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Registered Member #2310
Joined: Wed Aug 19 2009, 08:04PM
Location: Santa Catarina - Brazil
Posts: 169
Hey guys I'm back with some news... I got a transformer that gives me 30vAC, properly rectified and filtered it results in 43vDC. I've tested the circuit with it and the power of it raised a little. I've measured the current and it was at 12,8A at peak. But the voltage drop was giant, When the current was 12.8 the voltage was 28,00v so it gave me aproximately 360w of power...
So the question is: what do I need to change on the circuit to get more current flowing?
The parts are: IRFP260N 13v Zeners BYV26E Ultra fast rectifiers Maximum trr of 75ns
The tank capacitor consists of 27x 0.068uF MKP + 3x 0.47uF + 1x 0.22uF + 2x 0.1uF , resulting in: 3.666uF (all MKP)
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi gabriel,
well most importantly, what is the power rating of your transformer? And if you don't know it, what is the approximate area of it's core? It might just be too small for what you want, and buying a new transformer of the required power would be very expensive, hence I recommended a rewound MOT. The little transformers I used didn't have much more guts than yours did, I could get full 30V only when I cranked the variac to 11.
If you're sure your circuit is working well you could even consider using a welding supply if you have one, on lowest voltage setting you can get from it.
Registered Member #2310
Joined: Wed Aug 19 2009, 08:04PM
Location: Santa Catarina - Brazil
Posts: 169
Hey Marko! Thank you for the fast reply! I don't have a welding supply here =// The area of the transoformer is aproximately: 11.4cm X 10cm X 6,8cm It's pretty huge but i Know that it's Power rating is low, because it is a multi tapped transformer... So, do you thing that whats limiting the current flow of the circuit is the transformer only? changing the inductors inductance and the work coil's inductance are good ideas? or not? What about 3,666uF it's good?
A good ideia is getting a transformer capable of 36v and about 22A of current isn't it? or a variac... Or a MOT, but i've never experienced with it, and I don't know how to rewind it...
Registered Member #3888
Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649
for mot rewinding info. variacs for that kind of current are pretty expensive. audio amplifiers and subwoofer supplies usually have big transformers too (as long as they're not switch mode supplies)
Registered Member #2310
Joined: Wed Aug 19 2009, 08:04PM
Location: Santa Catarina - Brazil
Posts: 169
Thank you for the information on MOTs. I have one doubt, help me understanding the following: For example, I Get 2 transformers, both rated 36v output, one of them is capable of 300VA and the other 700VA... That means that the 300VA one is capable of 8,33A and the 700VA one is capable of 19.44A So, the transformer limits the current drain of the whole circuit, or the circuit will take as many amps as It deserve? In other words... Can a transformer be overloaded and start to heat due to high current consumption from the circuit? or it will only work within it's limits, giving only the current that it's projected to give?
Registered Member #2919
Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652
Gabriel35 wrote ...
Thank you for the information on MOTs. I have one doubt, help me understanding the following: For example, I Get 2 transformers, both rated 36v output, one of them is capable of 300VA and the other 700VA... That means that the 300VA one is capable of 8,33A and the 700VA one is capable of 19.44A So, the transformer limits the current drain of the whole circuit, or the circuit will take as many amps as It deserve? In other words... Can a transformer be overloaded and start to heat due to high current consumption from the circuit? or it will only work within it's limits, giving only the current that it's projected to give?
Thanks!
A transformer will work beyond its current ratings for short duty cycle. It will _not_ work beyond its voltage ratings (it'll saturate )
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi guys,
gabriel, I'm sorry that I didn't clarify that it's cross-sectional area of the core that is required, or whatever it's called (width*length of the centre leg).
Anyway if that's your 700VA transformer it looks like it has more than enough power for this circuit, but if it is the multi-taped transformer you mentioned then it might just have too much resistance in the particular winding you're using. Oh, and changing your DC link inductors will do pretty much nothing as long as they are not too small, at which the circuit will most likely go into latchup and die. It looks to me like you just need a bigger power supply.
I think someone mentioned that it's advantageous to use a separate 12V supply for the gates - I used to do it all the time when I tried to conserve every mW of power for wireless power circuits. But now here not only that this is a negligible loss, but I also suspect that big value of gate resistance with high voltage supply actually helps improve stability of the circuit. I'm not sure why is that so, but the circuit seems to like it's gate pullups act more like high impedance current source than a low impedance voltage source. Using discrete gate divers for amplification always turned out a disaster for me.
And yet with this circuit which is the only I've built by bastardly copying the original mazzili schematic, is by far the most stable of them all. It starts up nicely even when I bring the supply voltage up slowly by a variac, a feat unseen before in a royer world.
Registered Member #2310
Joined: Wed Aug 19 2009, 08:04PM
Location: Santa Catarina - Brazil
Posts: 169
I Agree Marko! I've experienced with some variations of the Mazzili's driver too , and the most stable one is this, with just one power supply, and using mosfets like IRFP250 & 260N (This secon one, is even better, because of the lower RDS on and higher current capabilty.
So... I think its time to get some MOTs here... But... I'm sacred about how it sags... I hope that the rewinded MOTs don't sag too much... Because if everything works, I pretend to apply something near 60v and see how much power the old and good Royer driver can deliver...
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi gabriel... probably a good time to start using water cooling, as your coil looks quite like a roast to me? I also don't think your mosfets are going to last long once you push 20A through them continuously with their current heatsink. I used simple pieces of 2mm copper as cooling blocks with pipe soldered underneath which worked well enough, but if you truly wished to be a bastard you could even solder the mosfet backs directly to the pipe... you'd need to have confidence not to have to replace them too often that way, though.
I've also heard some people (including Celem cap manufacturers) complain about the specific arrangement of caps you use, with all of them in a long line from a work coil. I suspect the problem might be in the all extra inductance the cap furthest away from the coil sees, which may be comparable to the work coil inductance - this inductance in turn cancels a portion of the faraway cap's capacitance, making them appear smaller and causing those closest to the work coil to hog current.
I see you placed some of your largest caps on the far end though, which may cancel out this effect to some extent, but I would consider it for future. If you use double sided PCB for your interconnections, inductance is low enough that you can orient the caps a more relaxed way.
Or you can use another common approach, with one work coil end going around the cap bank and connecting to it from back (some extra copper pipe usage).
Regarding the MOT: I've never rewound a MOT, so I don't know what it's voltage regulation characteristics are, but due do separation of primary and secondary I presume they are poor. Make sure to knock out the magnetic shunts from the mot too along with the secondary, and if it heats up a lot with no load add a 10-20 more primary turns to it somehow.
You could pull more juice if you wound the transformer for a bit high voltage, but just enough that it sags to 60V with royer unloaded. It would sag further when you put a workpiece in, but there's little you can do about that. Note that this way you would have to power your monster up and down from the mains side, to avoid slamming it with too high voltage from overcharged bus caps - and this is in contradiction with royer circuit's inherent desire to latch up and explode when it's supply voltage rises too slowly, as with charging bus caps after turn-on. I don't think it will be a problem in this case though, I couldn't force my circuit to latch up yet no matter what I did :)
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Royer induction heater
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