JohnSwenson wrote: > OK, now that actual PS design is being talked about I'll jump into this > thread. (I actually have a couple hours not at work today!!) > > I've done a huge amount of PS testing, design and building in regards to > the Touch and other devices, what I'm presenting here is not just > conjecture, its been tested many times. > > My conclusions are that a large percentage of improvements with PS > design for the Touch are related to high frequency noise sent BACK into > the mains and picked up by other components. power sent into the Touch > itself has little impact on what goes on inside the Touch (not zero, but > quite small). The differences in mains injected noise is vastly greater > than any changes in the power actually delivered to the components > inside the Touch. > > Given this the focus of PS design should be on decreasing what gets sent > back down the AC line, not making the absolute lowest noise, lowest > impedance feed to the Touch. Many of the attempts at getting the "best" > power to the Touch increase the noise sent down the line. > > For a linear supply there are primarily three things that contribute to > noise sent back down the line: reverse recovery noise when the diodes > switch, transformer ringing and current spikes when the diodes conduct. > > > Using Schottky diodes can get rid of the reverse recovery noise, > transformer ringing can be dealt with with an RC network across the > secondary which damps the resonance. This damping of the transformer > ringing is very effective but almost never done. If you go and look at > 100 different linear PSs there will be a very high probability that NONE > of them have damped the transformer resonance. Its intriguing that the > higher the "quality" of the transformer the worse the resonance is, thus > using expensive "high end" transformers is one of the worst things you > can do. (unless you damp it) > > The current spikes are the hard part. These are caused by the > traditional PS design of transformer, diodes and a big cap. The diodes > only conduct when the voltage from the transformer is greater than the > voltage on the cap. Thus the power coming from the transformer is in > short high current spikes. For example a common design for the 5V 2A for > the Touch will actually have 20A spikes coming from the transformer. > These 20A spikes have all kinds of high frequency components which get > sent right back through the transformer and into the mains. They ALSO > excite the above mentioned transformer resonance causing the transformer > to ring like a bell, even if you use Schottky diodes. > > The DC signal coming out of such a design is a sawtooth wave. This also > has large amounts of high frequency components. Unfortunately most > rectifier designs have very little input rejection at high frequencies, > they do great at low frequencies, but at high frequencies not so good. > This is one reason that discrete regulators have been used in audiophile > designs, they can have much better high frequency input rejection, which > is needed to correctly handle the sawtooth. > > There is a solution, but its hardly ever used in low voltage PS designs. > Its called a choke. Not the little high frequency things designed to run > at 50KHz and up, but big heavy things which have significant inductance > at 120Hz. If properly designed they allow continuous conduction through > the transformer and rectifier, eliminating the high current spikes. > Another advantage is that the output waveform is a pure sine wave, no > high frequency harmonics. > > Put the choke filter, transformer damping and Schottky diodes together > and you have a supply that injects almost nothing back into the AC line > and delivers a clean pure sine wave to the rectifier. With this you > don't NEED a complicated expensive regulator. > > I've put together a design using these principles, the schematic is at: > > http://home.comcast.net/~johnswenson1/stereo/SB_5V.GIF > > All the parts should be available at Mouser or Digi-Key. I'm sure there > are other distributors in other parts of the world where you can get > these parts. > > You should be able to get all the parts for $75 or so. > > Give it a try I think you will like this design. > > John S.
Possible to a link that works to this PS ? ------------------------------------------------------------------------ R1200CL's Profile: http://forums.slimdevices.com/member.php?userid=63832 View this thread: http://forums.slimdevices.com/showthread.php?t=82648 _______________________________________________ Touch mailing list Touch@lists.slimdevices.com http://lists.slimdevices.com/mailman/listinfo/touch
