Power supply?

[Magnus Danielson]

>      On 11/19/96 e6erik at etek.chalmers.se wrote:
>      
>      >
>      > Power supply for a modular:
>      > (taken from computers I think)
>      >
>      > 5V/100A 5V/15A 12V/5A -12V/5A Qulidyne switched
>      >
>      > 5V/2A 12V/1.5A -12V/0.3A
>      >
>      > 5V 20V 12V -12V
>      > linear
>      >
>      > Could something like this work at all?
>      >
>      > Could I make use of a +/- 12V for any applications?
>      >
>      
>      Most analog circuits will work fine on +12/-12 rather than +15/-15. 
>      Some loss of range may result, but that totally depends on the 
>      specific circuit. I built a Minimoog ladder filter once (+10/-10) and 
>      it worked fine on either 12 or 15 volt systems.
>      
>      Switching power supplies (like you find in PC's) can be used to power 
>      analog audio equipment, no problem. They may need some filtering on 
>      their DC outputs to reduce noise. Also they will radiate 
>      electromagnetic waves at high frequencies, so they might need to be 
>      located further from the audio circuits than a linear supply needs to 
>      be. It also may be a good idea to enclose them in a grounded metal box 
>      to help minimize emissions.
>      
>      Switchers can be tempermental - some will shut off if a certain 
>      minimum load is not present, some will shut off if the load is changed 
>      rapidly, and all will shut off if their output is shorted. Almost all 
>      of them have all voltage regulation functions tracking the +5 output 
>      *only*, which means if you load down the +5 to close to it's current 
>      limit, it will be working hard and the +12 and -12 outputs will be 
>      around 14 volts each! Pretty sucky regulation. Also if something goes 
>      wrong inside, they will appear to be dead to the world, and (in my 
>      opinion) it's cheaper to throw them away than to try to fix them. Most 
>      cheap switchers have a lot of design tricks in them just to get them 
>      to work in the first place, so it's difficult to just look at the 
>      components and then figure out how the thing works and what 
>      replacement components can be used. Besides much of their guts consist 
>      of special custom transformers and such.
>      
>      The 5V/100A 5V/15A 12V/5A -12V/5A must be huge. I would think that 
>      this power supply would be way overkill for a typical DIY project. It 
>      is really intended to power many logic circuit boards, probably a 
>      whole cabinet the size of a minicomputer. Also one hundred amps is 
>      quite a dangerous amount of current to have around - if you don't take 
>      many, many precautions you can cause a fire! Even though the supply 
>      will certainly have a shutdown feature on this output, it is designed 
>      to drive extremely low impedance loads so it can easily turn a wire 
>      harness into a flaming molten mass. Also clouds of sparks a couple 
>      feet in diameter could be made. Hey sounds like fun, huh.

Also, this is how you get the _rigth_ warm sound out of your synth... have you
onced heard the sound of bubbling and poping silicon you are hooked for life...

I have a friend that has a - uh - EPROM Burner... it's a neat little device
which will pop the top piece of cheramic of (usually goes 1.5 - 2 meters up in
the air). All the bonding threads (they are usually made of gold) are being
reduced in length (with a bluish white ligth). Also, the two legs in each 
corner
of the chip is also reduced in length (burned of that is). This is a very
effective EPROM burner and those available on the market does not come close in
as much fun per buck as this one...

It's also fun to see how current take new passages on the chip...

>      One thing that needs to be kept in mind when using standard 
>      off-the-shelf switchers for audio, is that analog circuits usually 
>      only need +12/-12 or +15/-15 volts, and there is no need for the +5 at 
>      all. Well the +5 voltage is really the main reason for the switching 
>      supply's existence in the first place - the +12/-12 outputs are just 
>      secondary outputs to power the RS-232 ports which don't need much in 
>      the way of amps. Also the -12 output is usually the wimpiest of the 
>      bunch, because it's hardly ever used for anything (most designers have 
>      a subconcious bias towards using positive voltages all the time). Many 
>      analog circuits (such as synthesizers) center everything around ground 
>      so that the plus and minus supplies are best made symmetrical.

You can still have some use for the +5 V stuff if you put in some digital 
control stuff which happends not to be standard CMOS which lives in +12V.

>      However much merit must be given to the idea that since switching 
>      power supplies (especially PC power supplies!) are cheap and widely 
>      available, they may be useful for synthesizer projects. Now that this 
>      topic has come up I think it may be a good idea to examine a typical 
>      PC supply to see how it will work for analog audio. I think I'll go 
>      mess with one the next chance I get.

I'd care about high frequency noise with switching supplies. Useing plastic
bypass caps over the electrolyts may be worth the effort.

Also, about the bootstrap problem. Some (most standard PC/AT switching 
supplies)
will not start unless there is a minimum load. It is even a problem with some 
of
these lowpower discs that we have now. There is one solution to this, useing a
dummy load. The dummy load is typhically a resistor which can emitt 5-10 Watts
or so. I usually mount one resistor onto the same contact as being used by a
harddisk or floppy. Beware of those contacts BTW. They are not really _that_
good.

Magnus