The control circuit uses a method known as ZVS (zero voltage switching) to activate the transistors which allows for an efficient transfer of power. In the circuit you see here, the transistors barely get warm due to the ZVS method. Another great thing about this device is that it is a self resonant system and will automatically run at the resonant frequency of the attached coil and capacitor. If you want to save some time, we have an induction heater circuit available in our shop. You might still want to read this article though for some good tips on getting your system working well.
This great little project demonstrates the principles of high frequency magnetic induction and how to make an induction heater. The circuit is very simple to build and only uses a few common components. With the induction coil shown here the circuit draws about 5A from a 15V supply when a screwdriver tip is heated. It takes approximately 30 second for the tip of the screwdriver to become red hot!
When a magnetic field changes near a metal or other conductive object, a flow of current (known as an eddy current) will be induced in the material and will generate heat. The heat generated is proportional to the current squared multiplied by the resistance of the material. The effects of induction are used in transformers for converting voltages in all sorts of appliances. Most transformers have a metallic core and will therefore have eddy currents induced into them when in use. Transformer designers use different techniques to prevent this as the heating is just wasted energy. In this project we will directly make use of this heating effect and try to maximise the heating effect produced by the eddy currents.
If we apply a continuously changing current to a coil of wire, we will have a continuously changing magnetic field within it. At higher frequencies the induction effect is quite strong and will tend to concentrate on the surface of the material being heated due to the skin effect. Typical induction heaters use frequencies from 10kHz to 1MHz.
DANGER: Very high temperatures can be generated with this device!
The circuit used is a type of collector resonance Royer oscillator which has the advantages of simplicity and self resonant operation. A very similar circuit is used in common inverter circuits used for powering fluorescent lighting such as LCD backlights. They drive a centre tapped transformer which steps up the voltage to around 800V for powering the lights. In this DIY induction heater circuit the transformer consists of the work coil and the object to be heated.
The main disadvantage of this circuit is that a centre tapped coil is needed which can be a little more tricky to wind than a common solenoid. The centre tapped coil is needed so that we can create an AC field from a single DC supply and just two N-type transistors. The centre of the coil is connected to the positive supply and then each end of the coil is alternately connected to ground by the transistors so that the current will flow back and forth in both directions.
The amount of current drawn from the supply will vary with the temperature and size of the object being heated.
From this schematic of the induction heater you can see how simple it really is. Just a few basic components are all that is needed for creating a working induction heater device.
R1 and R2 are standard 240 ohm, 0.6W resistors. The value of these resistors will determine how quickly the MOSFETs can turn on, and should be a reasonably low value. They should not be too small though, as the resistor will be pulled to ground via the diode when the opposite transistor switches on.
The diodes D1 and D2 are used to discharge the MOSFET gates. They should be diodes with a low forward voltage drop so that the gate will be well discharged and the MOSFET fully off when the other is on. Schottky diodes such as the 1N are recommended as they have low voltage drop and high speed. The voltage rating of the diodes must be sufficient to withstand the the voltage rise in the resonant circuit. In this project the voltage rose to as much as 70V.
The transistors T1 and T2 are 100V 35A MOSFETs (STP30NF10). They were mounted on heatsinks for this project, but they barely got warm when running at the power levels shown here. These MOSFETs were chosen due to having a low drain-source resistance and fast response times.
The inductor L2 is used as a choke for keeping the high frequency oscillations out of the power supply, and to limit current to acceptable levels. The value of inductance should be quite large (ours was about 2mH), but also must be made with thick enough wire for carrying all the supply current. If there is no choke used, or it has too little inductance, the circuit might fail to oscillate. The exact inductance value needed will vary with the PSU used and your coil setup. You may need to experiment before you get a good result. The one shown here was made by winding about 8 turns of 2mm thick magnet wire on a toroidal ferrite core. As an alternative you can simply wind wire onto a large bolt but you will need many more turns of wire to get the same inductance as from a toroidal ferrite core. You can see an example of this in the photo on the left. In the bottom left corner you can see a bolt wrapped with many turns of equipment wire. This setup on the breadboard was used at low power for testing. For more power it was necessary to use thicker wiring and to solder everything together.
