Heat Controlled Soldering Iron

This circuit prevents overheating of soldering iron bit and subsequent damage. It keeps the soldering iron suffiently warm but not hot during the interval between soldering works. The circuit also saves electric energy and protects delicate components through heat controlled soldering

A Triac and a Diac are used to control the AC power through the soldering Iron. Diac adjusts the gate current of Triac and thus its firing angle. As a result, current flows through the soldering iron in a controlled manner. This controls the temperature of the soldering iron element. When the power is switched on, diode D1 forward biases and provides DC to Diac. It conducts only when the voltage across it increases above 34 volts. The conduction of the Diac can be adjusted through the 100K Pot. When Diac conducts, Triac gets gate current and it also conducts. This completes the current path through the soldering iron. The firing angle of the triac determines the rate of current flow through it and thus the heat. Resistor R1 and Diode D2 bypasses the current when Diac turns off and C1 stabilize the pulsing of Diac.

Connect the soldering iron to the AC socket as shown in the diagram and adjust VR for the required heat.

Caution: The circuit uses high voltage AC and there chance of lethal shock if handled carelessly. Do not touch any parts when powered and use a plastic knob for VR. Enclose the circuit in a shock proof case with the AC socket on the top.