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from PC power supply to an induction heater

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Dinges

Tue Dec 22 2009, 03:47PM

Registered Member #2511 Joined: Mon Dec 07 2009, 02:46AM
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Posts: 36

Yes... it turns out you *can* use a PC powersupply as a small induction heater....

1261496537 2511 FT0 Induction Heater Test Setup

The test setup: a 200W PC PSU without fan, so I didn't dare to put in more power than 110W and even that warmed the tiny cooling fin more than I felt comfortable about.

1261496537 2511 FT0 Induction Heater Tank Circuit

The work coil consists of 5 turns of copper brake pipe (5mm diameter) with an inductance of 0.57 uH, the capacitor bank consists of 13 pcs. 0.15uF/440Vac capacitors for a total of nearly 2uF. The resonant frequency of the tank circuit is 155 kHz, as determined by measuring and by experimenting.

Replaced the timing resistor of the TL494 (which originally worked at 76kHz) by a 10-turn potmeter so I could increase frequency up to 200kHz and tune the PSU to the tank resonant frequency of 155 kHz. The TL494 was powered independently from a 12V battery.

Initially I ran the PSU with a series lightbulb and a variac. Slowly increased voltage as I scoped the tank circuit and monitored behaviour and temperatures of all components. Felt confident enough after a while to remove the series lightbulb and just use the variac - took it up till about 150V, but power increased only marginally (10W extra input) over 110-120Vac. Above about 150Vac oscillations stopped - I attribute this to saturation of the coupling transformer (powdered iron toroid, using it now for something it wasn't meant to do). There's not really room for more turns, but I'll try glueing two of them on top of eachother and rewind with the same number of turns. The material is probably MicroMetals type-52 (blue/green); at these frequencies it suffers from quite large losses, but it didn't become uncomfortably warm.

A M8 bolt was, after about a minute, warm enough to start giving off smoke. After a few more minutes of heating it up I turned the machine off to feel if anything (besides the bolt) got warm - the cooling fins got pretty warm, and the lousy powdered-iron toroid (meant for RFI suppression) was warm too - but much less than I had initially feared. After a few minutes I touched the bolt - still hot. Turned out it was still warm enough to easily melt some PbSn solder, so at least 183C - I estimate it was over 200C. Not too bad, considering the very small RF output (estimated 60-70W) of the setup.

After about an hour of experimenting one of the switching transistors gave up the ghost - not a surprize, considering how warm the tiny cooling fin had become in the meantime. The isolation pads probably didn't improve heat transfer as well. The increased frequency the supply was running at, 155 kHz vs. the original 75 kHz, probably also contributed to the overheating.

Tank voltage was 10Vrms - how can I calculate circulating current from this?

All in all, I'm very pleased with the results so far. It gives hope for further experiments with a more powerful induction heater - have started construction of a machine that is supposed to become a 2kW unit. But this PSU was my very first trial of induction heating and it didn't disappoint. All it took was an old PC PSU, some copper tubing and a handful of capacitors.

Peter.

IamSmooth

Tue Dec 22 2009, 05:12PM

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

Dinges wrote ...

Tank voltage was 10Vrms - how can I calculate circulating current from this?

Zc = 1/(2piFC)

I = Vtank/Zc

GeordieBoy

Tue Dec 22 2009, 05:14PM

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

Neat!

Although what you really want is a high-permeability Mn/Zn ferrite core for the coupling transformer instead of low-perm (and lossy) iron-powder. Iron-powder is not the right material to use here as you want tight coupling and energy transfer in this series-fed arrangement, not energy storage like you do in the LCLR topology.

I agree that the switching devices in the PC supply probably have a hard job coping with the higher operating frequency, and range of different load conditions that induction heating can present them with! Are they bipolars or MOSFETs?

You also really need that forced air cooling. The heatsinking in most PC supplies I have seen is next to useless without a decent airflow across it.

> Tank voltage was 10Vrms - how can I calculate circulating current from this?

About 19.5 amps RMS if you measured the voltage across either the work-coil or the tank capacitor. Or did you measure across the coupling transformer?

-Richie,

IamSmooth

Tue Dec 22 2009, 05:43PM

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

GeordieBoy wrote ...

Although what you really want is a high-permeability Mn/Zn ferrite core for the coupling transformer instead of low-perm (and lossy) iron-powder. Iron-powder is not the right material to use here as you want tight coupling and energy transfer in this series-fed arrangement, not energy storage like you do in the LCLR topology.

Richie,

I used iron powder toroids from here

It is the #3 material and good from 0.05 - 0.5Mhz. I bought two of them for my coupler and it works very well for my low power tests. I am waiting for a fountain pump so I can water cool my coils for a full power test. What do you think of these? If your recommendation is that much better, where can I get it?

GeordieBoy

Tue Dec 22 2009, 06:23PM

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

Type -3 is a much lower loss material than -26 or -52. I would think a large high permeability Mn/Zn ferrite toroid would work better here as good coupling and high specific inductance would seem to be the desirable properties. But, hey if Type -3 works stick with it. Don't fix what ain't broke.

-Richie,

IamSmooth

Tue Dec 22 2009, 06:32PM

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

do you have a source to get one? It's easy enough to wind another and compare side by side.

Dinges

Tue Dec 22 2009, 06:41PM

Registered Member #2511 Joined: Mon Dec 07 2009, 02:46AM
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Posts: 36

Thanks Imsmooth - actually had jotted that equation down before I posted, but thought that Xl and Xc compensated eachother at resonance - which they do... but still it's what determines circulating currents, of course (slaps forehead). Looks like about 20A circulating current then, in this case.

Geordie - I moved about 1/2 a year ago. For years I've been saving some very large toroids and never used them. Threw them away before the move, thinking 'I don't have a deathwish, I'll never experiment with high-power switchmode electronics'. Here we are, 1/2 a year further in time.... and to say I regret me throwing them away would be an understatement.

BTW, made a miscalculation: the TL494 originally oscillated at 76 kHz, but as it operates in push-pull, the transformer operated at 38 kHz. Now I've pushed the PSU up to 155 kHz (oscillator of the TL494 at 310 kHz).

The failure mode of the switcher transistors was interesting - much due to bad thermal conduction, bad workmanship of the original manufacturer. The transistors were basically sitting loose from the heatsink.... Hm.

And yes, I know about the forced cooling they require.

As far as ferrites vs. powdered-iron goes: I've read here (http://www.neon-john.com/Induction/091609_Update.htm) that powdered iron was supposed to be better at those high frequencies - am I missing something here?

The half-bridge consists of bipolar transistors, not MOSFETs.

I'm learning constantly here - most of this high-power switchmode stuff is new to me. Thanks for all the feedback, it is very much appreciated.

Peter.

IamSmooth

Tue Dec 22 2009, 07:21PM

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

I have a PLL based, integrator feedback design that I am almost done with. I put the schematic on here along with some video when it is finished. It has shown some very stable resonance tracking ability.

Davebmx

Mon Feb 06 2012, 07:50PM

Registered Member #4394 Joined: Mon Feb 06 2012, 07:43PM
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Posts: 40

Hi Dinges in my first venture into induction heating i have made an attempt to interpret your heater and make my own. So far I have built my series tank circuit and work coil albiet a little crude compared to yours and i am currently adapting a psu using the tl494 chip. Unfortunately i cant make head nor tails of the tl494 chip spec and i was hoping you could post some closer photos of how you modified the psu so that i might work out where to add my 12v supply and where to replace the frequency mod resistor with the potentiometer.

regards

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