Introduction: Powerful Yet Simple Induction Heater

Picture of Powerful Yet Simple Induction Heater

21/06/2014: Updated power supply section.

This is my first instructable and I'm not really good in explaining things, so forgive me if it sounds like too much.

As this whole thing is a work in progress make sure to check for updates, which I will add at the end of each step.

In case there is need to change the original step I will include a seperate note at the beginning of the changes.

You can find quite a lot of schematics and instructions on how to build yourself a nice induction heater,

but I noticed most of them simply lack the understanding of how everything actually works together.

If you tried it before you might have noticed your capacitors getting so hot that they pop or despite a good power supply it takes forever to get a small nail glowing red.

That is the reason why I wanted something better...

Please excuse that I repeated myself a few times, but it is only for the thing that are important.

The basic circuit is based on Mazzolli's flyback driver, so thanks to him for the initial design.

Part 1: How does it actually work?

Induction heating works basically like a transformer inducing so called Eddy currents into a conductive workpiece.

This works at all frequencies but in the ragne of 20-250kHz ferrous metals heat the best.

Every time the magnetic field of the work coil changes the resulting Eddy currents will heat the workpiece.

Considering the size, shape and mass it quickly becomes clear that for bigger stuff you need more power or it will take forever.

I don't want to go too much into the details as Wiki can supply you with all details about it ;)

Part 2: What are the dangers?

Induction heating is surprisingly safe as it is a form of non-contact-heating, meaning your workpiece never has any contact to the insane powers in your workcoil - no touch.

Also the heating stays localized inside the coil, so longer workpieces can be held in your hand.

But! The voltage produced inside the tank circuit will go as high as Pi multiplied by your input voltage.

This is one reason why I limited thisversion to only 30V input voltage for the time being.

Currents go the way of least resistance and unless you touch the workcoil and some grounded earth at the same time nothing at all will happen as it is much easier for the currents to go back into the circuit.

Of course I recommend to insulated the coil with paint or a silicone sleeve to be on the safe side.

Part 3: Consideration for the selection of parts to build

I fried quite some caps and killed a bunch of mosfets on my way to succes, so I will try to help you avoiding these downfalls.

Many tutorials give you completely different values for the parts used.

The circuit in question is a so caller "Royer Oscillator" which is basically a flip-flop that gets the timings from the attached tank circuit.

There are several ways to make sure the circuit resonates properly and does not kill your mosfets, so I decided to go the most simple way with some modifications.

If you have access to electronic scrap from hard rubbish collections or similar you don't have to buy any parts, except maybe the copper pipe and a water pump for cooling.

The mosfets are not eally critical as long as they meet or exceed these specs:

At least 150V, 10 or more amp and a low internal on resistance below 0.15Ohm- IRF540 or IRF840 are commonly available for cheap otherwise check the datasheets of the mosfets you could salvage from power supplies, TV's and similar.

You need two identical mosfets.

Either small heatsinks with a fan or big passive heatsinks are recommended to mount the mosfets.

The caps seemed to be of great importance to me in the beginning of this project.

I constantly had them heating up or even explodig on turn on.

So going for better quality was my first order of business until I realised that even expensive pulse caps and caps designed for induction cooktops won't solve the overheating.

It was only I perfected the work coil setup and the electronics when I realised there are several hundred amps going through the caps I used.

Running time of the heater was limited to a few seconds which meant a new way had to be found.

The ideal capacitance for a work coil following my desing is between 2 and 4µF.

The safest way to get there is by using a lot of smaller caps connected parallel and directly to the exteded pipe of the coil.

So you end up with a coil that has two long "legs" with a lot of small caps between them.

MKP snubber caps used for mains filter work great in the range of 100-330nF - the higher your input voltage the smaller and more numerous the single caps should be.

Don't try to solder them onto some cable or copper sheet to connect them as I can tell you even 6quare mm burns trough in a few seconds.

For input voltages over 15V you will need thicker copper pipe, I used 5/16" below that 3/8" will do fine.

To get the power needed to run I removed the HV side from a MOT and rewired it with thick cable to get two windings, one for 15V and one for 30V, running through a 25A rectifier and 7200µF caps for filtering.

Part 4: Hints on making it easier.

The mosfets usually have a metallic back which is connected to the drain, so if you want to avoid big heatsinks you can braze a copper sheet onto the pipe, mount the mosfet on it and have a perfect connection plus water cooling for the mosfet at the same time - plus you get it all a bit smaller :)

All connection to and from the tank should be oversized!

With mosfets mounted directly to the pipe this leaves the connection to the choke coils (more on that later!).
The reason for this is that if there is a problem with the resonace your mosfets might pop but also your soldering connections resulting in quite massive sparks.

Using thicker cable can help avoiding this.

A lot of the available tutorials will state the use of Zener diodes and parallel resistors from Gate to source on the mosfets to keep the gat voltage at around 12V no matter what the input voltage is.

This can work quite well but I found that the use of a 12V regulator to drive the gates is much better as it allows for much more current and therefor higher and more accurate switching times.

I came to this conclusion after frying a few 5W zener diodes...

A small heatsink is recommended for the regulator to avoid overheating.

Part 5: Getting it all together....
Please check the schematics carefully before proceeding!

You can use any power supply that is capable of delivering at least 10 amps at 30V - lower voltages down to 12V are possible but reduce the output power.

I used no circuit board but direct wiring with standard house installation wire as it is the easiest way, especially if you use the heatsinks to connect the mosfets to your tank.

Again: make sure all connection to work side (what is connected to the output of the mosfets) is done properly!

