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Royer induction heater

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Marko
Sat Aug 13 2011, 11:48PM Print View
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi guys

I remember a while ago some members complaining they can't find an easy to build and yet powerful enough IH to play with. This has been done before and I just reiterated it in my own way, built in one afternoon to see if it would work. To keep short here are some of the most important hints:

- The circuit is the very same as ZVS mazilli flyback driver (along with zeners and 470 ohm resistors), except it uses two inductors going from V+ to each end of the coil instead of one going to center of the coil.

- Use water cooling. Your mosfet drains are electrically connected to each end of the coil and you can use this as an opportunity to cool them too without investing in big heatsinks. I soldered some pieces of copper to loose ends of the copper pipe and bolted mosfets to them - not the best waterblocks in existence but worked more than well enough.

- Use a MMC of many parallel FKP caps for your tank cap. If you want to push a lot of power you'll need a lot of capacitance. I used 16 * 270nF, total over 4uF. The MMC has to handle very large current so you can't save there.

- Don't push your supply voltage, 50-60V would be ideal along with IRFP260's. I used up to 30V and could push over 600 watts and melt aluminum and ignite steel coins quite fast. If I used bigger mosfets like IRFP260's and 60V input, I could do likely over 2kW. This would ruin the caps very quickly though as they get very hot and were a limiting factor in my design. If you want an industrial power IH prepare to buy hundreds of small caps of a conduction cooled cap (both might turn out costing about the same).

Here is a video for now showing melting a brass coin, melting/burning up a steel coin and melting a piece of aluminum heatsink (shown uncut), along with some additional aluminum re-melting.



It was fun and I'm thinking about building a bigger version, with an unique water-cooled MMC. I don't have a proper power source though, I feel sorry killing my only MOT to rewind it. I'll post schematics (if required at all?) after some interest shows up.

*Some pics and a schemo*










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jnbrex
Sun Aug 14 2011, 02:10AM
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Location:
Posts: 51
That thing is awesome! Some schematics would be great too. That is a very interesting project, but I think it looks a little too complicated for my novice soldering skills...

On another note, does the copper pipe that is the heater get hot? I thought that induction heaters worked through eddy currents produced on the object to be heated and didn't actually heat up themselves.
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radhoo
Sun Aug 14 2011, 07:43AM
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Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 698
looks and works great!

a few months ago I built something similar, schematics and photos here:
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Conundrum
Sun Aug 14 2011, 12:46PM
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Joined: Thu Feb 09 2006, 05:37PM
Location: Republic of Guernsey
Posts: 3805
Its a shame you can't buy cheap magnetic solder (yet!)
Something like this would be great for emergency repairs etc and for people who aren't allowed to use a conventional soldering iron for safety reasons.

-A
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Gabriel35
Wed Aug 17 2011, 02:33AM
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Joined: Wed Aug 19 2009, 08:04PM
Location: Santa Catarina - Brazil
Posts: 168
Incredible!
How do you got so much power from a simple Royer oscillator? I'm very very interested on the schematic *-*
I got some nice IGBT's and 30vDC Supplys here, and I can't wait to try this scheme out!
Can IGBT's be used instead of mosfets?
I have here a pair of IRFPG40 4.3A @ 1000v
and a Pair of IRFZ48N 64A @ 55v

Can some of them be used?
It's really the same mazilli's driver only with two inductors and strong caps?

Thank you!!
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Dr. ISOTOP
Wed Aug 17 2011, 03:13AM
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Location: Cambridge, MA
Posts: 652
Conundrum wrote ...

Its a shame you can't buy cheap magnetic solder (yet!)
Something like this would be great for emergency repairs etc and for people who aren't allowed to use a conventional soldering iron for safety reasons.

-A

Induction heaters will melt normal solder. It's a common misconception that they only heat ferrous objects.
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Gabriel35
Wed Aug 17 2011, 12:52PM
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Can you post some photos of your circuit too?

Thank you!
I can't wait to buy some mosfets and try it out!
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Marko
Wed Aug 17 2011, 01:12PM
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Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Gabriel35 wrote ...

Incredible!
How do you got so much power from a simple Royer oscillator? I'm very very interested on the schematic *-*
I got some nice IGBT's and 30vDC Supplys here, and I can't wait to try this scheme out!
Can IGBT's be used instead of mosfets?
I have here a pair of IRFPG40 4.3A @ 1000v
and a Pair of IRFZ48N 64A @ 55v

Can some of them be used?
It's really the same mazilli's driver only with two inductors and strong caps?

