Summary

Weller TCP Hack

Replacing the faulty magnetic switch in a Weller TCP is a costly affair, this project is a workaround for just that. The upgraded soldering iron has a thermocouple sensor and triac heater control. Optionally, the temperature can be displayed on a cheap DVM module.

Soldering iron hack : remove the magnetic switch. Get a china-made temperature sensor from a cheap (I had defect irons with good thermocouple) Chinese soldering iron or order a replacement heater containing it (Aliexpress search "Ceramic Heating Element"). Form an omega-shaped 1 cm wide brass plate in which the sensor fits tight in the middle, and the outer part of the "omega" fits in the heater tube (5.6mm). Position the plate holding the sensor in the tube so that it touches the soldering tip bottom. Use eventually a weak spring to push the sensor against it. Attach new 4 wire cable to the soldering iron. The sensor output voltage 41 uV / C degree Red + Blue -. I have not used the old connector socket (the iron is one of a kind anyway) but soldered the cable on the PCB directly.

The original transformer provides the stabilized 12V DC supply for the electronics. The first LM358 amplifies the sensor signal. The sensor output is 41uV per C degree, amplified 244 times we get 10mV/C on pin1 of the amplifier. Since the 3 digit DVM module's resolution is 10mV, we can display this voltage directly as Celsius. Pin1 output is compared on the second LM358 with the setpoint from a trimmer potmeter. If the temperature is low, the LM358 pin7 fires the triac and heats the soldering iron. Reaching the set temperature, pin1 goes higher than setpoint, pin7 output goes low, triac and LED goes off, heating stops. There is always a time lag between heating up and sensor response, so the temperature fluctuates up and down around the setpoint as it did with the original magnetic switch.
The triac I used is a MOC223 and the triac opto driver is TLP160. Any other similar component can be used.

If the control does not work, swap the thermocouple wires, perhaps the polarity is wrong. The + side of the thermoelement is ground, - is the signal, so after the inverting opamp it results +ve output.

Decimal point is unnecessary on the voltmeter module, pin 5 on the chip (AtTiny 44) was the driver, I lifted it from the PCB with a sharp knife so it makes no contact.

The DVM module can display (set by jumper) the setpoint voltage if you prefer that. I have set it to 370C, this was the temperature of the Weller 7 series tips. The DVM has 115 kohm input resistance so it distorts the voltage on the setpoint pot, this means that if you adjust it and switch back to show the output temperature, you remove this extra load and the setpoint will actually rise on the pot.

Built the electronics on a perfboard PCB. Cut a rectangular hole for the 0.28" LED display 10x23mm. For the adjustment pot used a 3/16 thread trimpot holder. The cable input is a 18.5 mm plug 6mm hole, LED glued on the back. Both are 3D printed.

The schematics is a downloadable PDF file.
DVM on Ebay , Aliexpress: search "0.28 Inch 3 wires 2.5V-40V Mini Digital Voltmeter"