Too thin wires or bad connection will result in massive arcs!

Leave enough pipe on the coil so you solder the caps directly between the two pipes - this will drastically reduce the need for copper sheets or finger thick wires to connect the cap bank to the coil.

Part 6: Ok, I have it all connected but it looks like it is one big short and can't work....

Well, in one that is true, but you have to keep in mind the circuit is self resonating.

This means both mosfets switch when the amps are at zero.

Due to the high frequency the workcoil, caps and choke coils oscillate in resonance.

The impluses are extreme short and it will work fine.

Part 7: Troubleshooting...

As I mentioned I killed a few parts on the way of developing this heater, so here is what to look out for.

The caps should be MKP types rated at 275 or more volts, Wima caps are ideal.

If your caps overheat (and you got the right type) your single values might be too high, try 220nF or 100nF caps.

The mosfets should only warm up with big loads in the workcoil, if they get hot with no load you might have to use better choke coils - I used yellow ferrite cores with quite thick magnet wire.

The diodes should be really fast, there are many ultra fast recovery diodes available - as long as they are rated for 200V or more you should be fine, if they are too slow your mosfets heat up and you have poor performance.

All looking good but no output power at all? Check if the 12V supply to the gates is ok ;)

Last word of advise: Never hold small workpieces by hand, a drill bit gets red hot in seconds, which can be quite painful for your fingers ;)

Step 1: Explanations on Getting It All Together - the Coil and Tank Caps

Picture of Explanations on Getting It All Together - the Coil and Tank Caps

Since not everyone is familiar with mosfets, soldering and pipe bending I will add some more info here.

First off the coil:

You see the finnished coil and how it looks with the capacitors and wires for the mosfets attached - not the thickness of the wires!

I do recommend to use quite thin piping and not the 1/4 inch I used here.

But as I plan on using a ferrite mantle (there will be a tutorial on making proper ferrite for this project at a later time) and a higher input voltage I used this thick pipe.

For short term use you can do without water cooling, especially for smaller workpieces but I recommend to include a little water pump to avoid overheating, which can actually cause your solder joints to fail with the high currents.

Use annealed copper pipe, this is the stuff that comes in a coil anyway, the straight pices of pipe are not good for our job.

There are several ways of bending pipes, most involve tools like benders or springs.

Other ways are using salt or fine sand to fill the pipe before bending.

They all work but I don't like any of them as you first have to straighten the pipe to fill it and than tap it hard to comapct the sand/salt, tools cost money...

I simply leave the whole coil as it is and fill it with water - you need a good seal and a bit of pressure so all the air comes out.

Put it in the freezer over night and do the bending the next day.

If you don't have the option to weld a little bar on a piece of steel pipe to hold your copper pipe in place:

Put the coil over your steel pipe and straighten the inner winding a bit - about 40cm is good.

Now use both hands and bend both the straight bit and the still coiled up copper around your steel pipe.

You will see as soon as you complete the first turn the bend around the pipe with start to follow the shape of the steel pipe.

Now hold one end and wind the other so you end up with a straight bit long enough for your caps and connections.

The number of tunrs affect the output power and frequency - 3 turns minimum - 8 max, I opt for 5.

Hint: You might notice when you bend the pipe a few times that is becomes hard to bend and might even get cracks.

Heating the section of pipe until glowing red and letting it cool down fixes that problem and softens the copper.

Update! :

Heating vs melting, coil size and pipe diameter.

I did some test with my previous coils to compare output power levels.

My intial thoughts on coil size and magnetic flux density have been confirmed.

For ferrous metal and normal heating the coil diameter is not really that important, meaning it is easier to use 1/2" pipe than 5/16" not only for the cooling.

But when it comes to non ferrous metals or melting you want to maximise the flux as only the eddy currents will heat the workpiece.

So a small diameter coil with little gap to the workpiece is prefered for this purpose.

Since melting is not an option at current input voltages I did not really bother to make different coil diameters to match my workpieces.

To get a figure on the water cooling:

With 1/2" pipe reaching forging temps for small object like a 8mm bolt is no not problem without water cooling, continous use or short cycles however will require water cooling as the copper not only heats up fast but also stores the heat for quite some time.

There is not need for huge amounts of water, a small aquarium pump or similar will do, you only need a slow water flow at these levels for cooling.

Soldering on the pipe....

At this stage you might have encountered the problem that your soldering iron is incapable of heating everything for a proper connection.

There are many was to overcome this problem so I will reduce it to the two most simple.

a) Put the coil in the oven at about 200° celsius for about 15 minutes to get an even temperature throughout - make sure to wear gloves when handling it to prevent burns!

You now have abot 15 minutes to solder the caps and cables on until the copper cools down.

Working fast and on both ends of the pipe makes it easier.

b) Use a blow torch :)

Heating the coil or the pipe ends with a blow torch will heat the copper enough for proper soldering, just make sure to keep the flame away from the caps!

This is also good if you used a) and the coil cooled down too much to finnish the soldering.

Be aware that oxidization will happen and that normal flux solder won't penetrate this layer, so you might need to give it a quick rub with some stainless stell wool.

You need to make sure every cap is soldered properly to avoid sparks or melting connections during use of the heater.

It pays off to first put a layer of solder on the pipe and to add the caps one by one while adding enough solder for a proper connection.

If you have enough of these terminal strips as used on lamps or to connect two cables, you can remove the plastic around them and braze them onto your copper pipe so you can use the srew connection for the caps.

This is only useful though if you want to experiment with a lot of different caps for the tank or have the need to change the working frequency without soldering.