As there were so few components involved, we soldered all the connections directly and did not use a PCB. This was also useful for making the connections for the high current parts as thick wire could be directly soldered to the transistor terminals. In hindsight it might have been better to connect the induction coil by screwing it directly to the heatsinks on the MOSFETs. This is because the metal body of the transistors is also the collector terminal, and the heatsinks could help keep the coil cooler.
The capacitor C1 and inductor L1 form the resonant tank circuit of the induction heater. These must be able to withstand large currents and temperatures. We used some 330nF polypropylene capacitors. More detail on these components is shown below.
The coil must be made of thick wire or pipe as there will be large currents flowing in it. Copper pipe works well as the high frequency currents will mostly flow on the outer parts anyway. You can also pump cold water through the pipe to keep it cool.
A capacitor must be connected parallel to the work coil to create a resonant tank circuit. The combination of inductance and capacitance will have a specific resonant frequency at which the control circuit will automatically operate. The coil-capacitor combination used here resonated at around 200kHz.
It is important to use good quality capacitors that can withstand large currents and the heat dissipated within them otherwise they would soon fail and destroy your drive circuit. They must also be placed reasonably close to the work coil and using thick wire or pipe. Most of the current will be flowing between the coil and capacitor so this wire must be thickest. The wires linking to the circuit and power supply can be slightly thinner if desired.
This coil here was made from 2mm diameter brass pipe. It was simple to wind and easy to solder to, but it would soon start to deform due to excess heating. The turns would then touch, shorting out and making it less effective. Since the control circuit stayed relatively cool during use, it seemed that this could be made to work at higher power levels but it would be necessary to use thicker pipe or to water cool it. Next the setup was improved to tolerate a higher power level&#;
The main limitation of the setup above was that the work coil would get very hot after a short time due to the large currents. In order to have larger currents for a longer time, we made another coil using thicker brass tubing so that water could be pumped through when it was running. The thicker pipe was harder to bend, especially at the centre tapping point. It was necessary to fill the pipe with fine sand before bending it as this prevents it from pinching at the sharp bends. It was then cleared out using compressed air.
The induction coil was made in two halves as shown here. They were then soldered together and a small piece of pvc pipe was used to connect the central pipes so that water could flow through the whole coil.
Less turns were used in this coil so that it would have a lower impedance and therefore sustain higher currents. The capacitance was also increased so that the resonant frequency would be lower. A total of six 330nF capacitors were used to give a total capacitance of 1.98uF.
The cables connecting to the coil were just soldered onto the pipe near the ends, just leaving room for fitting some PVC pipe.
It is possible to cool this coil simply by feeding water through directly from the tap but it is better to use a pump and radiator to remove the heat. For this, an old fish tank pump was placed in a box of water and a pipe fitted the outlet nozzle. This pipe fed to a modified computer CPU cooler which used three heat-pipes to move the heat.
The cooler was converted into a radiator by cutting the ends off the heat pipes and then linking them with PCV pipes to the the water would flow through all 3 heatpipes before exiting and going back to the pump.
If you do cut some heatpipes yourself, make sure to do it in a well ventilated area, and not indoors as they contain volatile solvents that can be toxic to breathe. You should also wear protective gloves to prevent skin contact.
This modified CPU cooler was very effective as a radiator and allowed the water to remain quite cool.
Other modifications needed were to replace the the diodes D1 and D2 with ones rated for higher voltages. We used the common 1N diodes. This was because with the increased current there was a larger voltage rise in the resonant circuit. You can see in the image here that the peak voltage was 90V (yellow scope trace) which is also very close to the 100V rating of the transistors.
