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T12 STM32 V2.1S Soldering Station Controller - schematic etc.

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Here's a schematic I drew of the KSGER soldering station controller V2.1S
Please PM for any corrections and I will keep it updated.

Known Issues:
-The RTC battery goes dead in a few weeks. Remove resistor R10 10k, it adds unnecessary 0.28mA load.
-The encoder body is not grounded which is a problem for ESD.
-The heater and wand cable has ~4uH inductance and -ve spikes occur switching which stress the MOSFET and thermocouple amplifier. A small cap C8 is across the MOSFET to absorb this, but I added a fast recovery reverse diode across the heater.
- The vacuum desoldering pump MOSFETs/diodes/connectors are not populated.
- Silkscreen has duplicate ref. designators.
- JBC option likely doesn't work, T245 thermocouple overloads op-amp and option R11 pcb trace is wrong on some boards.
- S/W 2.10 my cold junction temperature from NTC is not working despite A/D voltage changing. A replacement controller works.

Controller board H/W versions:
OLED-V3.0 (black PCB) same as 2.1S but no pump control, 6-pin OLED, no R20 battery killer, ref. designators shuffled, no resistor packs, battery on board, JBC option R0 may be fixed. Looks like a cheaper build.
2.1S (blue PCB) has pump control and buck-converter. JBC option is bungled, I think you must cut a trace.
2.1S (green PCB) has pump control and SPX2954 LDO for 3.3V
2.1  (sometimes called 2.1S) has no pump control and an LDO for 3.3V, not buck-converter.
2.01 fixed the (2.0) LDO overheating, added copper-pour heatsink.
2.0  is a failed design using SOT-89 LDO for 3.3V that overheated from no heatsinking, and no snubber on the MOSFET.
1.5  oddball design using dual buck/LDO vregs, 4-pin GX12 connector like Quicko, and dual op-amp for thermocouple amp. PT Dreamer's Blog reverse engineered it and made custom open firmware for it.

Lately, firmware needs an activation key first run. STM32 CPU ID1, ID2 and then activation codes.

The T12 thermocouple is oddball metallurgy, measured empirically in Hakko Patent US6087631A for the look up table, 20-21uV/°C accurate from 200°-600°C. Exact curves here US6054678A.

edit: uploaded the wrong sch (huge file), fixed, added thermocouple info, V1.5 url.
edit2: updated sch Rev. 2 - 8/2020 added JBC pinout and fixed the JBC thermocouple breakout, my pcb has the trace on wrong side of R11. Other boards do not have this error.

Voltlog was showing some of the station issues in his recent video

You might want to preemptively replace C8 (Chongx brand 100μF 35V electro) with something from Japan. Mine was open circuit. Edit: oops I meant C4

After a hint I got from another user on twitter, I assume R10 was intended as protection to prevent reverse leakage current from flowing through the diode into the battery when system vcc (3V3) is present. This is sometimes shown in RTC chip datasheets but KSGER somehow got that wrong and the resistor is slowly draining the battery. I have removed R10 on my unit, problem solved.

Another known issue (but related to the power supply, not the controller board) is poor isolation between primary and secondary, caused by the output diode heatsink which extends on the top side from the primary high voltage tracks up to the secondary low voltage side.

Power Supply schematics I drew of the KSGER soldering station, V2.04 and V2.05, 24VDC.
KSGER 96W 24V 5A Electric Power Supply Unit For STM32 STC OLED T12 Digital DIY Soldering Station Controller.

KSGER did change their power supply's PC board grounding.

Soldering Power V2.05 (black pcb) has mains PE ground from IEC connecting to a large pad and open jumper/pin3 output connector, to the 24VDC secondary side GND, so
the controller board and heating-element are NOT grounded. The enclosure is not either. They fixed the PCB trace HV spacings to the heatsink, larger 82uF primary cap.
Safety mod is to connect that jumper, and add a ground wire to the case which also covers ESD to the knob/encoder.

Soldering Power V2.04 (green pcb) has mains PE ground from IEC connecting to the 24VDC secondary side GND, so the controller board and heating-element is grounded. The enclosure is not.
Safety mod is add a ground wire to the case, which also covers ESD to the knob/encoder. The heatsink over the HV PCB trace, add some Kapton insulating tape or lift the heatsink for some airgap.

If you want your soldering iron floating and trust the "double insulation" of the power transformer etc. add a 1MEG resistor from PE to DC GND. There's no safety approvals here, no idea how the transformer is wound if it would pass the few kV hipot test for the double-insulated test. But this did not work, see scope traces 122Vpk so I'd definitely just add a wire as a hard ground.
To check, just measure ACV tip-PE when it's running to confirm the tip is not floating up to some high voltage that can damage semiconductors or people.

*Caution there are no bleeder resistors for the big primary cap, so always ensure it is discharged (check with a voltmeter) and safe to poke around before going in.
edit Feb. 26, 2022: updated both PSU schematics- had X-cap in the wrong spot.
EEVblog member Hugo.Froh has contributed PSU V2.05 voltage measurements, pics, annotated schematic, and at different loads with the V2.05 PSU.


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