tatapoum Posted June 5, 2006 Report Share Posted June 5, 2006 Hello all,I have a transformer with two secondaries at 6 volts. I need 5 V and 12 V.Is this design correct ? Or should I link secondaries ?Thanks,ludo Quote Link to comment Share on other sites More sharing options...
illogik Posted June 5, 2006 Report Share Posted June 5, 2006 hifor the following bits you really need a second opinion, they are guesses basicly; :Pi think you should link secondaries (now you have 6 volts going to a 7812), and only use one bridgerectifierof this i am more sure;a regulator causes a voltage drop; so for a 7812 you need 1 or 2 volts more, even the 12v you get when linking secondaries isn't enough cheers, marcel Quote Link to comment Share on other sites More sharing options...
tatapoum Posted June 5, 2006 Author Report Share Posted June 5, 2006 No problem with 12V as rectifier output voltage is 1.42 higher as input.You're right about 6V on 7812. In fact, I have to admit that the actual design is the following. But I'm not sure it's here the best design. Diodes get hot, but output voltage are correct.ludo Quote Link to comment Share on other sites More sharing options...
doc Posted June 6, 2006 Report Share Posted June 6, 2006 Hi,just to simplify your design a little bit ... (only a suggestion)... sorry for the drawing. Not enough time to make it on CAD.You don't need the second rectifier then. You also can save some caps. The values of the caps are just suggestions.The only thing is, as mentioned before, if the 12V input AC are enough to drive the 7812.greets Doc Quote Link to comment Share on other sites More sharing options...
tatapoum Posted June 6, 2006 Author Report Share Posted June 6, 2006 Thanks for your suggestion. However with your design, the 7812 is overload. The 4.2V used for the backlight consume 450mA and main 5V for logic and LED has a similar need.Moreover, I've read somewhere that cascading 78xx may produce a lot of noise. As 7812 provide current for SID module, this not, in my opinion, acceptable.My major concern is about the rectifiers. Two diodes are parallized and I feel that's not a 'clean' design. But it works even if they come hot.As I've said in my previous post, 12VAC if perfect to provide 12VDC. The rectifier drops the voltage to 17V (1,42 x 12, rectified current rules) and the voldate drop down of a 7812 rectifier is about 2V.ludo Quote Link to comment Share on other sites More sharing options...
doc Posted June 6, 2006 Report Share Posted June 6, 2006 The 4.2V used for the backlight consume 450mA and main 5V for logic and LED has a similar need.Oh, sorry, you're right with this values. I don't know the current until then.For the diodes: Why don't you choose a "bigger" rectifier or diodes with higher load?eg. B80C3700 (3.7A) or B80C5000 (5A), or if you prefer diodes: BY251 (3A) or P600A (6A)How much current delivers your transformer ?greetsDoc Quote Link to comment Share on other sites More sharing options...
ganchan Posted June 12, 2006 Report Share Posted June 12, 2006 Moreover, I've read somewhere that cascading 78xx may produce a lot of noise. As 7812 provide current for SID module, this not, in my opinion, acceptable.ludouhm.. so, if we realize an Optimized c64 PSU for SIDs, and we left out the SID rectifier, 7809 of psu and 7812/7809 of sid are cascaded.. uhm.. Quote Link to comment Share on other sites More sharing options...
tatapoum Posted July 17, 2006 Author Report Share Posted July 17, 2006 After spending a long time on this PSU, I found the cascaded 78xx solution is the best in term of noise. My initial design produced a lot of buzz.Thanks for your help,ludo Quote Link to comment Share on other sites More sharing options...
Mr modnaR Posted July 19, 2006 Report Share Posted July 19, 2006 hi, just looked through your flikr pages: very impressive stuff! about the PSU you'#ve made though, you say you updated the design because you weren't getting enough power? is the design in this thread the updated one?cheers Quote Link to comment Share on other sites More sharing options...
tatapoum Posted July 19, 2006 Author Report Share Posted July 19, 2006 The Doc's hand schematic is the one I used. Thanks Doc !ludo Quote Link to comment Share on other sites More sharing options...
Mr modnaR Posted July 19, 2006 Report Share Posted July 19, 2006 cool, so you've built this design then? how quiet is it? can it be compared to the c64 psu? also, what was the point of having a separate output for the LCD backlight? sorry, i'm new to all this complicated stuff! :-[ Quote Link to comment Share on other sites More sharing options...
tatapoum Posted July 19, 2006 Author Report Share Posted July 19, 2006 cool, so you've built this design then? Yep.how quiet is it? The voltage oscillated at less than 10 mV. And the sound seems to have not too much buzz.can it be compared to the c64 psu?I never have a c64 PSU, so I can't compare. also, what was the point of having a separate output for the LCD backlight? sorry, i'm new to all this complicated stuff! :-[Yes, that's complicated. It's an adjustable regulator LM317 fixed at 4,2V with the pot. It's the best system I found to power up the LCD backlight. It needs 460 mA, that's quite amazing, and a serial resistor of 5 ohms at 3 Watt is not the prefered solution because the pots allow to low the luminosity.ludo Quote Link to comment Share on other sites More sharing options...
Mr modnaR Posted July 19, 2006 Report Share Posted July 19, 2006 and a serial resistor of 5 ohms at 3 Watt is not the prefered solution because the pots allow to low the luminosity.ludoout of curiosity, why did you include it in that case? Quote Link to comment Share on other sites More sharing options...
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