Step 2: Power Supply

Picture of Power Supply

You can use whatever you available to power this circuit as long as you stick to voltages suited for your mosfets.

As you can see I used an old microwave transformer that I rewired.

I used a angle grinder to open the welding lines on the sides and removed the secondary, high voltage coil.

The wire used for this was standard house wire, but please don't rip your walls open to salvage it, find other means of supply! ;)

For testing purposes I have a center tap to allow the usage at 15V and 30V - currently running on 30V.

There is no use to try low amp supplies as you need at leat 5amps and for bigger coils and workpieces you might go as high as 15amp - so choose wisely.

As you can see I also used this wire for the connections to the rectifier and filter caps - although here the term filter caps is not correct.

Rectifier is a standard 25A type mounted on an old CPU heatsink - for higher voltages a fan is required for cooling during long use.

To get proper power levels you will need a stable DC supply voltage that can handle the fast switching of the mosfets.

Without big capacitors your power supply might get into trouble in terms of supplying a stable DC.

In my case 2 7200µF electrolypte caps from the voltage doubler stage of an old amplifier are used.

Anything below 5000µF will reduce you output unless a proper power supply is used.

PC (ATX) power supplies might work but I have not tested them yet.

Update! :

After some feedback from someone in my forum who complained about low output power I did some tests and would like point out some important things.

From the electrical grid we get 50 or 60Hz depending where you live.

In the current setup with 2µF tank caps I run at 75kHz.

Unless you have an overrated lab power supply you might run into the same problem with low outpout power levels, so here is how to fix it:

To make sure the mosfets can deliver the full current several thausand times per second the DC supply must be able to handle it.

In the rectifier stage you need caps with high µF values and high energy levels.

In my pictures you can see the massive size of my two caps, which are designed to store high energy levels as the original use was in a voltage doubling stage of an amplifier.

Using the standard, small, electrolyte caps you find in standard power supplies and DC filters you won't be able to provide the energy needed to reach max output power.

Same, by the way for the cap on the 12V outpur side for the gat drive - go for at least 1000µF rated for 25V otherwise you might have problems driving the gates properly.

As an alternative for expensive huge sized caps on the rectifier stage you could use lead-acid batteries.

For example to run on 24V you use 2 12V batteries (about 4-8AH) and a regulated power supply at 26V.

This is of course only a last resort option or for those that only have power supplies with low amp ratings.

Check old amplifiers from the rubbish collection or scrap yard - the bigger their pysical size the bigger the energy levels available.

You will see what I mean once you compared the normal caps with those ;)

Update 21/06/2014:

I use now IRFP260 mosfets and an old stick welder for the power supply.

The use of the 260's was necessary for the higher supply voltage and more power output.

As my former modified MOT was constantly overheating I did some more checks on the actual power usage with the welding transformer.

Using my big coil as used in this Instructable the current going into the rectifier at close to 40V is already 9.5Amps.

With that in mind I strongly suggest to use a power supply with suffient reserves as bigger workpieces will need more current.

It also shows that mosfets with a high enough rating should be used, even with my oversized heatsinks the mosfets do warm up with longer use.

As they are supposed to only switch when the current is zero it nicely shows that the circuit is doing it best to at resonance, otherwise the mosfets would just go up in smoke.

Once the ferrite coating is complete I will upload another video showing the power levels and heat up time using the workpieces from previous videos.

Step 3: Driving the Mosfet Gates

Picture of Driving the Mosfet Gates

To do it properly a technician will tell you to use gate driver IC's and some logic for the frequency control.

Although this will be a very safe way to protect your mosfets, the additional electronics actually make it more complicated than necessary.

You can see I used a simple 12V regulator with a 1000µF electrolypte cap, the small cap is only for filtering and the diode for general reverse protection - not a must except for the big cap.

The use of 12V Zener diodes instead of a regulator might seem easier but in my experiments I had a lot failures with them and as a result killed the mosfets.

Using a 12V regulator capable of delivering 1A solved this problem for good.

The mosfets are mounted on oversized heatsinks, you can use smaller ones, I had these at hand and was too lazy to look for smaller ones.

Although a bit hard to see in the pic, the resistors and fast switching diodes are soldered directly to the pins of the mosfets.

The thick blue and red wires are the connections from source of the mosfet to the negative output of the power supply - they must be quite thick as a lot of currente runs through them!

Update! :

I'd like to point out that using a good sized cap at this stage is vital to make sure the gates of the mosfets are driven properly!

This is especially true if your caps on the rectifier stage are insufficient to deliver the power you need!

1000µF is the lowest you should use here, go for 2200 or 4700µF if possible.

Update 21/06/2014 :

In some schematics for the Royer Oziallator or similar circuits used as a flyback driver you will see the use of 10kOhm resistors parallel to the Zener diodes.

They are supposed to help achieving faster switching times.

I tested different values between 7.5 and 20kOhm and advise against their use.

The circuit often fails to oscillate properly resulting in blown mosfets if you don't act fast.

This is more dominant when high power mosfets are used, with IRF540 or IRF840 mosfets the circuit swings fine but I could not see any benefit at all with the added resistors.

As I also use a welding transformer for the power supply now I realised with the added power the caps on the DC side of the rectifier really need to be quite big - both in value and in size.
If your DC supply breaks down during the times when the AC supply is in the zero range for the current, your caps need to be able to provide the power for the mosfets to switch properly.

A good sign of failure is that the resonance frequency fluctuates and that one or both mosfets heat up too much.

If you are only working with a 12V supply you can use these caps used for the audio equippment in your car.

