Temperature controlled soldering iron

Temperature controlled soldering iron

My Weller TCP magnastat iron broke, the magnetic switch stuck open circuit. This is the second time in three years this has happened. Needless to say I was feeling a little hacked off about this given the price of spares for this iron, the magnetic switch retails at approximately 30.00.

Twenty four Volt AC, 48 Watt Chinese temperature controlled irons ( heating element + temperature sensor ) can be purchased on Ebay for 9.99 inc. shipping from a UK distributor - an incredible price. Taking a gamble, one of these irons was purchased with the view if I get twelve months out of it and it packs up it is cheaper than fixing the Weller.

A temperature controller circuit was designed and tested for this iron using junk box parts, the result of which is shown here.

No data for the iron was available so a little experimenting was carried out. The heater element connections are the first two on the left as you look at the 5 pin din plug ( flex facing away from you ). The centre pin is the iron ground. The remaining two pins are the temperature sensor which is a thermocouple, a K type i think .

The controller circuit uses a 741 op amp configured as a comparator to switch the relay contacts feeding power to the iron heating element. A pot is used to vary the comparator reference voltage so various temperatures for the iron can be set and maintained due to the closed loop servo action of the controller circuit. The reference voltage source for the comparator is derived from a small regulator ( Tr1 ) producing 3.5V. Using a low reference voltage gave better results than just potting down the 24V rail as a reference voltage. The minimum and maximum temperature range can be altered by changing the resistor values in the potentiometer network to suit ones preference.

All of the components came from the junk box. RLY1 has a 24V coil and should have a contact rating of three amps or better. The 24V transformer was removed from a ancient scrapped midi system

The temperature control pot was roughly calibrated using a K thermocouple resting on the iron tip connected to a multimeter, if one is not available just mark the pot scale min and max as any one who is experienced at soldering will know when the iron is the correct temperature for the job in hand. In use the iron maintains a steady average tip temperature. A LED is illuminated when power is applied to the iron heater.

A ground lift switch is included as sometimes it is handy to use the iron with out the tip grounded, the ground should be permanently wired if this feature is not required.

The soldering iron itself has a very good ergonomic feel - as good as the Weller, in fact it looks like a copy. Time will tell as to its reliability.

Circuit diagram
The iron
Controller PCB
Inside case
Complete station

Barry Zarucki M0DGQ