Thank you!!


Yes it is, and if you wanted you could even center-tap the coil and use a single inductor if you wished. Another advantage of this would be that you could drive the water supply through both ends of the coil and having it exit through the center tap, removing the temperature difference that would normally be experienced between mosfets cooled with same water flow. But this didn't really matter at this power level and used only a tiny fraction of possible flow rate, so you can do as you wish I guess.


Regarding switches, I would still best recommend IRFP260's and a rewound MOT for the power supply.


Those IRFG40 high voltage mosfets are pretty useless for this circuit, you want low voltage mosfets with ON resistance of 50m ohms or less which is important for keeping the circuit stable. IGBT's, especially 1200V ones have high voltage drop which may again result in the circuit ceasing to oscillate and shorting out the power supply. You can try them but at your own risk, and they will probably not be as efficient as mosfets at such low voltages.

IRFZ44/48 or whatever are another extreme, with them you would be limited to like a 12-15V supply (car battery)? providing tens of amps, and you would have to put a step up transformer between your driver and the tank circuit, or use a 4x larger tank cap than I did which would be impractical.

The radhoo's schematic which uses the transformer is wrong, the tank cap should go after the transformer and not before it by the way.








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Gabriel35
Wed Aug 17 2011, 02:36PM
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Location: Santa Catarina - Brazil
Posts: 168
Ok!
So the secret of the huge power of the classic Royer Oscillator isn't the two inductors?

I have here a bank of 32x 0.068uF @ 630v - P612 w722 EPCOS Model (P6XX MKP capacitor)
Resulting a capacitance of 2.176uF. I can buy those IRFP260's and try it out!
I've experienced with Royer Oscillator a long time ago using two IRFP250 and a 36v Supply. But it was so weak, It was just capable of making a screwdriver redhot. I think that I still don't know what is the BIG difference between this circuit and the classic ones, like the one I've tryed... It is pretty incredible the power amout that you achieved on that video =O
What about the number of turns and diameter of work coil? are them important variables?
About the water? It's just to maintain the whole thing as cool as possible or it affects the Pratical Power of the circuit?

The capacitors? Can i use the ones mentioned? Do I need more capacitance?
I have a transformer here that is 24v. Rectified and with 8x 470uF capacitors it results in 36vDC, this tranformer is capable of 10~15amps. Can I use it?

About the Zeners, I plan to use 1N5349B 5w.
And the Fast Rectifiers, BYT11-1000
The 470 Ohm 5w Resistors, OK
And the 10K 0.5w too. =D

EDIT: I've found some IRF540N's here...VDSS = 100V RDS(on) = 44mΩ ID = 33A... Can I use them with 36vDC?

Thank you, and nice pics too!
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Forty
Wed Aug 17 2011, 04:00PM
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Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649
ah the pictures certainly help. I can close the paused youtube video tab now lol. I wonder how the dual inductor scheme would work with driving a flyback.
I imagine you formed your work coil around a pipe. Did you fill it with sand or anneal it to help with even bending? I've got the tubing sitting here, I just don't want to screw it all up by bending it improperly.
what do you guys think about fairchild's fdp2552
for zvs circuits like this? or should I finally give in and buy the proper irf's?
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Inducktion
Wed Aug 17 2011, 05:17PM
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Joined: Fri Jan 21 2011, 11:07PM
Location: Houston, Texas
Posts: 1035
When I made my induction heater's coil, I filled it up with sand to make sure it didn't bend. Make sure you have it full, with no pockets otherwise you'll end up ruining the pipe. (I have a few kinks in mine because of that, but it still works fine)

and, the dual inductor thing probably wouldn't help with a flyback, because the windings on the core are technically inductors/an inductor as well.
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Forty
Wed Aug 17 2011, 07:02PM
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Location: Erie, PA
Posts: 649
i wonder if filling it with water and freezing it would work the same as the sand (that's what they do for brass musical instruments, thanks "how it's made")
I didn't mean to improve the driving of a flyback, i just wondered if it would work equally as well as a single inductor. with less turns actually required on the core, I could use thicker wire with some of my smaller transformers.
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Marko
Wed Aug 17 2011, 11:58PM
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Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hy everyone,

Gabriel: It's important to use a large tank capacitance, 4uF in my case, if you keep 0.68uF or whatever was used for a mazzili driver the power output will likely be pathetic. Using higher supply voltage as well as a step-up isolation transformer also helps. On the other hand it's completely the same whether you use one DC link inductor or two!

