old PC power supply - Model Railroader Magazine

 I agree. If you don't knwo a resistor from a light bulb, and don't knwo which end of the sodlerign iron to hold, do NOT go about attempting to modify a PC power supply. It WILL 'bite' you. And a sudden electric shock, even if the shock itself doesn't harm you, can make you do things lik jerk your arm involuntaroly and plot the sodlering iron in your eye. Not only is AC line voltage applied any tiem the cord is plugged in, but a switching power supply, as all computer power supplies made in this century are, has large value capacitors that can store quite a charge for longer than you might thing, ready to zap the unsuspecting tinkerer.

 If you really do understand the hows and whys of pwoer supplies, then use one of the sites linked above for directions. A PC power supply can indeed make a useful source of power, but one thing to be aware of is that they are NOT isolated from the AC input like the typical DCC power supplies. The ground carries through. Do not mix items powered by the PC power supply with ones powered by an isolated supply or another one that may also tie the ground to a different outlet.

 As for amount of power - the total pwoer suppyl rating generally cannot be drawn by a single voltage. That 500 watt power supply cannot put out a full 500 watts at 12V. Look on the sticker, there are generally maximums for each different voltage. But since a Tortoise draws about 15ma, a 15 amp 12V output can drive 1000 of them.

 Another note - yes all the yellow wires are the same voltage. But do not try to draw the full current from just one wire, it's not rated for it. Parallel a bunch of them. And run MULTIPLE SMALL FUSES. 12 volts at 15 amps is a good welder. ANd can easily fry electronic things or melt wires. Break it up at the power supply into multiple circuits with 1-1.5 amp fuses or circuit breakers.

                                                       --Randy