EVER WISHED YOU COULD CONTROL THE TEMPERATURE OF YOUR SOLDERING IRON? When you switch to a fine solder are you also having to switch to another iron? Sometimes it's just too hot for those delicate jobs. Or maybe you've just been struggling with a high-temp iron when doing your small stuff.

THIS SOLDERING IRON TEMPERATURE CONTOLLER will make you a pro at a fraction of the cost of what you would pay for a professional soldering station. Sure to become a useful part of your toolkit it will help to increase productivity as well as the quality of your work. It offers a wide range of temperature settings, from less than 20 watts to near maximum temperature when using a 40-watt soldering iron with a grounded option. And by changing different capacitors a dual or even triple temperature range can be had.

Circuit Description

Through the on/off switch AC voltage is applied to the rectifier bridge.

The rectified negative voltage is then applied directly to the cathode (K) of the SCR. The controlled trigger of the gate (G) allows the SCR to deliver bursts of DC pulses from the SCR anode (A) to one side of the iron connection.

The rectified positive DC voltage from the bridge is processed in three ways:

From the bridge it is connected directly to the other side of the iron connection allowing the current to flow through the iron to the SCR cathode (K) thus completing the circuit.

From the bridge the positive DC, approx. 105 volts, is dropped down through R7 and clamped to 8vdc by a ZD1 regulating diode. It then passes through D8 and is filtered by C4. This voltage is used as the positive supply for the electronic circuitry.

From the junction of R7 and ZD1 pulses from the full-wave rippling, unfiltered DC voltage are fed through a voltage divider consisting of R8 and R9. This, in turn, is connected to gate A of CD4011.

Circuit Analysis

Note that without the controlling function of CD4001 the output of gates C&D of CD4011 would be a constant and uninterrupted stream of pulses, that is, the rectified rippling voltage of 120 pulses per second (PPS). As these pulses are used to trigger the SCR gate (G) through R9 the iron would maintain its full temperature.

The controlling element in this unit is R11. It allows for settings of short pulses for low heat as well as longer firing periods for those jobs requiring a higher temperature. As the potentiometer is adjusted, by moving it clockwise toward D1, the ON duration is increased. Turned toward D2 the timing is decreased. Note that C3 controls the range of CD4001 oscillation and therefore the minimum and maximum range of temperature control. Accordingly a dual or even triple range can be obtained by switching different capacitors in the circuit.

Test point 1 (TP1) shows the result of the oscillator output. This is applied to gate A of the CD4011. The substracting funtion of this trigger pulses at TP2 via gate B of CD4011. The resulting firing pulses to the gate of the SCR at TP3 . The neon connected across the iron connections gives a visual indication of the SCR on/off SCR firing periods .

For DC application see Modified Circuit