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OMG Induction Heater

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Experimentonomen

Mon Mar 17 2008, 06:05PM

Registered Member #941 Joined: Sun Aug 05 2007, 10:09AM
Location: in a swedish junk pile
Posts: 497

Hey conner! How much ringing did you have on the transformer input ?

I am trying series feed and have this far blasted 200VAC in to the thing but i get loads of turn on rings on the transformer input, but this could be explained by the 0.5+ meter long leads between it and the inverter.

Raka

Tue Feb 10 2009, 08:03PM

Registered Member #1838 Joined: Tue Dec 02 2008, 06:01PM
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Posts: 38

Is it really necessary to have the Schmitt trigger in the circuit? Isnâ€™t it possible to use only an inverting mosfet driver like the TC4421 (with clipping diodes on the input), its got build in hysteresis as well.

Tonskulus

Thu Feb 19 2009, 11:39AM

Registered Member #1223 Joined: Thu Jan 10 2008, 04:32PM
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Posts: 133

Raka wrote ...

Is it really necessary to have the Schmitt trigger in the circuit? Isnâ€™t it possible to use only an inverting mosfet driver like the TC4421 (with clipping diodes on the input), its got build in hysteresis as well.

Yes, because trigger also acts as "starting oscillator". Its self oscillating frequency is adjusted close to tank circuit resonant frequency by tuning feedback resistor.

My induction heater is still fine. Its been used for many times and many hours continously at 2..3kVA power.
So is seems to be reliable enough. No need to change blown mosfets or igbts every 5minutes.
I also made small curcible to melt some metals:

Only things that gets hot is cooling water and, ofcourse, workpiece itself. IGBT stays always under 40celsius, just slightly warm after 30mins at full power run.

Raka

Thu Feb 19 2009, 08:14PM

Registered Member #1838 Joined: Tue Dec 02 2008, 06:01PM
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Posts: 38

Canâ€™t you give the TC4421 some positive feedback, to make it self resonant?

Edit: I meant self oscillating

Paragon

Sat Jul 17 2010, 02:44PM

Registered Member #3007 Joined: Sat Jul 17 2010, 02:35PM
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Posts: 12

Hello All,

I have been playing around with a similiar setup to Kim Lahda and would like to ask for some advice.

The snubber capacitors values for the half bridge that Kim states on his schematic are 7 x 1uf for each side.
Could someone please explain how this value was arrived at and also how critical this value is?

The reason I ask is that I am currently employing IRFP460's as switches due having aprox 70 of them that I picked up cheap and was wondering if I would require different value snubbers!

Thanks
SRG

IamSmooth

Sat Jul 17 2010, 05:49PM

Registered Member #190 Joined: Fri Feb 17 2006, 12:00AM
Location:
Posts: 1567

SRG,

Refer to the middle of this page that shows the capacitors:

They filter out the DC component. I tried various combinations with my levitation coil that has near 50A on the primary side.

If the value is too high there is still some DC current that gets through. I found that when I got under 0.5uf I did not have a significant difference in the DC current. I just wound up with more expensive capacitors. My final cap values are 0.44uf. Each 0.44uf "capacitor" is made from two 0.22uf capacitors. Each capacitor can handle 750vac and 12A. This gives each leg 750vac/24A. So, I can handle 48A (full current cycle), with 50A pushing the limit.

So, in answer to your question, I choose my capacitors so they could handle the current and voltage, and filter out the DC current.

Here are the caps:

Paragon

Sun Jul 18 2010, 06:00PM

Registered Member #3007 Joined: Sat Jul 17 2010, 02:35PM
Location:
Posts: 12

Firstly, thanks very much for the information you supplied IamSmooth,

Could you (or anone else) kindly help with L-match?
My inverter appears to be drawing way to much current, reading from the dc bus at 10v it is drawing around 2A!

Am I correct with my understand that the L-match inductor is to match impedance of the inverter to the load and by doing so also limits the current draw through the inverter?

According to the diagram below taken from RB's site the L-match inductor 45uh would you know how this value is arrived at?

Kind Regards,
SRG

GeordieBoy

Sun Jul 18 2010, 07:35PM

Registered Member #1232 Joined: Wed Jan 16 2008, 10:53PM
Location: Doon tha Toon!
Posts: 881

The matching inductor forms a conventional L-match circuit with a portion of the tank capacitance. It transforms the parallel load resistance of the tank circuit from what would be quite a high number of ohms down to a something in the region of 10 ohms load to be presented to the inverter. This lower impedance is much more suited to being driven by switch-mode power MOSFETs running directly off the mains line.

The L-match arrangement is just a combination of an inductor and capacitor connected in series that transforms one AC impedance into another AC impedance. It is commonly used in RF engineering to achieve impedance matching between things like power amplifiers and antennas in transmitter setups.

I don't have the equations in front of me now, but you should be able to do a google search for L-match equations/calculations whatever. Or i'm sure most of the usual amateur radio text books will cover this calculation in the early chapters.

I hope this helps,

-Richie,

803

Mon Jul 19 2010, 02:57PM

Registered Member #2807 Joined: Fri Apr 16 2010, 08:10PM
Location:
Posts: 191

WOW, A 3 YEAR LONG THREAD!! Go melting molten steel!

Paragon

Wed Jul 21 2010, 02:11PM

Registered Member #3007 Joined: Sat Jul 17 2010, 02:35PM
Location:
Posts: 12

Thanks for your informative reply Richie.

Yep a three year thread that just won't die

I was wondering if these capacitors would suffice for tank capacitors.
MEA Capacitor 224k 0.22uF 2000V +/-10% Metallized Polyester as opposed to polypropylene?
The reason I ask is that I can get my hands on aprox 50 quite cheaply!

I don't think I can stretch to the Celem caps at this time, although saying that I have no idea what two csp150/200 would cost!?

Kind Regards,
SRG

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