madammim

Thanks a lot Staigen! I have been thinking about somethings like that, but did not understand how to do it (did not think as far as using two transistors). So there is no problem feeding my 2. transistor with "-" on the "gate" and "+" on the "source"?

Thanks both. You know your theory well enough to ruin my project :'( I will change my frequency to 10Khz. It obviously will give me a better chance for success both with the curciut and the transformer. I just hope it will not give me an annoying 10Khz tone. If I still need to wire the two transformer circiuts on the same leg, will it then be any different if i put the 325V outside the 12V?

Ok. I am a little bit on thin ice here, but total gate charge for this mosfet is 63 nC. Using this converter: http://www.translatorscafe.com/cafe/units-converter/charge/calculator/coulomb-to-ampere-second/ I find that the current is much lower than the max rating (200mA) for the 555. I have also been running the 25 mosfets in my test setup, and it seemd to be running fine (messuring the frequency between "D" and "+"). Can anyone please underline where I have got this wrong by doing the calculations showing the 25 mosfets are too heavy load for the 555. ???

Thank you for responding Indulis and Staigen. I think you got me wrong, it is "only" 25 mosfets... But I am not that experienced in calculating this so I asked people with this skills, and they said 25 mosfets on a 555 shuld work fine. I think the wiering of my transformer is ok, but please check the drawing. I am not sure what you mean when saying a "snubber". Please explain for me. (My english and skills...) I have used B<0.3 in my calculations for the transformer, and the frequency is set to 25 Khz due to advices about the noice. But I can easily change it to 10 or 15 Khz. I have put in the diode on the drawing for the circute. Will this work ok?

I have now ordered some diodes. Will come back with the test results. Thanks a lot for now. :) BTW I normally dont dress up in womans cloths, but if I ever shuld, I would brobably look just like her... :P

Thanks again. I am a huge Disney fan (as you?), and choose to identify my self with a lookalike... ;D If I have to follow your suggestion, witch I probably shuld, I will have to re-wire my transformer, buy new transistors, and make the occilator from scratch again... The time is not a broblem, but I am allmost at the budget limit as it is. I have been searching for clamp diodes, and found some from my supplier: http://www.elfa.se/pdf/70/07040900.pdf http://www.elfa.se/pdf/70/07012081.pdf Is there any chance I can use some of these or any similare?

Ok. Thanks for taking your time, Audioguru. You know I did a lot of research before I started this project. (Maybe I shuld have asked first here :'() I found that recomended maximum frequency for a normal steel transformer was about 25 Khz. (...use a frequench of 20-25 Khz due to no noise...) But still even if I reduce the frequency to 10Khz (N1_min 43 turns) I will have the problem with the flyback voltage. Do you think this is not do-able at all? By the way, here is a data-sheet for the mosfets: http://www.elfa.se/pdf/71/07115835.pdf

Ok. But I am a little surprised. I make 325V DC, cut it up to 25 Khz, feed this "almost AC" to the transformer. I have calculated the N1_min to be about 22 turns at 20Khz. I dont know where I got this wrong, but I am not so experienced. I found a scetch for a SMP supply on the net, rearanged and recalculated it, and started building it "my way". Hope tou can take your time to explain a bit more, and maybe someone can give me a tip for a good WORKING SMP. Thanks

Thanks Audioguru. You say that this setup wont work cause a high voltage flyback that kills the mosfets? Is there any thing I can do to prevent this from happening? Can I rearange it in any way? I am not so experienced with such conections, but I think it shuld work in theory at least...

I am sorry. Here are a sketch. I can post a complete one later, but I need to draw it first. My original one have been changed some since I started this project... When I first testet this I used a 60W bulb lamp in series with the transformer. I had 25 Khz out from the transistors, and was able to adjust the duty sycles. Then I removed the lamp and started it with just the transformer. I heard the 50 Hz humming for about 0.5-1 sec, and then the fuses blowed.

Hi everyone. I am afraid i have started something i cant finish. I am trying to make a heavy duty powersuply using a NE555 timer and a set of 25 IRF740 mosfet. The timer uses a 12 V external supply. I plan to pull out about 90 A at 12-15 V. But i run into some problems. The transistors are running at 25 Khz, and the duty sycles is controlled from 50-75%. All of this is tested and working ok. I am useing a homemade transformer made of 1 mm steel plates (50 pieces) with walls of 50x50 mm. On the primary side i use 280 turns of 1.4 mm wire and on the secondary side i use 23 turns of 2 mm wire, 6 of them in paralell. I am useing a 10 A 700 V bridge and a 2200 uF 400 V cap to make DC. The negative side is feed through the transistors. Ok. So the problem. When I start up this supply, some of my transistors break down, and I get 50Hz on the primary side, makeing my fuses blow. I dont know what happens first, 50 HZ or transistor breakdown. I have a theory that the startup current is so high it makes the transistors brake, feeding my transformer with DC, until the cap gives up, feeding the system with 50Hz. If so, how can i prevent this from happen? Any other ideas about whats wrong? Thankful for all answers.