Don't like the use of batteries but a ggod sized one parallel to your power supply should eliminate all problems until you have a proper power supply.

With about 400 to 600W currently on the input side we are looking in the 2-5kW range inside our tank circuit.

Step 4: The Choke Coils

Picture of The Choke Coils

Bigger is better, same for the choke coils.

Check the size and color!

I used yellow toroid cores as they are rated for the frequencies we use - standard black ones or othe colors can work but I recommend the yellow ones.

Most people won't have the means to check for the inductivity of their choke coils, so again: more/bigger is better!

The choke coils should be as close as possible to your tank (the coil with the caps) !

To get the connection of thick wires, choke coils and my pipe I had to use hose clamps as my soldering iron could not supply enough heat.

The wires and coils are soldered onto the thick part of the clamp and slides over the pipe where it is thightened up good for a proper connection.

A word of warning for those that already built a heater based on the standard Royer Oscillator and a coil with center tap:

The choke coils for the normal Royer only need to filter the high frequencies, in this setup they also have to store a lot of energy and "block" the direct connection from the mosfet drain to the positve rail of your power supply!

You need a higher inductance for this project!

Why do the choke need to be of such high inductance?

Well, it all comes down to resonating.

The caps in the tank store the energy which comes from the mosfets and the the work coil.

This work really well in resonance as the resstance of the coil drops to zero.

As the work coil also releases energy into the choke coils we want to re-use it.

Take a normal relay for example.

It turns on and stores some energy, when it turns off this energy is "killed" by a protection diode so it won't harm the circuit driving it.

You can check the high voltages generated from a small relay by holding a one wire of the power supply to the contact while the other is soldered on.

When you remove the wire you can see a little spark, do it very gently and you have a vibrating relay with big sparks on the contact pin.

Same priciple as your ignition coil in the car.

When the mosfet switches off the choke coils release their stored energy back into to tank.

Due to the resonance only tiny amounts of energy are wasted.

This works because the choke coils block the high frequencies while allowing the pure DC to pass.


Using 1/2" copper pipe and 5-6 windings you will end up around the 2µH region for your coil.

To make sure the mosfets are save and enough energy is supplied to the work coil, our choke coils must be able handle the currents provided by the mosfets.

Don't be confused with the massive currents inside the tank here ;)

If you make your own choke coil on a donated core you should start with a lot of windings for your initial test if you have no means to test the inductivity.

In my tests on these big yellow cores I killed my mosfets when using only 8 windings of .75mm^2 speaker wire on them.
With 12 windings all was fine and power output good.

Outpur levels decreased once I had over 30 windings on it - so you see the safe region is quite large making it easy to check.

Simply reduce a few windings and check the output level (take the time to heat an object).

Once you notice no significant improvement you are good to go.

Step 5: Seeing It in Action

Check out how the complete setup works to heat a steel bot of 8mm till glowing red.

The massive noise from the transformer reminds me to clamp the core pack before welding it....

In the background you can see the frequency produce in the work coil in kHz.


Step 6: Last Words and Troubleshooting

Althoug I stated it before:

Check and double check your connections and soldering !!

Only use quality MKP caps of small size in big numbers, e.g. 20 100nF caps to get to 2µF for the tank.

Use single caps of similar sizes at your own risk as I noticed they all heat up too much due to the insane power levels produced.

If you yre unsure about your mosfets test them this way:

Connect a diode tester (every good multimeter has it) with the negative wire to the source.

Tap the gate for about a second with the positve wire - if it beeps or shows nothing the mosfet might be dead.

Move the postitve to the drain - it should show a quite high value, now use a finger (while still holding the positive on the drain) and place it onto gate and drain - the resistance should go up until it is too high to display - this is good mosfet.

If at any stage your multimeter beeps or shows a short your mosfet is dead.

All connections that carry the currents to and from the mosfets must be oversized!

You don't want the cables to heat up or have a too high resistance, after all we are operating the circuit close to a short anyway (in DC terms).

If you can keep the connection from mosfet to tank as short as possible or mount the mosfets diectly onto the pipe using a brazed on copper sheet - in this case you ust have a proper water cooling to prevent the mosfets from overheating!

Any heat up of the mosfets with no load inside the work coil indicates a problem!

Do your initial tests either with a current limited power supply or for a few seconds only.

Check the temp of mosfets and caps but only when switched off!!

There is still a good chance that some caps fail over time and blow up, a single one is no problem an can be replaced later on, if they all heat up and blow you got the wrong type of cap ;)

Start with small work pieces!

Never hold the workpiece with your bare hands! Especially small pieces can heat up in seconds causing serious burns!

Be aware that with the power level available here you won't be able to melt metal.

Once the so called Curie temperature is reached the steel looses it's magnetic properties.

This is good for forging but bad for melting it ;)

Although non ferrous metals like brass and aluminium can be heated with this setup you should be aware that they have not only a lower electrical resistance but also a much better heat transfer, apart from the problem that they are non-magnetic.

Step 7: What Come Next?

To make full use of the magnetic field and to get even more power out of it my next tutorial will include an update on higher supply voltages and the use of a ferrite mantle on the coil.

Using a higher input of voltage causes several problems:

1. The resulting voltages in the work coil are already considered to be dangerous and having around 1000V in the tank at higher input levels not only requires special caps but also a properly insulated coil and most likely a load transformer to protect the power circuit.

2. Cooling becomes an issue and requires a proper pump and destilled water.

Also insulating the driver part, water supply and coil is a must.

