Tools and Components
To do the mod, I need:
- PSU with sense wire (see below);
- Soldering iron, wires, and other accessories;
- 10,000Ohm variable resistor (1-3 pieces);
- 50Ohm fixed resistor (1-3 pieces);
- Insulation tape or heat-shrinkage pipe.
As mentioned above, the mandatory requirement says the PSU must have sense wires. Alas, if you’ve got a very cheap or two-or-three-years-old unit, you have a big chance of finding no feedback circuits at all. For example, the above-mentioned Codegens (at least, the three samples I use as stands for my bookshelf) have no sense wires, and thus cannot be modded. This is the worst case.
The best case is when you have three independent sense wires for all the three power rails. If your PSU cost you less than $80-100, there’s a little chance for that. I can only think of Antec, who equips its TruePower series cases with such a feedback circuitry. Brands like PC Power & Cooling and OCZ Technology can afford such solutions, too, since quality is more important for them than manufacturing costs. I also saw such a scheme in PSUs from Sweex (Gold Series 650W) and Antec’s OEM units under the CWT brand (the 550W model, to be exact).
PSUs from Sirtec that are selling under a score of different brands including Thermaltake and Chieftec, have one sense wire on the +3.3v circuit and control the circuits independently. That is, making the proposed modification you can only control the +3.3v rail, while the other two rails will be working independently, without any control on your side.
Some manufacturers use a scheme with two sense wires, one of which is responsible for +3.3v, while the other controls the two remaining circuits at once. That is, increasing the voltage on the +5v rail will result in a similar increase on the +12v rail.
Lastly, there is a scheme with one +3.3v sense wire that is used to control all the three circuits at once. This is the most inconvenient, but the cheapest solution from the ease-of-realization point of view. That’s why you are most likely to see it in cheap PSUs. Well, that’s anyway better than nothing!