I have a question, Can 500 Watt Power Supply can be converted into model railroad power?
No, or a least without a lot of reengineering to make it work. The problem is the way the switching power supplies work. A simple model railroad power supply is a simple rheostat that controls the output voltage on the DC side, or a varistor on the AC circuit. A computer power supply is a switching power supply which works totally different.
Without going into too many more details than this, the computer supply needs a known-good feedback loop in order to stay on, and a constant load to power up. Without the load, the switcher can blow up if it is not configured properly.
Please don't consider modifying a switching-type power supply. These have high voltages and currents, and are very dangerous to work on.
See this here:
Surplus PC power supplies are widely available and inexpensive. However, what do you do if 5 V isn't exactly what you need for a project?
(From: Winfield Hill (hill@rowland.org).)
Some of the PC power supplies I've dissected do have pots, by they have a limited voltage-adjustment range. One interesting thing, every design used a TL431 chip, which is a 3-pin TO-92 regulating IC, as the voltage reference and opto-feedback component. Find this chip and trace out the resistors connected to it to determine which part to change to make a higher voltage. But, watch out for the SCR over-voltage circuit in some supplies. This is usually set to trip around 6 to 6.5 volts, and its trip point would need to be modified as well. As far as the step-down transformer turns ratio, there's little trouble one will encounter here, because the power supply is no doubt designed to function properly with reduced AC line voltages. The penalty one will pay for turning up the output voltage is a higher minimum AC voltage.
In most designs, the +12 and -12 V supplies merely track the 5V supply, and are not separately regulated. They may soar to higher voltages anyway if unloaded, but will be additionally increased in voltage by the ratio of 5V output increase. Even though the rating of the 5V electrolytic may not be exceeded, and still have a sufficient safety margin, this may not be the case for the 12 V outputs. So that issue should be examined as well.
Finally, a reminder for any reader tempted to break open the box and start experimenting. Voltages of up to 320 V are present, so be careful. Know what you're doing. For safety, stay away from open supplies when plugged in, or always keep one hand behind your back when probing. Remember a the AC bridge and HV DC and flyback transformer portion of all these supplies is operating straight from the AC line, so don't connect the ground of your oscilloscope to any of that circuitry. A battery-operated multimeter is best.
All taken from:
http://www.repairfaq.org/sam/smpsfaq.htm
Power supplies are a dangerous lot to work on. I can't repeat this enough. They can blow up! Seriously! As a technician, I was zapped a few times from residual power in some capacitors in CRT circuits. This was only voltage with low current, but it was enough to hurt and jar my hand enough to cut it on the chassis. I've also been zapped by what were thought to be "dead" power supplies. They were left in a box to be repaired when we had time to do so. Some of these power supplies were in that box for about 3 or 4 months. Guess what? The capacitors still held a charge, and one of them put me on my backside! Getting hit by 320V at 1A, really, really hurts!
John
