Indicator for Weller Soldering Stations

Many of the soldering stations produced by Weller/Cooper Tools Group use the ‘Magnastat’ principle to control the bit temperature. The interchangeable bits are fitted with a magnetic cap which pulls on a contact in the iron and completes a circuit to switch power to the heating element. When the magnet in the tip reaches a predefined temperature (the so-called ‘Curie temperature’) it loses its magnetism and releases the switch contact. The process is reversible so that the contact is remade as the temperature falls. A number stamped on the cap identifies its operating temperature: 5 = 260 °C, 6 = 310 °C, 7 = 370 °C and 8 = 425 °C. When used with lead based solders a 370 °C bit is the usual choice. The heating element is switched on when the tip is below this temperature and off when it is above it, keeping the tip temperature constant.

It is fair to say that the Weller solder station is probably the most commonplace piece of test gear you are likely to encounter in labs and electronics departments up and down the country. They have a good reputation for reliability and the circuit suggested here is an add-on indicator lamp to show when the soldering tip is up to temperature. The circuit described here is intended to be installed in the housing which forms the base of the soldering iron. An LED fitted to the front panel indicates when the iron is heating. The circuit works by measuring the voltage difference dropped across a shunt resistor fitted in series with the heating element in the iron. It is not necessary to carry out any calibration on the circuit. This design can be fitted to the WTCP-S, WTCP 50 and WTCP 51 soldering iron stations from Weller.

Indicator for Weller Soldering Stations Circuit Diagram

Indicator Circuit Diagram

The add-on indicator circuit can be seen in the uppermost dashed box of the circuit diagram, the lower box represents the internals of the soldering iron station. A transformer in the base supplies 24 V to the heating element in the iron and is connected via a cable and three pin plug/ socket on the base unit. The heating element has an impedance of 12 Ω which produces an average current of 2 A and a peak value of 2.822 A. Using a 33 mΩ resistor for the shunt (R1) gives a voltage drop of 93 mV (peak) when the element is heating.

IC2 is a LM358 type dual operational amplifier. The amplifiers are powered from a single ended power supply and IC2.A is con-figured as an amplifier with a gain of 100. It amplifies the positive half waves of the voltage dropped across the shunt R1. The resulting output signal charges up capacitor C4 to approximately 10 V via diode D1 when the element is on. IC2.B is configured as a comparator and resistors R5 and R6 set the reference voltage to around 2.1 V. When the element is heating the comparator output is positive and the LED lights. As the operating temperature is reached the magnetic switch opens and the voltage across C4 is discharged through R4 (time constant = 100 ms) and the LED turns off.

Power for the circuit is derived from the 24 V transformer in the solder station. Diode D1 performs half wave rectification and C1 is a reservoir capacitor to produce a DC voltage for the 12 V voltage regulator (IC1). The maximum offset voltage for the LM358 is only 7 mV, with a gain of 100 this can produce an output offset of 0.7 V which is well below the 2.1 V comparator thresh-old and is not likely to be a problem so it is not necessary to fit any form of offset adjustment.

Author : Heinz Kutzer - Copyright: Elektor