I have a good and expensive power supply from an old lap top. It provides 2,5A 20 volts DC current. It's absolutely perfect for another mini-itx project. However, it was a long time since I left school, and can hardly just remember Mr Ohm. I'm not in the mood for calculating on tyristors etc. I wonder if: a) Is there a dc-dc power supply for ATX that is satisfied if it's feeded with 20V DC? b) does anyone having a scheme on how to get decen 5v and 12v out of 20V without too much heat?
They make them for 19V input like this one but your supply is only 50W which seems a bit low, especially as the converter will lose at least another 10-15%. Same goes for if you try building your own. Plus the price of the converters are a lot more than a plain mains psu.
My supply is enough for a regular LapTop, should work for an VIA EDEN aswell in my opinion. I'm at work now, will doublecheck when I come home. 50W do sound a bit low even in my rusty ears. Thanks for the link, that DC-DC converter is a bit too much. I only need something to get the current down to 12V, I don't need anything to clean it up or so. Can I go with a tyristor, or will ut just give me heat and trouble?
A thyristor (silicon-controlled switch) is a switchable diode, you need a lot more bits for a switch-mode power supply.
I'm sorry if I'm hard to understand. My english isn't what it could... What I have here is a switch mode power supply, generating 20v with 3.5 (update from above). This current is well balanced error free. What I want is to use this power supply, but to get the voltage down to 12. After what I understand I wont need another switched power supply, or do I? If I do, isn't my existing completely useless then?
Im not so intelegent when it comes too this type of electronics. Digital cicuits im more familiar with. Transformer? Resistor w/ heatsink and fan Not sure exactly what your trying to do.
Using linear voltage regulators you can only get 3.5A total from both 5V and 12V lines added together, so most of the 70W will be wasted as heat. To do it efficiently needs another switching psu (a DC-DC converter) which will still waste over 10%, and AFAICS may need a high-frequency transformer winding to produce both 5V & 12V (or 2 separate DC-DC converters). Not easy to design, and the fact that the input is clean & accurate doesn't help much. You could search the Maxim site, I think they have free software to help SMPS design.
