Why add the resistors if all I need is 12Vdc and that is what the output reads?
PC Power Supplies are built around a "switching" circuit. Because of this design, most of PC power supplies require a load for the unit to power up properly. Without a load, usually on the +5V line the PC power supply is built to shut itself off. This is especially true of the newer ATX style. The purpose of using resistors is to imitate the load of the hard drive, so the power supply cycles up to full output power. If the power supply doesn't sense a load the ATX styles shut off.
Some PC power supplies can be damaged if you try to power up with out a load.
By the way--remember if you plug in the power supply to house current the black cord of the older AT style power supplies always has 110 AC in it even when you have it switched off.
The power supply total watts (200 watts, 300 watts etc) is a rating of the max. design wattage of all circuits coming from the power supply. There is a smaller maximum wattage for each 3.3 v, 5 v, and 12 v line generated by the power supply. Some power supplies have circuits that also supply "stand by" power to the computer mother board. There are also outputs that are measured in "negative volts!" You will never get all the watts on one line ( for example on the +12v DC line).
Most power supplies have info on the unit labeling that will tell you how many max amps are designed to flow on each voltage line. Even on a 300 watt PC power supply, 8 amps max is typical for the 12 v DC line.
Watts = Volts X Amps
12 volts X 8 amps = 96 watts
So you can see that the power from these PC power supplies is limited. You will never be able to run high amperage devices from a single PC conversion. IF you try to pull to many amps through a converted power supply it will over heat and probably shut down. Be aware of the max amp rating and keep within the design capacity of the wires for that line. I have found that I will be able to run cabin lights and a fan off my conversion. Not much but it is all I want.
There are helpful links at the bottom of my page. Read about all the various approaches to this project. There are different solutions depending on what kind of power supply you are working with.
http://www.nfdc.net/home/cbdb/12%20V%20Power%20Supply.htm
It is possible to boost the volts and the amps, thus increase the watts but it is a more extensive conversion as the following link shows. On this web the way watts were increased was by combining three power supplies. Make sure you know what you are doing if you try this or you'll just get smoke.
http://www.qsl.net/i0jx/pcsupply.html
Take it step by step. Be Safe. Don't be afraid to read and learn. Hope this helps!
DEE
Grace happens...