Inductance Heating – Switching Circuits
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Inductance Heating – high frequency switching
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As with most circuits published on the web it is advisable to make sure that you analyse and understand them before putting solder to component. It is interesting to note that a lot of the circuitry used in these designs is not unique, there is no patent and therefore should not be a hindrance if one were to steal the design and enhance any weaknesses – however because you are working with high voltages, high energy and circuity designed to generate heat you can burn down your workshop and/or electrocute yourself. Don’t attempt these circuits if you are not comfortable with high voltage circuits or have only built a 9V astable multivibrator. Sie wurden gewarnt!
Inductance heating is not new – industry found a way to use this concept in foundaries many decades ago. In actual fact if one had to build a mains transformer without laminatiing the core to limit eddy currents you will quickly discover the concept. Where to from here? Well, one of the beauties of modern electronics is that component pricing has not increased like your basic foodstuff or clothing or beer. Electronic components have become more reliable and cheaper. So basing your purchases, dollar for dollar you can’t go wrong with electronics as a hobby. It is not unheard of to hear engineering students starting their own business from home: knowledge x creativity = wealth. Inductance heating nowadays use very fast switching circuits running at over 200kHz. This equals smaller components equals lower costs. A 300W power supply costs under 20U$ to build. (in Asia, that is). Home brewed inductance heaters can cost as little as 100U$ for a 1kW unit. (Update: A pre-purchased unit in the 3rd quarter of 2013 cost under 50 U$D for a 2kW heater).
The most expensive single component on mine was the copper tubing (to heat bolts and nuts to white hot).. Your inductance heater consists of 5 main blocks – the frequency generator, the gate driver, the switchers, a DC blocking capacitor (I put this in a separate block because there are safety concerns without this, like frying yourself instead of the work object) and of course, the most important block of all the tank circuit. For those budding electronic engineers it is advisable to learn up on tank circuits from our friend Wiki – I was fortunate enough to go to radio school in 1980 and resonant tank circuits were covered ad nauseam. Yes, the good old Conqueror transmitter and Apollo receivers (Marconi). The LC networks used in the inductance heaters of most of our linked circuits all had one common problem – damage to the capacitors due to the very (read: very, very) high currents through them at resonance. Careful planning and layout with air and water cooling in the tank circuit is one workaround. Oil filled capacitors are very expensive but in theory a better alternative.
The People
From my own research four engineers have contributed more than most to the field of inductive heating on the web and they are Tim Williams, John De Armond (Neon John), Jonathan Kraidin and Richie Burnett. If readers of this article know of anyone else’s work I’d be only too happy to submit a link here as well to their website.
The Tank Circuit
Typical Tank circuit block showing parallel LC network (also see Richie Burnett’s website on induction heating). The coil surrounding the white hot bolt is known as the work coil. In an efficient circuit such as this the bolt will become red hot within a matter of seconds. Richie’s website covers the theoretical aspects used in parallel and series resonant circuits. Maybe it may be a good idea to look up resonant tank circuits on the web as well. Radio transmission (or receivers, mind) has always intrigued me and of course what is radio transmission without LC tuning tanks.
Below: A simple block of a typical inductive heater
I found Tim’s experimentation and interesting articles a definite must read. Although all the links here are excellent I found Tim and John Neon’s a load of fun to read and for anyone whom has a definite interest to build his own heater Tim does cover all the “nasties” associated with the tank and switching circuitry. Tim has split his explanations, experience and working model over nine pages. Do read them all starting with page one. Tim’s schematics on page six are copyright so be warned.
Below is a picture of Tim’s 10kW induction heater which he also manufactures and sells. The tank “capacitor” can be seen beneath the meter. The half bridge switchers are four FGH80N60DTU IGBTs, water cooled – powered by a near 2000ufd 400V smoothing circuit.
John’s disclaimer is the funniest I have read in a long while. John supplies schematics and tons of information. Neon John as he is known professes to be a baby boomer like this parts-ring scribe but obviously has many talents that most youngsters of today only dream of. It’s good to see wit and ingenuity thrown in. BTW he links to his friend Jonathan Kraidin whom supplies stunning information on his own project. Very well written, clever, informative – a must read. Here something else can be learnt – PLL or phase locked loop circuitry. Many of the circuits available on the web are switched from an inverter driven by an oscillator set to the resonant frequency – this one changes the frequency to lock into the new inductance and hence resonance frequency (combined work coil which inductance changes according to temperature and work piece inductance). Johnathan’s schematic can be found here. The entire series can be read from here.
Inductance Heating: Some more information:
If you are a newcomer to electronics there are some things which one must know in order to understand the inductive heating process:
- Power supplies, full and half wave rectification.
- Power supplies and voltage regulation. Symmetrical or split rail supplies.
- The Curie Effect. What happens to a magnet when it is heated?
- What is an inductor?
- What is a capacitor?
- What is an LC circuit?
- What is Power Factor Correction and Q?
- Fast switching semiconductors: Mosfets and Bipolars.
- Switched mode power supplies.
Yes, there is plenty more and the order of importance is not important. As one moves through the basics you will find they all have their own little niche. Readers should find that Wiki will supply all the answers but unfortunately, such is the nature of electronics formulae can be offputting. Stick to the basics and then once fully understood move onto the more intricate part – formulae and some maths. Years ago I was overwhelmed by the amount of maths involved in simple radio antenna – nearly put me off radio for good. Unfortunately it’s all part of the learning and understanding aspect of any technical subject, be it human blood to cutting down trees.
[Ed – some of these images may be copyright infringed – for any concerns or ceases please contact our webmaster @ this website.com]
Whilst inductance heating is certainly not a new ‘discovery’, one can only be inspired by the marvellous work of Richie Burnett, Tim Williams and John de Armond.
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