In this post we learn 2 easy to build induction heater circuits which work with high frequency magnetic induction principles for generating substantial magnitude of heat over a small specified radius.
The discussed induction cooker circuits are truly simple and uses just a few active and passive ordinary components for the required actions.
Update: You may also want to learn how to design your own customized induction heater cooktop:
Designing an Induction Heater Circuit - Tutorial
Induction Heater Working Principle
An induction heater simply is a device that uses a high frequency magnetic field to heat up a iron load or any ferromagnetic metal.
During this process electrons inside iron are unable to move as fast as the frequency, and this gives rise to a reverse current in the metal termed as eddy current. This development of high eddy current ultimately causes the iron to heat up.
The generated heat is proportional to (current)^2 x resistance of the metal. Since the load metal is supposed to be made up of iron, we consider the resistance R for the metal iron.
Heat = I2 x R (Iron)
Resistivity of Iron is: 97 nΩ·m
The above heat is also directly proportional to the induced frequency and that's why ordinary iron stamped transformers are not used in high frequency switching applications, instead ferrite materials are used as cores.
However here the above drawback is exploited for acquiring heat from high frequency magnetic induction.
Referring to the proposed induction heater circuit below, we find the concept utilizing the ZVS or zero voltage switching technology for the required triggering of the mosfets.
The technology ensures minimum heating of the devices making the operation very efficient and effective.
Further to add, the circuit being self resonant by nature automatically gets sets at the resonant frequency of the attached coil and capacitor quite identical to a tank circuit.
Using Royer Oscillator
The circuit fundamentally makes use of a Royer oscillator which is marked by simplicity and self-resonant operating principle.
The functioning of the circuit could be understood with the following points:
- When power is switched ON, positive current begins flowing from the two halves of the work coil towards the drains of the mosfets.
- At the same the supply voltage also reaches the gates of the mosfets turning them ON.
- However due to the fact that no two mosfets or any electronic devices can have exactly similar conducting specifications, both mosfets do not turn on together, rather one of them turns ON first.
- Let's imagine T1 turns ON first. When this happens, due to heavy current flowing through T1, its drain voltage tends to drop to zero, which in turn sucks out the gate voltage of the other mosfet T2 via the attached schottky diode.
- Here, it may seem that T1 might continue to conduct and destroy itself.
- However, this is the moment when the L1C1 tank circuit comes into action and plays a crucial part. The sudden conduction of T1 causes a sine pulse to spike and collapse at the drain of T2. When the sine pulse collapses, it dries down the gate voltage of T1, and shuts it down. This results in a rise in voltage at the drain of T1, which allows a gate voltage to restore for T2. Now, its the turn of T2 to conduct, T2 now conducts, triggering a similar kind of repetition that occurred for T1.
- This cycle now continues rapidly causing the circuit to oscillate at the resonant frequency of the LC tank circuit. The resonance automatically adjusts optimally depending on how well the L1C1 values are optimized.
However the main downside of the design is that it employs a center tapped coil as the transformer, which makes the winding implementation a bit trickier. However the center tap allows an efficient push pull effect over the coil through just a couple of active devices such as mosfets.
As can be seen, there are fast recovery or high speed switching diodes connected across the gate/source of each mosfet.
These diodes perform the important function of discharging the gate capacitance of the respective mosfets during their non-conducting states thereby making the switching operation snappy and quick.
How ZVS Works
As we discussed earlier, this induction heater circuit works using the ZVS technology.
ZVS stands for zero voltage switching, meaning, the mosfets in the circuit switch ON when they have minimum or amount of current or zero current at their drains, we have already learned this from the above explanation.
This actually helps the mosfets to switch ON safely and thus this feature becomes very advantageous for the devices.
This feature could be compared with the zero crossing conduction for triacs in AC mains circuits.
Due to this property the mosfets in ZVS self resonant circuits such as this require much smaller heatsinks and can work even with massive loads upto 1 kva.
Being resonant by nature, the frequency of the circuit is directly dependent on the inductance of the work coil L1 and the capacitor C1.
The Frequency could be calculated using the following formula:
f = 1 / ( 2π * √[L * C] )
Where f is the frequency, calculated in Hertz
L is the inductance of the Main Heating Coil L1, presented in Henries
and C is the capacitance of the capacitor C1 in Farads
You can use IRF540 as the mosfets which are rated at good 110V, 33amps. Heatsinks could be used for them, although the heat generated is not to any worrying level, yet still it's better to reinforce them on heat absorbing metals.
The inductor L2 terminating from center of the main induction coil is a kind of choke for eliminating any possible entry of the high frequency content into the power supply and also for restricting the current to safe limits.
Relatively the value of L2 should be high enough, a 2mH will do the job well. However it must be built using high gauge wires for enabling high current usage through it safely.
The Tank Circuit
C1 and L1 constitute the tank circuit here for the intended high resonant frequency latching. Again these too musts be rated to withstand high magnitudes of current and heat.
Here we can see the incorporation of a 330nF/400V metalized PP capacitors.
Circuit Diagram of the first Design
Now comes L1, which is the most crucial element of the whole circuit. It must be built using extremely thick copper wires so that it sustains the high temperatures during the induction operations.
The capacitor as discussed above must be ideally connected as close as possible to the L1 terminals. his is important for sustaining the resonant frequency at the specified 200kHz frequency.
Primary Work Coil Specifications
For the induction heater coil L1, many 1mm copper wire may be wound in parallel or in bifilar manner in order to dissipate current more effectively causing lower heat generation in the coil.