The PSU used was set to 30V so it was also necessary to feed the voltage to the transistor gates via a 12V voltage regulator. When no metal was inside the work coil, it would draw about 7A from the supply. When the bolt in the photo was added, this went up to 10A and then gradually dropped again as it heated up beyond curie temperature. It would certainly go over 10A with larger objects, but the PSU used has a 10A limit. You can find a suitable a 24V, 15A PSU in our online shop.
The bolt you can see glowing red hot in the photo took about 30 seconds to reach maximum temperature. The screwdriver in the first image could now be heated red hot in about 5 seconds.
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In order to go to higher power than this, it would be necessary to use different capacitors or a larger array of them so that the current was more distributed between them. This is because the large currents flowing and high frequencies used would heat the capacitors significantly. After about 5 minutes of use at this power level the DIY induction heater needed to be switched off so that they could cool down. It would also be necessary to use a different pair of transistors so that they could withstand the larger voltage rises.
In all this project was quite satisfying as it produced a good result from just a simple and inexpensive circuit. As it is, it could be useful for hardening steel, or for soldering small parts. If you decide to make your own induction heater project, please post your photos below. Please read through the other comments before making your own as it could save you time later on.
If you wish to simulate this project for testing different inductance values or transistor choices, please download LTSpice and run this DIY Induction Heater Simulation (Right click, Save as)
It is difficult to say how hot you will be able to get something as there are many parameters to consider. Different materials will react differently to induction heating and their shape and size will affect how the heat up or shed heat to the atmosphere.
You can get a rough idea using some basic calculations with the formula below, or if you prefer, we made a handy Heater Power Calculator that can work it out for you. This form includes materials (like water) that can not be directly heating using induction heaters, but it is still useful if you are trying to work out for example the power needed for heating a pan of water using a induction heater.
Simply stated, induction heating is the cleanest, most efficient, cost-effective, precise, and repeatable method of material heating available to the industry, today.
Precisely designed induction coils teamed up with a powerful and flexible induction power supply produce repeatable heating outcomes specific to the desired application. Induction power supplies designed to accurately quantify material heating and respond to a material&#;s property changes during the heating cycle make achieving diverse heating profiles from a single heating application a reality.
The purpose of induction heating may be to harden a part to prevent wear; make metal malleable for forging or hot-forming into the desired shape; braze or solder two parts together; melt and mix the ingredients which go into the high-temperature alloys, making jet engines possible; or for any number of other applications.
Induction heating takes place in an electrically conducting object (not necessarily magnetic steel) when the object is placed in a varying magnetic field. Induction heating is due to the hysteresis and eddy-current losses.
Hysteresis losses only occur in magnetic materials such as steel, nickel, and very few others. Hysteresis loss states that this is caused by friction between molecules when the material is magnetized first in one direction, and then in the other. The molecules may be regarded as small magnets which turn around with each reversal of direction of the magnetic field. Work (energy) is required to turn them around. The energy converts into heat. The rate of expenditure of energy (power) increases with an increased rate of reversal (frequency).
Eddy-current losses occur in any conducting material in a varying magnetic field. This causes heating, even if the materials do not have any of the magnetic properties usually associated with iron and steel. Examples are copper, brass, aluminum, zirconium, nonmagnetic stainless steel, and uranium. Eddy currents are electric currents induced by transformer action in the material. As their name implies, they appear to flow around in swirls on eddies within a solid mass of material. Eddy-current losses are much more important than hysteresis losses in induction heating. Note that induction heating is applied to nonmagnetic materials, where no hysteresis losses occur.
For the heating of steel for hardening, forging, melting, or any other purposes which require a temperature above Curie temperature, we cannot depend upon hysteresis. Steel loses its magnetic properties above this temperature. When steel is heated below the Curie point, the contribution of hysteresis is usually so small that it can be ignored. For all practical purposes, the I2R of the eddy currents is the only way in which electrical energy can be turned into heat for induction heating purposes.