3. Protection of the power outlet needs to be considered. Without this there will be high chance of blowing your fuses all the time :)

So you might "only" see an Instructable on how to make good ferrite mix that won't saturate and focus the magnetic flux trough the work coil.

I want to keep it simple, so before I start a project with more input power a lot of resear will be necessary so I can be sure it can be done safely and by everyone.


The Instructable for making your own ferrite start here:

Make your own ferrite to improve magnetic flux


JohnC430 (author)2017-10-25

the schematic you showed says the coil is 1.5mH. in reality it is more like 1.5uH or less could be even half of that.

apart from that you did a great job. did it work the first time you turned it on or did a few transistors blow up before you got it working? i have seen a few of these designs and peoples attempts at getting it started and quite a lot of blown transistors. i want to try this out but it seems to me that perhaps the circuit is not that reliable to start with hence the large number of blow-outs. Please enlighten me. thanks for your reply.

tjh20 (author)2017-10-18

Make flyback transformer and other components from old CRT televisions and monitors? Can you do make a version using scrapped CRT devices? I literally have tons of them warehoused. Plenty of beefy electronics, transformers, wire, and ferrite.

Downunder35m (author)tjh202017-10-18

If I would have that sort of access I would not revert to home made LOL
Real ferrite is like ceramic and unless you just want to cut it it is a pain to work with.
To make something new out of it you need to grind it first...

DylanF48 (author)2016-11-14

So what would happen if i stuck my hand in one of these? I'm curious and i don't think just doing it would be a good idea.. lol

crafter2u (author)DylanF482017-06-26

nothing unless you have a ring on your finger

Downunder35m (author)DylanF482016-11-14

Nothing unles you are a terminator or touch the coil and ground at the same time....

lizardd (author)2016-10-25

Just to be sure, you choke coils are 2.2 micro Henry, not 2 milli-Henry right?

I wrapped 8 winds of 14 gauge enameled copper wire on a black ferrite core and got 5 micro-Henry according to my meter. Does that sound right?

Downunder35m (author)lizardd2016-10-25

It should be fine, although I prefer a few more turns.
The choke coils make sure there is no back emf going into the mosfets and they also seperate the work coil from the the unused mosfet during the switching.
Having low values here means max power for the work coil but being too low can mean you blow your mosefets.

lizardd (author)2016-10-18

I found a good-sized oval ferrite core, about 4 cm on the long side and 3 cm tall. I was thinking of using that, but if I wrap the work coil perpendicular, like you usually do on cores, there will not be enough space on the inside to use it. If I wrap the work coil around the outside, then I have a lot od work space, but I don't know if that will help the efficiency.

Do you think that it would help to wrap the work coil around the outside of a large oval ferrite core?

Downunder35m (author)lizardd2016-10-18

The magnetic field has to go through the core not around it.
Check Wiki for magnetic fields and hand rules to fully understand what I mean.

lizardd (author)Downunder35m2016-10-18

So you are saying that the ferrite core needs to be between the work coil and the workpiece?

Downunder35m (author)lizardd2016-10-18

No, but think about this:
You add a core inside the work coil.
By doing so you offer a path for the magnetic field.
If your workpiece it now inside the round core there will be little to no magnetic flux be available to heat it.
On the other hand, if the core is on the outside you offer an easy "return path" while all the flux still goes fully through your workpiece.

If you split the coil in two halfs that are filled with a core than the max flux will be between those cores - great for sheet metal work for example.

lizardd (author)Downunder35m2016-10-18

Thanks, I think I can picture the last part.

lizardd (author)2016-10-07

Thank you for that info.

I was not thinking of the chrome plated tubing to avoid cooling but as a way to improve the cooling while still using the water or air inside it. If it won't be an additional help, then the extra cost is not justified.

If you have the time, could you please illustrate what you mean but your ferrite improvements for the work coil?

Also, what is Litz wire?

Downunder35m (author)lizardd2016-10-07

It is called flux control or in the industrial area flux director.
Like a transformer core the ferrite will focus the magnetic field to where it is needed.
And for Litz Wire just check Google to see tons of images that explain it much better than I can.

Comes down to surface area ;)

lizardd (author)Downunder35m2016-10-11

I did look this up and it suddenly dawned on me that thick speaker wire, like for car stereo, is a Litz wire. Just in case someone is looking where to find some, but I don't know how much less hot it will be.

Downunder35m (author)lizardd2016-10-11

That wire is looking very similar but not Litz Wire.
For Litz Wire each individual strand is coated and isolated, speaker wire is just bare copper wire.
You need the insulation as otherwise you won't have the benefit of the combined surface area of the single strands.

lizardd (author)Downunder35m2016-10-11

When, the guy at the local car audio store is a liar. He said that the reason the reason that the 4 gauge car stereo wire was so expensive was that is was specifically made for AC conduction. Maybe they just say that to justify the price. It could be tested it since you could easily check for cross conduction. At least it looks right being braided.

lizardd (author)lizardd2016-10-11

I was thinking that you could make your own Litz wire. Unwrap an old transformer and make 20-40 loops, each would be long enough to make the coil. Twist each bundle, then braid enough bundles to make the gauge you want for the work coil.

Downunder35m (author)lizardd2016-10-11

That would not be a problem if you have the space and time for it.

lizardd (author)2016-10-06

Thank you for this design and tutorial, it is excellent. I have not made it yet, but I have a few questions. If you have design updates, please share here or privately.

Can you drive this circuit with an external oscillator to be able to tune accurately?

Would modifying the caps and work coil to increase the resonant frequency help with efficiency?

Is it possible to make an oval instead od round work coil?

