Introduction: DIY Induction Heater
When you think of a way to heat up a metal object, you think of fire -right? Fire is an inefficient, old fashioned, and slow way to heat up metal objects. It wastes lots of energy as heat and creates lots of dirty smoke. Well, what if you could have a way to heat up metal objects that solves all these issues -it would be great, right? In this instructable, I am going to show you how to build a ZVS induction heater. This is a device that heats most metals using a ZVS driver circuit and electromagnetism. It is very efficient, produces no smoke, and can heat up objects like paperclips in a matter of seconds. The video below gives a demonstration of this induction heater in action, along with another type of instruction on how to build it.
Lets get started.
Step 1: How It Works
Many of you who are reading this may be asking "What is a ZVS driver"? Well, it is an extremely efficient oscillator circuit that is able to create an extremely powerful electromagnetic field that heats up the metal. It is the backbone of the induction heater that this instructable is showing you how to make.
To understand how this power supply works, I will explain the different sections of it. The first section is the 24 volt power supply. The power supply needs to produce 24 volts at a current of 10 amps. For my power supply, I will be using two sealed lead acid batteries wired in series. The power is then fed into the ZVS driver board. The ZVS oscillator pushes and pulls current though a coil around the object that is being heated. This constant changing of the current's direction creates a fluctuating magnetic field. This induces many small eddy currents in the metal(refer to the diagram above). All of these currents are relatively high, and because of the low resistance of the target metal, heat is generated. According to ohms law, power converted to heat in a resistive circuit is P=I^2*R.
Now, the metal type of the object that is being heated is very important. Ferrous metals have a higher magnetic permeability, so they are able to harness more energy from the magnetic field. This allows them to be heated quicker than other materials. Metals, like aluminum, have a lower magnetic permeability, so it takes longer for them to heat up. Things that have a high resistance and low magnetic permeability, like a human finger, will not be heated at all by an induction heater. The resistance of the material is also very important. If you have a higher resistance in the target metal, then less current will flow, so the power converted to heat gets exponentially smaller. If you have a metal with a lower resistance, then the current will be higher, but power loss will be lower due to ohms law. It is a little bit complicated, but because of the relationship between resistance and power output, the highest power output is achieved when the resistance of the object approaches 0.
The ZVS oscillator is the most complex part of this circuit, so I am going to explain how it works. First of all, when the current is switched on, it flows though 2 inductive chokes into each side of the coil. The choke is to make sure the circuit does not draw to much amperage on start up. The current also flows thought the two 470 ohm resistors into the gates of the two Mosfets. Now, because no component is perfect, one Mosfet is going to turn on first. When this happens, it hogs all the gate current from the other Mosfet. It will also draw the drain of that Mosfet that is on to ground. This will not only let current flow though the coil to ground, but it will also let current flow though one of the fast diodes form the other gate of the other Mosfet, locking it off. Because there is a capacitor in parallel with the coil, it creates a resonant tank circuit that starts oscillating. Because of this resonant action, the drain of the other Mosfet will swing back and forth in its voltage, eventually reaching 0 volts. Once this voltage is reached, the gate charge from the Mosfet that is on will discharge though the fast diode into the drain of the opposite Mosfet, effectivly shutting it off. With this Mosfet off, the other Mosfet has the opportunity to turn on. After this, the cycle repeats thousands of times per second. The 10K resistor is meant to deplete any excess gate charge on the Mosfet, because it is like a capacitor, and the Zener diode is meant to keep the gates of the mosfets at 12 volts or under so they do not explode. This high frequency high power oscillator is what allows metal objects to be heated.
Now, its time to build this thing!
Step 2: Materials
To build this power supply, you will need a few parts, luckily, most can be salvaged for free. If you have ever seen an old CRT TV laying on the side of the road, pick it up, because it has most of the parts needed for this project in it. If you want higher quality components, you can buy them at the LCSC online store. Click the parts to bring up the product links in LCSC.
You will need:
Step 3: Tools
Step 4: Transistors and Cooling
In this circuit, because the transistors switch at 0 volts (Hence the name, Zero Voltage Switching ZVS) they do not get very hot, but they should still be mounted on a heat sink if you are planning on running this circuit longer than 1 minute. I mounted both of the transistors on one heat sink. When you do this, make sure you isolate the metal backs of the FETs from the heat sink. If they both touch, it will short out and blow out your FETs. My heat sink was from a computer power supply, and already came with a piece of insulating silicone, so my transistors are isolated. To make sure that your transistors are isolated, touch your multi-meter to the middle pin of both transistors, the drain. If you get continuity, then your FETs are not isolated.