Note that in this circuit the device has to be rated at least pi*supply voltage, hence 30V is pretty much the macimum for a 100V mosfet. Your supply voltage will probably sag under 30V even without workpiece, but I would use a variac anyway to bring the voltage up if it sags too much when you put the workpiece in. You would need a huge transformer with good regulation if you want to go without variac and drive the circuit to full power.

Also if you have only a 2uF tank cap, your power throughput might be somewhat disappointing if you don't get 200V mosfets. But your current ones should work for the first try.

Forty,Inducktion: Yes, FDB2552 looks great if you have some, although a 200V mosfet would be even better as I think.

I filled my coil with table salt before winding it, and blown it out (with some difficulty) with a compressor later. I didn't think of freezing the coil with water in it, it looks like even better idea actually.

Marko



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Adam Munich
Thu Aug 18 2011, 12:27AM
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Location: Cali-forn. i. a.
Posts: 2237
Marko wrote ...




Funny, I've built that same circuit in the past as an experiment. It worked, but my mosfets kept exploding so I didn't bother to post it.

Maybe I just used crappy mosfets, but good job for getting it working marko!
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Dr. ISOTOP
Thu Aug 18 2011, 12:56AM
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Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652
The giant Eurofarad Snubbers on Ebay are excellent tank caps; I've run mine to 4KVA with only mild heating (cooled through the two large metal terminals).
This has got to be the most beautiful Royer project I've seen
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Marko
Thu Aug 18 2011, 02:00AM
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Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hey bwang, those caps you discovered do look superb for induction heaters, so depressing they don't ship them out of US. I see some of them have large flat terminals, are those designed to be conduction cooled? And by 4kVA I assume you mean apparent input power, not the reactive power handled by the cap

WIMA caps do surprisingly well though, I was very surprised to calculate that current in my work coil with no load is over 100A, all of which is handled by just 16 small paralleled caps! It's surprising they are surviving it at all without bulging up or showing decreased capacitance.


Grenadier, your circuit might have been unstable if you used high RDS-on mosfets like IRFP450's. That's the main reason why I enforced low voltages here.

marko
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radhoo
Thu Aug 18 2011, 03:48PM
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Joined: Sun Jan 25 2009, 12:44PM
Location: Romania
Posts: 698
Marko wrote ...

The radhoo's schematic which uses the transformer is wrong, the tank cap should go after the transformer and not before it by the way.

I think you got it a little wrong. You can't move the capacitor as the oscillator will not work.
The purpose of the ferrite core is as insulation transformer. It reduces stress on transistors and capacitor.

This approach can be used to power the original winding of flybacks as well, since we know that a concentric primary-secondary topology will put out more power.

Your current design is identical to my second variant, without the insulation transformer, just that you increased the power ratings with the bigger tank capacitor and work coil. Others willing to try this might want to look for mosfets with lower Rds instead, since keeping mosfets cool in this circuit is essential.
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Marko
Thu Aug 18 2011, 10:40PM
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Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
I think you got it a little wrong. You can't move the capacitor as the oscillator will not work.
The purpose of the ferrite core is as insulation transformer. It reduces stress on transistors and capacitor.


Hi radhoo,

the simple answer is, why would you want to have your transformer suffer all the reactive power in the tank circuit when you can have it handle only the real power instead? If I was to place a transformer in my circuit this way, it would need to handle the several tens of kVa present there, requiring a huge ferrite pole pig.
On the other hand having the transformer before the tank circuit it would only need to handle 600 watts of real power that are really going in, which could be done on a flyback core or similar.


I've used these transformers all the time, but on low power levels though (<100W). A 1:1 transformer produces absolutely no observable difference in operation, other than the output being isolated from the input. You can center tap the primary and use a single inductor instead of two and everything stays the same again!

If you use a step-up transformer, it will make the impedance of your tank circuit appear lower to the driver section, divided by square of sec/pri ratio: in other words, the oscillator will feel like you're using more capacitance and less inductance, with everything else staying the same.


I would still take care to minimize the leakage inductance on the transformer as much as possible, which may destabilize the oscillator but simply overloading it could do the same. If I slapped a 1:2 transformer to my circuit I'm pretty certain it would explode, but 1:1 should make no difference at all.

