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In Reply to: RE: Power supply design for low power PC posted by John Swenson on January 03, 2009 at 16:46:07
Thanks for your great work, but fit-PC Slim specifications declares power 12V single supply +/-20% (9.6 minimal), not 9V.
http://www.fit-pc.com/new/fit-pc-slim-specifications.html
Follow Ups:
You can change the 300 to a 330 and you will get 10V. I would not go 12V with the speced transformer. But there is no need for 12V. The voltage just goes into a 5V regulator in the slim, all going to 12V gets you is a hotter box because the regulator has to drop more voltage.
If you really want to go with 12V use the VPS36-1200 and run the secondaries in paralel instead of in series as in the schematic. Change the resistor values to 430 and 51 that will give 11.8V. If you use 1% resistors you can tweak the voltage even closer. But again I don't think that is necessary.
John S.
Why wouldn't the 16VAC @2.7A rated Triad VPS16-2700 in your present design not be sufficient for a 12VDC regulated supply, especially with lower voltage drop Schottky diodes, low drop-out voltage 3-terminal regulator, and significantly lower current draw than the transformer's rated 2.7A for 16VAC. I don't see a need for going with an 18VAC secondary on the transformer.
When using 115V main the bottom of the ripple is going to be about 12.5V which is a little less than whats necessary for regulation. Yes the voltage will be a little higher due to less than max current draw, but not by all that much, these transformers actually have pretty good regulation characteristics. If you can guarantee to be running at 120V at all times then it would probably work fine. I'm just trying to give enough margine so that at worst case conditions (high current draw and low mains voltage) it will still be in regulation.
John S.
You can adjust the value of R2 and R3 to get 12vdc out if you like or use the fixed value version of the IC regulator: LT1085CT-12 or LT1084CT-12. (In the case of the fixed regulators, simply ground pin 1 and eliminate R2 & R3).
The power supply is pretty much text book & app note from the chip maker. It's a basic bridge rectifier followed by a pi section filter and low drop-out voltage adjustable regulator IC made by Linear Technologies. While it's more common to find a pi filter with choke in a high voltage tube power supply, nothing says you can't use one in a low voltage setup.
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