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Power supply design for low power PC

Here is a power supply design for the low power PCs I've been talking about. It will also run a squeezebox very well.

The design can be built for either 5V or 9V. The original fitPC takes 5V and the new "slim" version takes 9V. Also the squeezebox SB3 (now classic) takes 5V and the new duet takes 9V. So build it which ever way you want.

Most of the parts are the same, just a different power transformer and different regulator for different voltages. For 5V there is a 5V fixed regulator, for the 9V you have to get the adjustable version and use two resistors to set the voltage.

All the parts except for the choke can be bought at DigiKey. The chokes are a little harder to find, Angela instruments and Radio Daze have them.

As with any AC supply you need to add your favorite input connector and fuse. For 120V lands use a 1/2 A fast blow, for 240 lands use a 1/4 A.

The transformers specified have dual primaries, wired in parallel (as shown in the schematic) that are good for 120V areas. For 240V areas wire in series. Check the transformer spec sheets that come with the transformer for exactly which pins are which. Not all of them will match the pinout in the schematic

Use the parts specified, the design has been tuned for these parts, if you substitute other parts it may not work as well. Don't skip the choke, it is what makes this design work. The resistors can be 1/4W or 1/2W carbon or metal film. Cheap ones work fine here, they don't have to be super expensive audiophile types. C1 should be a film type, I use the Sprauge orange drop, but other will work as well.

It is extremely important to heatsink the regulator. Without it it will burn up, not good! The one I speced is what I use, but there are many others that will work, as long as they can handle 3-4 Watts. The case of the regulator is NOT isolated! So make sure you use an insulator between the tab and the heatsink. These days I use the synthetic pads rather than mica insulators since you don't need to use the messy grease with them. Digikey has quite a selection to choose from. Pretty much any of the TO-220 pads will work.

The design is a bit unusual, it uses a choke in a low voltage design. This has some interesting properties. Primary among them is the use of a fairly small first cap which radically cuts down on the peak currents through the diodes. With low peak currents and Schottky diodes there is essentially no RF noise in this design. And on top of that I added a secondary snubber (C1 and R1) which prevents the transformer from resonating as well. The upshot is that this is one of the cleanest power supplies you will ever find, it has no RF and has one of the lowest impacts on the main AC line as its possible to get with a PS.

Since this is a power supply that plugs into the AC power line its probably not a good idea to build one of these if you have never touched a soldering iron before.

Oh yeah, you are on your own to get the right connector to fit whatever you want this to power. Radio Shack has a line of adaptaplugs which might come in handy for this.


John S.


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Topic - Power supply design for low power PC - John Swenson 16:46:07 01/03/09 (26)

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