I actually wanted to try it out on this IH, but am unsure how to mount it to the existing configuration of the circuit, would have to isolate the mosfets from the work coil which is some trouble to do now :(

Still if you're going to use a 50Hz transformer like rewound MOT for a power supply it might make no sense to use an additional ferrite transformer - unless you have only very low voltage supply and low voltage mosfets like IRFZ44's and you want to push a lot of power.

Marko

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Gabriel35
Fri Aug 19 2011, 01:55PM
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Location: Santa Catarina - Brazil
Posts: 168
Hey Marko!
I'm experiencing with the circuit again.
I got some cheap as hell IRF540N's just to try out and but how you said, 36v seems to be so much for them to keep.
I'm waiting for my new IRFP260N's to arrive.
I have 8x 1uF 250 X2 Category Capacitors here.
can I use a capacitance of 8uF or something like that? How does it behave on the circuit?

Can you post a video of your oscillator melting a screw? and some photos of your Power Supply stage?

Can I use 13v Zeners instead of 12v ones?
Thank you!! =D
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Forty
Fri Aug 19 2011, 03:46PM
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the 13v zeners should be just fine. I think his power supply is just a MOT with a rewound secondary and then a big rectifier and some smoothing lytics (unless that's what you meant that you wanted to see)
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Marko
Fri Aug 19 2011, 04:52PM
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Hi guys,

In the video I'm actually using two UPS transformers wired in reverse, in series - rectified and filtered with a bunch of caps. I have a MOT too but I don't want to hammer out it's secondary and I'm actually building a transformer now from some huge cores I had around from old dead OBIT's.

I suggested a MOT as an easy solution for a power supply for soemone who has acces to lots of MOT's.

Gabriel: I don't know what type your caps are so I can't tell whether they'll be good or not, but also their capacitance seems a bit too high individually, unless they can handle 20-30amps per cap I wouldn't use them. If they are polyester, which they most likely are, the circuit might not work at all or they may just burn out.

Marko
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Goodchild
Fri Aug 19 2011, 07:27PM
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Beautiful construction, all your compact projects are great!

May I ask what the overall voltage and RMS current rating is of the MMC bank you are running? It must be rather high by the amount of power you are putting into it!
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Nah
Fri Aug 19 2011, 08:39PM
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I knew it was a good idea to get all those WIMA caps!

Also, would it be ok to use a back of caps with different values?

Thank you

Paul
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Forty
Fri Aug 19 2011, 09:22PM
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oh whoops. i thought by "I don't have a proper power source though, I feel sorry killing my only MOT to rewind it." you meant that you already had killed your mot and currently feel bad. sorry for responding incorrectly on your behalf.

i wonder too about using a bank of mismatched capacitors (i've got lots of salvaged mkp x2 box caps.) I suppose it would be fun to try it out and possibly explode a few.
anyone ever used an ac oil filled cap (like a MOC) in a high current tank circuit? i've got 3 10.5uf 520vac oil filled caps from a large ballast and no idea what to do with them.
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Gabriel35
Mon Aug 22 2011, 01:42AM
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Nah wrote ...

I knew it was a good idea to get all those WIMA caps!

Also, would it be ok to use a back of caps with different values?

Thank you

Paul


I Have the same doubt. about different cap values...

Thank you!
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Dr. ISOTOP
Mon Aug 22 2011, 02:18AM
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Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652
Forty wrote ...

oh whoops. i thought by "I don't have a proper power source though, I feel sorry killing my only MOT to rewind it." you meant that you already had killed your mot and currently feel bad. sorry for responding incorrectly on your behalf.

i wonder too about using a bank of mismatched capacitors (i've got lots of salvaged mkp x2 box caps.) I suppose it would be fun to try it out and possibly explode a few.
anyone ever used an ac oil filled cap (like a MOC) in a high current tank circuit? i've got 3 10.5uf 520vac oil filled caps from a large ballast and no idea what to do with them.

Different-valued caps will share current unequally, leading to possible failure of some of the capacitors.
Don't use oil/paper/mylar caps in a tank circuit. You'll be real sad when it fails and spits boiling oil at you.
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Marko
Mon Aug 22 2011, 03:17AM
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hi guys,

If you have the caps from same manufacturer, of same technology and voltage rating but different capacitance I think they should do reasonably well when paralleled. Caps will form a current divider and all of them will share portions of current proportional to their capacitance.