Two basic things for induction heating to occur:
Induction heating is particularly useful where highly repetitive operations are performed. Once an induction heating machine is properly adjusted, part after part is heated with identical results. The ability of induction heating to heat successive parts identically means that the process is adaptable to completely automatic operation, where the workpieces are loaded and unloaded mechanically.
Induction heating has made it possible to locate operations, such as hardening, in production lines along with other machine tools instead of in remote, separate departments. This saves the time of transporting the parts from one part of the factory to another. Induction heating is clean. It does not throw off unpleasant heat. Working conditions around induction heating machines are good. They do not give off the smoke and dirt which are sometimes associated with heat-treating departments and forge shops.
Another desirable characteristic of induction heating is its ability to heat only a small portion of a workpiece, offering advantages where it is unnecessary to heat the whole part. This advantage is critical in essential parts with a few localized high-wear areas during normal operation. Previously, a higher quality, more expensive material would be required to withstand the wear of operation. With induction, less expensive materials can be locally processed to achieve the durability required.
Induction heating is fast. A properly tuned induction heating machine can process high part volumes per minute by utilizing efficient coil design and part handling. Since induction heating machines are well suited to automation, they can easily integrate with existing part production lines. Unlike radiant heating solutions, induction heating heats only the part inside the coil without wasting energy on unnecessary heating.
Induction heating is clean. Without flame operations that leave soot or otherwise require cleaning after heating, induction is a choice for parts that require clean heating, such as in brazing operations. Because induction heating utilizes magnetic fields that are permeable through glass or other materials, controlled atmosphere heating through induction is a possibility.
Faraday (-) was familiar with the fundamental principles underlying induction. The emphasis at first was on the undesirable effects of the phenomenon. Much attention was paid to finding methods to reduce the effects of induction so that devices like transformers, motors, and generators could become more efficient.
Michael Faraday (-) is generally credited with the discovery of the fundamental principles underlying induction heating in . However, the focus of induction research was on finding methods to reduce the effects of induction so that devices like transformers, motors, and generators could become more efficient at first.
Interest in the possibility of melting metals by induction started in . One of the earliest commercial applications was the melting of small charges using spark-gap oscillators. Another early application was heating metallic elements of vacuum tubes to drive off absorbed gases prior to sealing.
For a few years prior to World War II, a number of companies, more or less independently, began to realize that induction held the solution to a wide variety of specialized heating applications. Although induction had not become an industrial process for long after its theoretical discovery, its growth was rapid during World War II when there arose an immediate need for producing large numbers of parts with minimal labor.
Today, induction has taken its place in our industrial economy as a means of speeding part production, reducing production costs, and achieving quality results.
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With the coming age of highly engineered materials, alternative energies and the need for empowering developing countries, the unique capabilities of induction offer engineers and designers of the future a fast, efficient, and precise method of heating.
As the technology of choice for rapid, clean heating that is repeatable, accurate, and efficient, induction has established itself firmly in the future of manufacturing as a cornerstone of the industry. Induction&#;s rapid maturity since its discovery has earned it a reputation of cutting edge technology, critical to discovering new processes that are more effective. Today, induction is synonymous with groundbreaking solutions, paving the way to a new paradigm in manufacturing technology.
Radyne technology is at the forefront of induction heating, innovating in new ways to further develop induction heating techniques and processes in new, previously abandoned areas. We are a world-leading manufacturer and pioneer in the development of advanced induction and controlled atmosphere heating equipment. Click here to learn more about TFD Power Supply.
Further discussion on the topic of the fundamentals of induction heating can be found by continuing on to our article on advanced induction heater concepts, covering topics that build on the foundation in induction heating theory established here. For even more induction heating resources, Radyne supplies a few resources for your convenience allowing you to leverage induction theory for informed operation: including posters for reference of common laboratory and shop floor charts and reference books on the topic of the basics of induction.
If you are looking for more details, kindly visit Induction Heating Equipment.