC of the soldering station. A potentiometer will do, eventually in between 2 resistances to limit minimum and maximum setpoint.

author: Lordandreus208
author_rank: Newbie
timestamp: July 05, 2025, 03:10:28 pm
content: Hi everyone,Thanks a lot for your detailed suggestions and help.I should mention that this is just a hobby for me — I’m still learning and not very experienced with electronics troubleshooting, so I really appreciate your guidance.First of all, I just discovered that my KSGER T12 station is most likely a cheap clone. I bought it from a friend some time ago and wasn’t aware of this detail—he didn’t mention it to me.The board has a “cow” logo with Chinese text, and I also confirmed that the op-amp on the board is the “621K” type, which was mentioned as low-quality.Here’s what I’ve tested so far, following your advice:I switched the “Cold End Adj” (CJC Source) setting from NTC to CPU as suggested.However, nothing changed — the station still heats up to the target temperature, then starts “overshooting” on the display (even though the tip is actually cooling down) and only corrects itself after a long delay (often minutes), with a huge drop in temperature (sometimes over 200°C below the setpoint).This suggests that the problem is not caused by the NTC sensor itself.I also verified the NTC wiring and connections as suggested (to rule out intermittent contact issues).I measured the resistance from the soldering iron handle to the GX12-5 connector and from the connector to the board (up to R12).All connections were fine, with correct resistance readings at room temperature.There’s no continuity issue or bad contact between the iron, connector, and board.At this point, it seems the issue is either caused by the cheap op-amp itself or something else in the analog circuitry.However, I don’t have an oscilloscope, and I also don’t have a potentiometer or other tools to simulate the NTC or inject test signals.I only have a multimeter available at the moment.I also don’t own another soldering iron or station, so if a test requires swapping parts or using another soldering iron, I’d have to go back to the friend I bought this from.If you have any further ideas for tests that could be done just with a multimeter (or any practical suggestions for my case), I’d really appreciate it!Thanks again for all your help!

author: Lordandreus208
author_rank: Newbie
timestamp: July 05, 2025, 03:13:12 pm
content: Thanks a lot for your suggestion.As you can see in the other message I wrote I tried one of the tests you told me but that part seems to be working fine.

author: floobydust
author_rank: Super Contributor
timestamp: July 05, 2025, 05:49:23 pm
content: Your friend ripped you off? How much did they charge lol.Then it's not the cold junction temperature sensor (NTC)- it's the main thermocouple/op-amp malfunctioning. You can tack solder a small wire to the output of the op-amp at pin 1 and look at voltage with a multimeter.My notes 450°C = 2.25V or roughly 5mV/°C.Typical op-amp is SGM8551XN marking code "8551", or OPA336NA marking code "A36".Crappy op-amp Runic RS621XF marking code "621K" - it's known people have problems with its high offset voltage and replace it with a better part.You could try a different cartridge. Some of the clones are low quality and have bad connections inside them that show up either when it's cold or when it's hot. Cold it can error message OPEN that goes away after a while as it heats up. The wand (socket) can have poor connections too.Very common problem with older control boards is the 3.3V voltage regulator overheating. You can carefully measure that to see if it is not sagging with heat.But I would guess the problem is the op-amp needs to be changed.

author: Lordandreus208
author_rank: Newbie
timestamp: July 06, 2025, 01:15:43 pm
content: Hi, thanks again.I bought the station, a few cartridges, and two rolls of solder wire (both partially used) for €45.Yes, I would’ve appreciated knowing it was a clone, but honestly, I wasn’t expecting top-tier quality either — and since meeting him, I’ve learned quite a lot from this friend, so I’m not upset.I’ll just take it as a lesson: I need to be more careful with these things in the future. And if I’d bought it from a random stranger instead, it probably would’ve been a lot worse.Now, following your suggestion, I measured the voltage directly at pin 1 of the op-amp (621K). Since I don’t have another soldering station, I didn’t tack-solder a wire — I used a small piece of electrical tape to insulate the other pins, and carefully held the multimeter probe on the correct one. (I’ll attach a picture.)Here’s what I observed during heating:With the tip cold, I measured around 0.50 V, which seems reasonable.When set to 300 °C, the voltage initially rose to 2.30 V, then quickly dropped to about 1.50 V. After a relatively long pause, there was a brief flash from D1, and the voltage started rising again — sometimes reaching 2.80 V or even above 3.00 V, despite the setpoint being only 300 °C.At that point, the screen showed 390–400 °C, and the station stopped heating. After a long delay, the temperature eventually dropped, and the station resumed heating — repeating this same loop.At 450 °C, the voltage went up to 2.80–3.00 V and, after some time adjusting, the behavior became noticeably more stable. The readings stayed closer to the target, and regulation was clearly better than at 300 °C.At this temperature, it was the only time I observed the D1 light turning on and off intermittently, indicating that the station was actively maintaining the tip temperature close to the setpoint.However, if I reduced the setpoint back to 300 °C from 450, it again took a long time to settle, with incorrect values showing for quite a while.I also tried swapping in a different T12 cartridge, but the behavior was exactly the same — so that seems to rule out a faulty tip.From what I can tell, the thermocouple is working correctly, and the op-amp is producing a plausible analog signal. But for some reason, the controller’s temperature reading and response seem to misbehave — possibly due to drift, ADC issues, or firmware quirks.I haven’t checked the 3.3 V regulator yet, but I’ll try that next.Let me know if these results clearly point to the op-amp as the culprit, or if there’s anything else worth checking before I attempt to replace it.Thanks again for your time and help!