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Soldering Iron Temperature Controller

Soldering Iron Temperature Controller Circuit & Working

If you are an electronic enthusiast, then you must be familiar with soldering iron device. This is generally used to design electronic circuits on PCB. If you are not using an adjustable soldiering iron for soldering, then chances are you might end up damaging your IC or even the device.

The voltage requirement of a soldering machine entirely depends on the solder ratings of the components used in the device. For example, a small device or IC needs only 5 Watts power whereas a large device might need 25- 30 Watts iron. Some of the huge devices also need even 50 Watts or more depending.

Soldering irons are of wide variety with different power capacity. Generally, the device runs at 230V AC mains with no temperature controller available. This is the reason we have decided to design a low-cost temperature controller for soldering iron in this article.

Sometimes, deterioration in the soldering iron tip may cause due to the constant energy consumption. To overcome this issue, we can use a temperature controller along with the iron to regulate the temperature as per requirement. The soldering iron present in the market with temperature controller is damn expensive and is not easily affordable by everyone.

In this article, we will be designing a temperature controller for soldering iron by using basic electronic components like resistors, DIAC, and TRIAC. Before starting with the design process of this circuit, let us discuss the prime components used in the circuits namely DIAC and TRIAC. As resistor and capacitors used in the circuit don’t need any explanation and is quite familiar with every hobbyist and we have already discussed them in details as well.


DIAC is a discrete electronic component that is also known as symmetrical trigger diodes. This is a bi-directional semiconductor switch that can be used in both forward and reverse polarities. DIAC are very often used in triggering of TRIAC, means used in the combination of DIAC-TRIAC. One of the most interesting facts about the DIACs is that they are bi-directional devices in which any of the terminals can be used as the main terminal.

Operation of DIACs

DIACs starts conducting voltage only after a certain breakdown voltage is exceeded. Most of the DIACs have a breakdown voltage of around 30V but the actual breakdown voltage entirely depends on the specifications of that component type. When the breakdown voltage is reached, the component resistance decreases abruptly. This leads to a sharp drop of voltage across the DIAC and the corresponding current increases in the result. When the current falls below the holding current, the DIAC switches back to its non-conducting state. Here, the holding current is a level at which the DIAC remains at its conducting state.

Each time the voltage in the cycle falls, the device is reset to its conducting state. DIACs provide equal switching to both halves of an AC cycle as the behavior of the device is equal in both the directions.

Construction of DIAC

DIACs are manufactured in three layers and five layers structure. Let us see the construction of both one by one.


Three-layer structure

In this structure, the switching occurs when the reverse biased junction experiences the reverse breakdown. This is the most commonly used DIAC in practical due to its symmetric operation. This three-layered DIAC can reach the breakdown voltage of around 30V in general and capable of providing enough improvement in switching characteristics.

Five-layer structure of DIAC

The five-layer structure of DIAC is very different in term of operation. This device structure forms an I-V curve which is like the three-layer version. You can say that this structure looks like two break-over diodes connected back to back.

Applications of DIACs

DIACs are of great use in electronics due to the nature of its symmetric operation. Some of the general applications include:

  • It can be used along with the TRIAC device to make the switching symmetrical for both the halves of AC cycle.
  • DIACs are widely used as light dimmers or domestic lightning
  • DIACs are also used in fluorescent lamps as starter circuits


As the name suggests, TRIAC is a three terminal device that controls the flow of current. It is used to control the current flow in AC for both halves. It is a bi-directional device, also a member of the thyristor family. TRIAC behaves as two conventional thyristors connected back to back with each other.


In simple words, TRIAC can be triggered into conduction by both negative and positive voltages with both negative and positive trigger pulses applied to its GATE terminal.

In most of the AC switching applications, the gate terminal of TRIAC is attached to the main terminal.

Construction of TRIAC

The construction of TRIAC is of four layers. This device can conduct in either direction when triggered by a single pulse.  The PNPN is placed in the positive direction and NPNP in the negative direction. It acts as open circuit switch that blocks current in its OFF state.

There are four modes in which TRIAC can be operated namely:

Mode I + : MT2 current is positive and Gate Current is also positive

Mode I – : MT2 current is positive and Gate Current is also negative

Mode III + : MT2 current is negative and Gate Current is also positive

Mode III – : MT2 current is negative and Gate Current is also negative


The TRIAC is triggered into conduction by a positive current applied at the Gate terminal. This is labeled as mode I in the above discussion. You can also trigger the TRIAC by a negative gate current, which comes into mode Ι–.

Following the same process, in Quadrant ΙΙΙ, triggering with a negative gate current, –ΙG is also common in both mode ΙΙΙ– and mode ΙΙΙ+. Modes Ι– and ΙΙΙ+ are, however, less sensitive configurations that require a great amount of current at Gate terminal to cause triggering than the more common TRIAC triggering modes of Ι+ and ΙΙΙ–.

TRIACs require a minimum holding current to maintain conduction at waveforms cross over point.

Applications of TRIAC

  • It is widely used in control and switching application used in household
  • It is used as a phase control device in most of the AC applications
  • This is also used for controlling the speed of fans
  • It is used in motors
  • It is also used as brilliance control in lamps

We hope that you have got a good knowledge of DIACs and TRIACs. We have discussed the working of both the devices in the above discussion to help you understand the use of both the components in the soldering iron temperature controller. Apart from these two, we have used a potentiometer in our circuit to control the temperature with a knob.

Gather the following components to design the soldering iron temperature controller circuit:

  • Resistor – 2.2 k (1 nos.)
  • Potentiometer – 100 K (1 nos.)
  • 400V capacitor – 0.1uF (1 nos.)
  • DB3 DIAC (1 nos.)
  • BT136 TRIAC (1 nos.)

Related Project: Traffic Light Control Electronic Project using IC 4017 & 555 Timer

Circuit Diagram of Soldering Iron Temperature Controller

This soldering iron temperature controller is very simple to design. The circuit is made using some of the simplest electronic components mentioned in the above list. One end of the 2K resistor is connected with the DIAC terminal and another end is connected with the 220 V power supply through a potentiometer to control the temperature. In other end, DIAC is connected with the TRIACs gate terminal to control the switching of the TRIAC.


Working of Solder Iron Temperature Controller

The temperature of this controller circuit can be varied from the maximum value, to regulate the heat dissipation. Connect this circuit to the soldering iron to make a rise in iron temperature quickly in no time. The TRIAC connected here in the circuit, switches the high current and voltages over both parts of an AC waveform. The TRIAC is fired in different angles to get different temperature levels from 0 degrees to maximum. The connected DIAC controls the firing in both directions. Here, you can use the potentiometer to set the temperature accordingly.

The working of this soldering iron temperature controller is very simple and easy to understand. You just need to connect the circuit with the soldering iron to vary the temperature accordingly.

Application of Soldering Iron Temperature Controller

The soldering iron temperature controller is used to control the temperature of a soldering iron. You can connect this controller to reduce the rising time of soldering iron temperature. This is very useful while you are soldering the sensitive components.

Bottom Line:

The soldering iron with temperature controller, are quite expensive and not affordable for everyone. Here, this temperature controller for soldering iron is designed with very low cost and basic electronic components. You can use this with your soldering iron to automatically control the temperature. We have also defined the working and specifications of the main components that is TRIAC and DIAC in our above discussion. This will be very useful in understanding the working of soldering iron with ease. We hope that now you will be able to design this low power and highly reliable circuit without any inconvenience.

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