Downunder35m (author)lizardd2016-10-06

1. No and not required.
2. No not really, changing the caps and coil only changes the operating frequency slightly.
3. Why not.

lizardd (author)Downunder35m2016-10-06

Thanks, what about updates?

Downunder35m (author)lizardd2016-10-06

See below for updates ;)

lizardd (author)Downunder35m2016-10-06

I was thinking about the first half of the first sentence below.

Also, what do you think of the using the chrome plated copper tubing, like for water delivery under sinks. There is an intense infrared emission during operation and the reflection may help hold down the heat transfer. I don't know if that would alter the capacitors for resonance.

Downunder35m (author)lizardd2016-10-06

The chrome stuf is no good as due to the high frequencies the current will be pushed to the outer skin of the conductor.
And no matter what you try it will get damn hot, that's why water cooling is strongly recommended.
At full power even 10mm copper tubing gets glowing red hot quickly if there is no water flowing through it.
Keep in mind that you are not dealing with a few amps but a few hundred of them ;)
If you don't like water cooling Litz Wire in really thick and with 200 or more strands will work.
For the heat insulation you can use ceramic pole insulators if you get your hand on them.
Put the coil on the insulator and the workpiece inside.

Downunder35m (author)2016-08-04

As this design was already commercialised (IMHO) I won't really post any bigger updates anymore unless I discover some nice and new way to keep things even simpler.
But for all of my friends following me here and maybe even using one:
I had some thoughts on the design parameters and some minor improvements.
a) The capacitor bank
My current design as well as the china version have a little flaw when it comes to multiple capacitors.
If you consider building one or feel your current model will benefit then please wire the capacitors slightly different.
Assuming all caps are next to each other in a long row you want one side of them connected to the coil directly and the other side after a another U-turn of the coil.
This way the other side will feed from the rear end and drastically improve the load balance and keeps the caps enevly laboured.
Just to make it clear: The first leg of the first capacitor connects to the closest point towards the coil, the other side of the coil passes all caps, makes a U-turn so that the other leg from the first capacitor from before will actually connect to the point furthest away from the coil.
It also helps to solder the legs onto some small copper bars as close to the cap as possible - the short legs and lots of solder will transfer heat away from the cap to the copper bar.

b) The power supply
IMHO there is no real limit for it other than what the mosfets can handle and as long as you don't use work coils with only 3 or four turns you should be fine in the 20A region.
Bu due to the nature of the beast you will need a lot of capacitors on the output side of the power supply to keep things stable at full load.
The circuit depends on a good and stable DC voltage so a power supply producing too much ripple under load might kill your mosfets under full load.
It always pays off to measure the current or better to monitor it especially in the testing phase.
The last thing you want to see is the voltage breaking under full load!

c) The mosfets
Everything else in non-critical apart from maybe the fast diodes.
And the mosfets also limit what you can handle in terms of input voltage.
They should be rated four times over the input voltage, e.g.: for 20V input you need at least mosfets cabable ov 80V.
Since we want to be save at all times we take the next higher class of 100V.
In the other case of having a mosfet collection already you want to make sure the input voltage is always below the above ratings.
Cooling should not really be that critical if the choke coils are sufficient and the work coil has enough turns.
If you notice that despite massive heatsinks you mosfets heat up far below their max ratings it usually means the chokes need more turns or if they have the workcoild needs more turns as the mosfets struggle to keep resonance.
If in doubt increasing the capacitor bank size will save you winding a new coil as this also lowers the working frequency.

d) The wiring
For some projects it does not really matter how it looks as long as it works.
But for high frequency applications like this you want to keep things as short as humanly possible.
For an idea of the ideal world here:
The copper bars for the capacitor bank and the coil connections should be desgned in such a way that you can solder the mosfets and choke coils directly between there without any additional wiring.
If possible SMD diodes and if used resistors should be placed directly between the legs of the mosfets.
All wire ends poking out should be removed and soldered over to prevent sharp egdes.
If you want to use exchangable work coils but can't mount it all close enough to your case please consider using Litz wire to make the connection from the tank to the outside work coil mounts.
Keep them as short as possible and if you can twist the two together.
You will need to make you own from old microwave tarnsformers or similar.
I recommend a minimum of 18 strands and the resulting diameter should match the amount of copper for the tube diameter.
Keep in mind that we have to handle insane amounts of current and if your Litz wire overheats it only means you need more strands and if possible enven more of thinner diameter.

e) Work coils
You find enough info on the web about coil designs for induction heating but what if you want to keep things smaller and without water cooling?
Again Litz wire is the answer.
But the real secret is to make the coil diameter as close to the max size of your work piece as you dare.
For example using high temperature mortar to create a crucible that the coil wraps around or using these high voltage pole insulators made from ceramic or glass.
So it really can pay off to be able to exchange the work coils depending on what you need to do - after all the circuit keeps the resonance unless you have coild with massive differences in the number of turns.
Forced air cooling is another option as even thick Litz wire will heat up qickly on 20A or more.
For added peace of mind you might want to consider placing a temp switch onto the coil to kill the power supply in case it overheats.
A microwave is usually a good source as well as using resettable temp fuses that require a manual push on a button - with the later you can be sure nothing turns on again by accident.