The downside is that not all of the caps may be able to take the same power dissipation punishment due to surface area of their package. Due to square/cube law a lot of small, low-value caps are of benefit here,

So if you have caps that are like 0.27uF and 0.15uF mixing them should work fine, but you wouldn't be able to run 0.15's to their maximum without cooking the other ones. So I say mix them and have fun - just watch them not to overheat. The general rule for polypropylene caps seems to be, if they're too hot to touch, they're too hot, as I found out the hard way.

Goodchild:

I'm not sure what exactly is the current rating of the caps, but dividing the tank voltage with it's characteristic impedance yielded a current value of as much as 150A - and without water cooling the coil gets sizzling hot in seconds. I'm very surprised how well are the caps holding without blowing up so far.

They are 160V DC caps and I'm running them up to 70V AC,which seems to be about the limit considering they get quite hot.

Cheers,

Marko
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Gabriel35
Mon Aug 22 2011, 02:58PM
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Can you post a video of your oscillator melting a screw?
Or Something like that Marko?
It's so nice to see things melting and burning with it, and quite impressive too.

Hey guys! I'm back, and got my new IRFP's260N and everything is working nicely, but now i have one problem... Like Marko said, when the workpiece is introduced on the work coil, the voltage drops a lot, in my case, I got a transformer very simmilar to the one that appears on Marko's video. It's rated 24V AC, Rectified it becomes 36vDC, when i turn the circuit on, it drops to 31, and with work, it drops to 24vDC...
I've made some calculations here, I got 11.8A as maximum current draw, and when the workpiece meets curie point it starts droping and keeps at 9A, so, my power source drops down to 24vDC, it gives me a real power of ~276W and aproximately 230W when curie point is reached...
With only 276W i alredy got the screw to become RedHot and I'm pretty excited to get more power with it, I think something around 1200W

As for the supplys, whats the advantage on using a separate 12v supply for the mosfets gates and a higher voltage for the power stage?

Someone knows a way to avoid that and make the thing work at full power?

As Marko noted, I can go up to 60v, because I'm using IRFP260N as switches.

Thank you!
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Arcstarter
Tue Aug 23 2011, 12:49AM
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Location: Beaumont, Texas, USA
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Gabriel35 wrote ...

Hey guys! I'm back, and got my new IRFP's260N and everything is working nicely, but now i have one problem... Like Marko said, when the workpiece is introduced on the work coil, the voltage drops a lot, in my case, I got a transformer very simmilar to the one that appears on Marko's video. It's rated 24V AC, Rectified it becomes 36vDC, when i turn the circuit on, it drops to 31, and with work, it drops to 24vDC...
I've made some calculations here, I got 11.8A as maximum current draw, and when the workpiece meets curie point it starts droping and keeps at 9A, so, my power source drops down to 24vDC, it gives me a real power of ~276W and aproximately 230W when curie point is reached...
With only 276W i alredy got the screw to become RedHot and I'm pretty excited to get more power with it, I think something around 1200W

As for the supplys, whats the advantage on using a separate 12v supply for the mosfets gates and a higher voltage for the power stage?

Someone knows a way to avoid that and make the thing work at full power?

As Marko noted, I can go up to 60v, because I'm using IRFP260N as switches.

Thank you!

To get more overall wattage, you can lower the inductance/resistance of the work coil for lower impedance which raises the current, or use more voltage which would also raise the current.

The 12v supply for the mosfet gates is a good idea. I doubt i will be making any more ZVSs without that kind of drive. With a 12v supply, the gate resistors have to drop less voltage, and thusly heat less. This means more efficiency and smaller resistors, i used a single 1/4 watt resistor for each gate and they remained well within their resistance tolerance range. However, i would probably use less than 500 ohms, because as the voltage drops, so does the speed the gates charge.

A big transformer is needed for the wattage you want, and even a MOT wound with too thin wire could be 50v output with no load and drop to 25v with 10 amp load. You can use ohm's law to calculate the amount of voltage that will be dropped across the resistance of the secondary, if you know the resistance of the secondary.
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Gabriel35
Tue Aug 23 2011, 02:05AM
Registered Member #2310
Joined: Wed Aug 19 2009, 08:04PM
Location: Santa Catarina - Brazil
Posts: 168
And about the two inductors noted 45~200uH , if I lower their inductance, will the current raise?
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