Simple DIY Induction Heater Circuit

Simple DIY Induction Heater Circuit

The control circuit utilizes Zero Voltage Switching (ZVS) to efficiently manage the transistors, ensuring minimal heat generation. This method makes the transistors barely warm during operation. One of the key advantages of this system is its self-resonant nature, which means it automatically runs at the resonant frequency of the attached coil and capacitor. If you are looking for a faster solution, you can purchase an induction heater circuit from our shop. That said, this article provides valuable tips for optimizing your system.

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The Basics of Induction Heating

Induction heating occurs when a changing magnetic field induces a current in a conductive material, generating heat. The generated heat is proportional to the square of the current multiplied by the material's resistance. This principle is common in transformers, where metallic cores induce eddy currents, though here the heating effect is unwanted. In this project, however, we harness this heating effect for practical use.

Applying a constantly changing current to a coil results in a changing magnetic field, which at higher frequencies strongly influences the surface of the material being heated due to the skin effect. Induction heaters typically operate at frequencies ranging from 10kHz to 1MHz.

WARNING: This device can generate extremely high temperatures!

The Circuit Design

This design uses a collector resonance Royer oscillator, known for its simplicity and self-resonant operation. A similar circuit is found in inverter circuits used in devices such as LCD backlights. In this DIY induction heater, the transformer is composed of the work coil and the object being heated.

The main challenge with this circuit is the use of a center-tapped coil, which can be more complex to wind compared to a standard solenoid. This coil is necessary for creating an AC field using a single DC supply and two N-type transistors. The coil center is connected to the positive supply, and each end is alternately connected to the ground by the transistors to allow current flow in both directions.

The current drawn varies based on the temperature and size of the object being heated.

Detailed Circuit Components

1. **Resistors R1 and R2**: Standard 240 ohm, 0.6W resistors. They determine the MOSFETs' turn-on speed, needing to be low but not too small in value.

2. **Diodes D1 and D2**: Use low forward voltage drop diodes, like Schottky diodes 1N5819, to discharge MOSFET gates effectively. They must handle the voltage rise in the resonant circuit, which can reach up to 70V.

3. **Transistors T1 and T2**: 100V 35A MOSFETs (STP30NF10) mounted on heatsinks. They remain relatively cool due to their low drain-source resistance and fast response times.

4. **Inductor L2**: Acts as a choke to keep high-frequency oscillations out of the power supply and limits current. The inductor should have substantial inductance and be constructed with thick wire. Our 2mH inductor was made from winding 8 turns of 2mm thick magnet wire on a toroidal ferrite core.

Due to the simplicity of the components, direct soldering without a PCB can be advantageous, especially for high-current connections.

Construction of the Induction Coil and Capacitor

The induction coil must be built from thick wire or pipe for handling large currents. Copper pipe is ideal as high-frequency currents mostly flow on the outer surface. The coil and a parallel capacitor form the resonant tank circuit. This setup resonated around 200kHz.

Quality capacitors that can endure large currents and high temperatures are crucial. They should be placed close to the work coil and connected using thick wires, as most currents flow between the coil and capacitor.

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Enhancing the System

The primary limitation is that the work coil heats up rapidly due to large currents. Using thicker brass tubing and water cooling can prolong operation time. Creating such a coil requires careful bending, often by filling the pipe with sand to prevent pinching at sharp bends. The coil was divided into halves, soldered together, and water-cooled via PVC pipe connections.

By reducing the coil turns, lower impedance, and higher currents are achieved. Increasing capacitance adjusts the resonant frequency; six 330nF capacitors provided 1.98uF total capacitance in this setup.

Cooling enhancements like a pump and radiator system can significantly improve performance. Our setup used an old fish tank pump and a modified CPU cooler converted into a radiator, ensuring the coil remained cool.

Higher voltage-rated diodes, like 1N4007, replaced D1 and D2 due to increased current peaks. When the PSU was set to 30V, about 7A was drawn without metal inside the coil, rising to 10A with a metal bolt.

For higher power levels, more robust capacitors and different transistors would be required to manage both larger currents and voltage surges.

If you wish to contribute, please share your own projects and experiences. Additional tips may save time for others attempting similar builds.

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