Even after this the coil could be subjected to extreme heats, and could get deformed due to it therefore an alternative method of winding it may be tried.
In this method we wind it in the form of two separate coils joined at the center for acquiring the required center tap.
In this method lesser turns may be tried for reducing the impedance of the coil and in turn increase its current handling capability.
The capacitance for this arrangement may be in contrast increased in order to pull down the resonant frequency proportionately.
In all 330nF x 6 could be used for acquiring a net 2uF capacitance approximately.
How to Attach Capacitor to the Induction Work Coil
The following image shows the precise method of attaching the capacitors in parallel with the end termianals of the copper coil, preferably through a well dimensioned PCB.
Parts list for the above induction heater circuit or induction hot plate circuit
R1, R2 = 330 ohms 1/2 watt
D1, D2 = FR107 or BA159
T1, T2 = IRF540
C1 = 10,000uF/25V
C2 = 2uF/400V made by attaching the below shown 6nos 330nF/400V caps in parallel
D3----D6 = 25 amp diodes
IC1 = 7812
L1 = 2mm brass pipe wound as shown in the following pics, the diameter can be anywhere near 30mm (internal diameter of the coils)
L2 = 2mH choke made by winding 2mm magnet wire on any suitable ferrite rod
TR1 = 0-15V/20amps
POWER SUPPLY: Use regulated 15V 20 amp DC power supply.
Using BC547 transistors in place of high speed diodes
In the above induction heater circuit diagram we can see the mosfets gates consisting of fast recovery diodes, which might be difficult to obtain in some parts of the country.
A simple alternative to this may be in the form of BC547 transistors connected instead of the diodes as shown in the following diagarm.
The transistors would perform the same function as the diodes since the BC547 can operate well around 1Mhz frequencies.
Another Simple DIY Design
The following schematic shows the second simple design which can be constructed quickly at home for implementing a desired induction heater coil.
R1, R4 = 1K 1/4 watt MFR 1%
R2, R3 = 10K 1/4 watt MFR 1%
D1, D2 = BA159 or FR107
Z1, Z2 = 12V, 1/2 watt zener diodes
Q1, Q2 = IRFZ44n mosfet on heatsink
C1 = 0.33uF/400V or 3 nos 0.1uF/400V in parallel
L1, L2, as shown in the following images:
L2 is salvaged from any old ATX computer power supply.
Modifying into a Hot Plate Cookware
The above sections helped us to learn a simple induction heater circuit using a spring like coil, however this coil cannot be used for cooking food, and needs some serious modifications.
The following section of the article explains, how the above idea can be modified and used like a simple small induction cookware heater circuit or an induction kadai circuit.
The design is a low tech, low power design, and may not be on par with the conventional units. The circuit was requested by Mr. Dipesh Gupta
I have read ur article Simple Induction Heater Circuit - Hot Plate Cooker Circuit And was very happy to find that there are people ready to help youngsters like us to do something ....
Sir I am trying to understand the working and trying to develop an induction kadai for myself ... Sir please help me understanding the designing as I m nt so good in electronics
I want to develop an induction to heat up a kadai of dia 20 inch with 10khz frequency at a very low cost !!!
I saw your diagrams and article but was a bit confused about
1. Transformer used
2. How to make L2
3. And any other changes in the circuit for 10 to 20 kHz frequency with 25ams current
Please help me sir as soon as possible ..It will be help full if u could provide with the exact components detail needed .. PlzzAnd lastly u had mentioned to use POWER SUPPLY: Use regulated 15V 20 amp DC power supply. Where is it used ....
The proposed induction kadai circuit design presented here is just for experimental purpose and may not serve like the conventional units. It may be used for making a cup of tea or cooking an omelet quickly and nothing more should be expected.
The referred circuit was originally designed for heating iron rod like objects such as a bolt head. a screwdriver metal etc, however with some modification the same circuit can be applied for heating metal pans or vessels with convex base like a "kadai".
For implementing the above, the original circuit wouldn't need any modification, except the main working coil which will need to be tweaked a bit to form a flat spiral instead of the spring like arrangement.
As an example, in order to convert the design into an induction cookware so that it supports vessels having a convex bottom such as a kadai, the coil must be fabricated into a spherical-helical shape as given in the figure below :
The schematic would be the same as explained in my above sevction, which is basically a Royer based design, as shown here:
Designing the Helical Work Coil
L1 is made by using 5 to 6 turns of 8mm copper tube into a spherical-helical shape as shown above in order to accommodate a small steel bowl in the middle.
The coil may be also compressed flat into a spiral form if a small steel pan is intended to be used as the cookware as shown below:
Designing the Current Limiter Coil
L2 may be built by winding a 3mm thick super enameled copper wire over a thick ferrite rod, the number of turns must be experimented until a 2mH value is achieved across its terminals.
TR1 could be a 20V 30amp transformer or an SMPS power supply.
The actual induction heater circuit is quite basic with its design and does not need much of an explanation, the few things that needs to be taken care of are as follows:
The resonance capacitor must be relatively closer to the main working coil L1 and should be made by connecting around 10nos of 0.22uF/400V in parallel. The capacitors must be strictly non-polar and metalized polyester type.
Although the design may look quite straightforward, finding the center tap within the spirally wound design could pose some headache because a spiral coil would have an unsymmetrical layout making it difficult to locate the exact center tap for the circuit.
It could be done by some trial and error or by using an LC meter.
A wrongly located center tap could force the circuit to function abnormally or producing unequal heating of the mosfets, or the entire circuit may just fail to oscillate under a worst situation.