f) Levitating or melting metals or why some metals won't work
This circuit was a piece of concept work for me.
Everything I found before to do the same either involved a center tapped work coil (what a pain!) or quite complex electronics to reach and keep resonant frequencies.
Even when going for a fixed frequency which would mean having a perfectly matched tank and work coil you would need a lot more lectronics and usually some circuit boards - for some people too much to do on their level as not everyone can etch a circuit board.
Due to the simple design we rely on the magnetisation of the work piece and the resulting eddy currents plus hysteresis losses for the heating.
Once the Curie temp is reach we can not only forge or quench the workpiece but we also loose the ability to further heat it up.
The remaining skin effect is not enough to overcome the heat losses to the outside enviroment.
To melt the steel the frequency would now need to go far higher and we would also need more power - one without the other won't do it.
Other metals like copper, aluminium, stainless steel (non magnetic!) won't work for the same reason they refuse to work on your induction cooktop.
Without the losses from changing the magnetic field and the resulting Eddy currents they won't heat up.
I got some question in regards to a coild design to allow levitating the workpiece.
Let me just say that this might work (at least in theory) with very small workpieces and quite small work coils the problem of keeping the frequency right still remains.
The hotter the steel becomes the less megnetic it will be.
So even if levitating would be possible on such small power levels it would drop the workpiece once it gets too hot as it looses the magnetism.

g) Improvement ideas for those who dare
I had some thoughts on how to increase efficiency even further, apart from using the full coil at each cycle instead of only one half.
1. Flux concentrator and flux directors - a fancy name for the use of ferrite matrials ;)
As you might know from the choke coils nothong works better to improve a magnetic field than using a suitable core material - the same is true for induction heating.
Take this circuit onto a smaller scale in the 5A range and use a work coild of small diameter that is wound in two sections with a large enough gap for the workpiece (flat steel prefered).
The coil halfs should be wound around a round ferrite bar that had pointy ends towards the work zone in the center.
If you add the flat steel and the gaps are quite small you see almost instantly that the steel gets to a glowing red between those tips.
The same works on a normal coil if applied to the outside.
Imagine that the workpiece will form the inner core but air the outer core!
If you add ferrite bars or a complete ferrite mantle around your work coil you can increase the real power on the workpiece by up to 200%.
2. Higher voltage and more power
Using a higher input voltage and mosfets rated for this will certainly result in far higher power outputs onto the workpiece.
The dowside is overall costs.
Up to around 20V you will deal will 20 to 35Ampere.
Mosfets for this region are quite cheap are easy to obtain.
Just going to an input voltage of let''s say 60V will mean that all needs to be oversized.
Hard to find cores for choke coils that can handle this...
Then there is the problem of operator safety!
With 60V input we have over 200V on the work coil - more than enough to be lethal which means we need proper insulation.
Isolating the oprator by means of rubber mats or similar is the industrial way by I don't like the idea of someone on a hobby level dealing with deadly voltages if it can be avoided.
After all the benefit of higher input voltages over costs for parts is not really enough unless you love to experiment and know how to stay safe doing it.
3. Optimised parts
I think here is the real secret on how to get the most power.
The caps need to able to handle the power and voltage but most importantly the energy from switching at these freqencies.
If you consider the usual working range of about 20-50kHz it makes sense to select mosfets that do best right here.
As an added option it is always a good idea to add the fastest possible protection diodes as usually the ones inside a mosfet are slower than what we call perfect here.
4. Double resonant circuit
This is an idea of mine that is still a work in progress.
Goal is to use a "work transformer".
So instead of powering the coil and tank directly the first "coil" is wound onto a transformer.
Our work coil can now have as little as a single turn as long as we get enough turns onto the transformer for a secondary winding.
The capacitor bank here can be configured in a similar way to a switchmode power supply.
If both the primary side and the secondary side of the transformer are equipped with matching tanks to keep both sides in resoance during the frequency shifts the output power onto the workpiece can be increased by about 500%.
Problem here is the workpiece and actual workcoil as the interfere with our perfect world.
Without additinal electronics it won't be possible keep resonace.
5. Theorectical only at this point
The choke coils currently only make sure the won't burn out the mosfets and limit the max current to some degree.
The stored energy from the switching however is mostly unused.
I was thinking of using and LC combination with a fast and powerful diode instead.
This way it might be possible to "direct" the resulting power into the capacitor so it can work with next half wave in sync onto the workcoil.

Enough food for thought for now ;)

Downunder35m (author)2016-07-18

Being my most succesful Instructable I was thinking of creating version 2.0 of the induction heater.
Technology moves fast and going a bit more modern would make sense too.
After checking prices for parts in the 1kW range I soon realised this project could be out of reach for the hobby tinkerer - especially when it comes to the required connections.
On a budget we are looking at around 250$ if a suitable power supply is already at hand.
Considering the parts cost and risk of damage if something was not done 100% correct I am playing with the thought to do the impossible:
Build a working induction heater by modifying a cheap 50$ induction cooktop.
Some people already tried with more or less success but usually not really with satisfactory outcomes in terms of speed and process control.
My aim would be to add water cooling, a suitable heating coil and proper user safety as these coils would run on around 1000V AC!

Let me think if you are interested and leave your suggestions too!
In case anyone here actually made my current induction heater it would also be nice to see some pics.

Have you seen these?

Cheap and I've used one to heat small steel parts. You can't put really large objects in them but all you need is a cheap 24V-36V switching power supply. Total cost about $60.

How much are you willing to bet that the circuit is a copy of mine?
The layout and coil design is a dead giveaway.
I admit the design is quite nice but I would love to see some detailed pics of the circuit board if you can provide them.
Not that I would have chance getting recognised by chinese "designers" but still...