Step 5: The Capacitor Bank
In this circuit, the capacitors get very hot. This is due to them always having current flowing thought them. Now, the capacitor value we need to make this circuit work properly is 0.47uF, so we will need the most amount of capacitors together to reach this same value, but have a larger surface area for heat dissipation. You also need to get the voltage rating of them above 400 volts due to inductive voltage spikes in the resonant circuit. What I did, is make a ring of copper, and add 10 .047uF capacitors in parallel around it. This makes the combined capacitor bank have a capacitance of .47uF, with lots of surface area for air cooling. This capacitor bank will be in parallel with the work coil.
Step 6: The Work Coil
This part of the circuit is what generates the magnetic field. It is formed using copper wire. It is very important that you use copper. I started this project using a steel work coil. It did not work very well. When it was running with no load, it was drawing 14 amps!! When I switched it out with a copper coil, it drew only 3 amps. I think that this is because the ferrous material in the steel coil had eddy currents induced into it. Its high magnetic permeability made the coil the subject of the induction heating, which wasted the power and stopped it from heating the material inserted. I am not sure if this is the exact reason this wasn't working, but it is the most logical argument based on the evidence provided.
To make your coil, use stranded copper wire, and wrap it around a piece of PVC pipe about 9 times.
Step 7: Building the Circuit Trial and Error
Building this circuit took a lot of trial and error. My number one issue was my original power supply and coil. The power supply is an 55 amp 12 volt switching supply. I think that this power supply drove the ZVS circuit with too high of an initial current, this blew out the mosfets. They exploded, like in the first picture. This probably could have been fixed by adding larger inductors, but I decided to just use lead acid batteries.
My second issue was the coil. In step 6, you saw that the steel coil did not work. This high current draw due to the steel coil blew a few mosfets too. In total, I lost about 6 mosfets to explosion. This may be bad, but I learned from my mistakes.
Over the course of this project, I built the circuit over again many times, but I will just explain how I built the most successful version.
Step 8: Building the Circuit
To build this ZVS driver circuit, you will need to follow the above circuit diagram. I first took the zener diode and twisted it together with the 10k resistor. You can then take this pair of components, and solder it between the gate and ground of the mosfet. Make sure that the black end of the zener diode faces the gate. Then, solder the mosfets to a piece to perf board. Use the bottom side of the perf board to solder two fast diodes between the drain and the gates of each fet. Make sure that the white line faces the drain(pin 2). Then, attach the VCC wire -from your power supply- through 2 220 ohm resistors to the gate of each transistor. Ground both sources. Then, solder the work coil and capacitor bank in parallel with each other and solder each end to a different drain. Finally, run power to the drains of each mosfet through 2 50uh inductors. These can be toroidial cores with 10 turns of wire. With that, your circuit should be ready to use.
Step 9: The Base
The base of your induction heater is just to support all of the components. I used a piece of 2x4 wood scrap. The circuit board, capacitor bank, and work coil were all hot glued to the wood. I think this setup makes it look cool.
Step 10: Testing Your Induction Heater
To power up your induction heater, just connect it to the power supply you have. Then, insert the part you are trying to heat up into the coil. It should start heating. I was able to get a paperclip to red hot temperatures in 10 seconds. Other things, like a nail, took about 30 seconds. With these objects inserted, the current draw rises by about 2 amps. This is a fun circuit to build an play around with. It can also be used very practically. It can heat up objects without any of the soot that comes from smoke. It can even heat up isolated metal objects, like the getter material in vacuum tubes. It is also human safe, so you will not get burned by putting your finger inside the coil. It will, however, burn you if you touch an object that has already been heated.
This heater has many possibilities, tell me in the comments what you plan to use it for!
Thanks for reading!
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If you change your coil to a bigger one let's say twice the length, will it still work or does this mean a drop in temperature? Ive got a 48v 1800 zvs with a 70mm coil thats 10cm long, i wanna make another coil thats 20cm but dont know if it will work. 20cm would be a better length for hardening 1095 steel