At 8:56 PM 10/20/4, [EMAIL PROTECTED] wrote: >In a message dated 10/20/2004 8:44:32 PM Eastern Standard Time, >[EMAIL PROTECTED] writes: > >> One main problem with using high >> frequency transformers in power supplies until fairly recently was >> rectification. Diodes drop in efficiency with frequency. These days the >> availability of high current low voltage FETs (with switching logic to >> achieve the rectification) permits efficient rectification, but even FETs >> still have frequency limitations, just much higher AFAIK. >> >> Regards, >> >> Horace Heffner >> >> > >Not so. Common variable speed drives now use switching transistors not >diodes. They commonly switch at about 10 kilohertz. This is done to >reform the >sin wave using a bunch of square waves. Rectification at megahertz >frequencies >is not a problem.
The subject, though, was 100 GHz rectification and power supply size. My point was that FETs overcame the drop in efficiency of diodes at high frequencies, but FETs too have their limitations and drop off in efficiency with frequency. High power switching at 100 GHz to achieve any kind of practical current rectification is a problem for a tiny power supply. > >This country started off with several standards. 25 hertz was used in the >steel mills and coal mines until the early 1980s. The low frequency produced >noticeable flicker in lights. I could see it out of the corner of my eye. It >was not visible looking forward. > >60 hertz produces no flicker, however, it is quite a feat to make a large >steam turbine that can spin at 3600 rpm. This was not possible in the early >days. Commercial generators, like those at Niagra, had many stators, so produced multiple cycles per rotation and thus did not rotate at 3600 rpm to produce 60 Hz. > >Aircraft commonly use 400 hertz. The spinning mass of the generator is small >and transformers are of a much lighter weight. > >Frank Znidarsic I remember in the early days of computing that some mainframes, like the IBM 360/65 used 400 Hz or 600 Hz motor-generator sets in order to isolate from the mains and reduce power supply size in boxes in the raised floor. Computing was a powerful thing in those days! 8^) Regards, Horace Heffner