Found some more pics online and it is definately my design.
The capacitor values and choke coils are even to my specs.
Since I see no way to pursue this matter I guess I can only say my design must be good enough to be stolen and marketed like this.
Only too bad that I am the one missing out on the money they make :(

JonathanS5 (author)2016-07-06

Can an induction heater such as this one be run on wild AC current? If not, is it possible to work around this by investing in some rechargeable batteries?

Downunder35m (author)JonathanS52016-07-06

You need a proper DC voltage and quite high currents.
Get a suitable transformer and rectifier ;)

moonspy79. (author)2016-03-04

Sorry Downunder35m for my late reply.

Yeah I cheked my head and realized that those currents are... well overkill :) I think I will continue with your desing and go from there to learn if I need more power. Heh heh I tend to get a bit crazy when designing stuff, but who want's to denide the scientists with in!!

I will absorb more info and continue. I really do appreciate you input on this and give you my thanks once again.

Downunder35m (author)moonspy79.2016-03-04

The current design is powerful enough to get a thick steel bar (4cm in diameter) glowing red in quite a short time.
If you really need more power just make sure the mosfets are rated for it and that you have a really beefy power supply.
You main problem however will be to find toroid cores big enough to cater for enough windings of much thicker wire.
Also the tank caps really should be either the expensive Wima ones or dedicated for the job, like from some salvaged induction hotplate.
But no matter how high you go with the available power, no matter how thick your copper tube is - you will never melt steel.
It is just a matter of loosing the magnetic properties when reaching the curie temp (hope that is the right name for it).
For anything more you need frequency control and that is an entirely differernt best.
If I ever find enough time I will make one in the 10KW range, which should be enough for most melting jobs up to 500gram.

moonspy79. (author)2016-02-26

Oh yes and of course I will need more than 46A current capacity from the fets if I want to run even close to 140A :)

moonspy79. (author)2016-02-26

First of! Excelent tutorial/guide/explanation, Thanks

I have couple of those ZVS ebay heaters (120W and 1000W version), but I want to make one that can fit to the container I have in mind. I'm going to use this as a car bolt heater.. Anyway, I'm thinking of using a welder inverter as a power supply. Do you think it will work? The idle voltage is around 70vdc, can't seem to find the ON LOAD voltage information, but the current is around 140A. I think the inverter would be ideal as I don't have to make a rectifier and also it will be wayyy lighter than welding transformer..

I have many 500V, 46A, 0.24ohm, 200ns MOSFETs so these should at least handle the input voltage increase?

How about the R1 and R2. How many wats are they?

How thick the choke winding should be? Haven't got any idea how thick it should be.

What else should I be thinking about?

Thanks in advance

Downunder35m (author)moonspy79.2016-02-26

If you amp up that much you need to oversize the rest as well.
Your choke clois have to handle the current so the wire must be thick enough - can be hard to find toroid cores of suitable size, you might have to revert back to the cores used on biggers switchmode power supplies.
Also the soldering on such big things will be a problem, you might need a blowtorch.
The resistors are pretty uncritical, 1/2W should do here.

Did a quick check with the wire from an old MOT and even the primary side wire is not thick enough to handle such currents.
I would try to get the biggest ferrite cores you can find and if in doubt use several to get enough windings.

JonD36 (author)2016-01-31

If I have a flowmax pump for water cool easyheat system.

Question what does the filter do with the pump and what's the harm using a old filter?

Downunder35m (author)JonD362016-01-31

Well, the filter keeps the water clean but in a closed system you can work without a filter.
For only short term use a direct connection to a tap works too.

JonD36 (author)Downunder35m2016-01-31

thank you

StevenG56 (author)2016-01-18

Hello again!

Can I use an ATX power supply unit as power supply to the circuit?


StevenG56 (author)2016-01-18


Thanks for the post!

Can you upload a picture showing the full circuit?

have a nice day!

SamirI (author)2016-01-07

transformer primary and secondary voltage?

gdawg98 (author)2015-11-08

you say that it gets to 1000V in the tank circuit is that at 30 v input or???
and then how much would it be and is it even possible to use thise setup at 240 380 plus input voltage to melt cast iron steel?? with a crucible obviously

Downunder35m (author)gdawg982015-11-08

You need a DC voltage for my type of induction heater, that also means due the design the Mosfets have to have 4 times the voltage capacity of the input voltage.
240V rectified gets far over the 250V, so you would need at least 1000V capable mosfets.
Apart from the price and complications making this work:
You won't get more power out of it.
What the circuit needs is AMPS, so a stick welder with rectifier gives about 60V with no load and around 40V @100A.
No chance to get those values directly out of your socket.
The 1000V I mentioned are a combination of the tank circuit, capacitors and frequency.
Keep in mind the circuit operates at resonant frequencies - normal math does not apply in this region ;)

gdawg98 (author)Downunder35m2015-11-08

So trying to make this circuit stronger wont really work, do you know of any schematic for say a 5 to 10 KW induction heater so I can make a induction furnace. You also say that "apart from the price and complications making this work" what complications

gdawg98 (author)gdawg982015-11-15

But what if I take my outlet supply of 240 v and make or buy a tansformer and amplify it so I get the power that I need

Downunder35m (author)gdawg982015-11-15

If you want to into the melting region you need frequency controlled induction systems, like those for levitation melting.
The parts cost an arm and a leg, the power requirements are massive...
And without a decent electronics lab in your hobby room you won't be able to tune it.

iglesigu (author)2015-09-29

Very nice work and advice, thank you.

I have a question related to the resonant frequency. I mean I need to increase the frequency up to 200 kHz, Do you think This is possible with this circuit? Can I have a variable frequency output. Changed the capacitor and the coil for each frequency?

thank you in advance,


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