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Using Computer Power Supplies

Another way to power your LCB is with one or more computer power supplies. The beauty of the computer PS is that it produces 3.3, 5 and 12 volt outputs (usually called “rails” in the computer PS world) with substantial current handling capability; up to 1200 watts combined (10 amps @ 120 volts) on really big power supplies. Computer power supplies are regulated and produce noise-free power. Its really the best possible power source on a budget, for under $100. A used computer PS in good condition will work well too; you may even have one lying around!

A typical 300 watt power supply compatible with older Dell desktop computers. The big white connector is the main power connector. This one sells for $42.

Should you modify a computer power supply to meet your needs? No, please don’t.

I’ve done it but I don’t recommend it for most people. There are some very large capacitors inside a computer power supply that can hold a big charge for a very long time. Serious injury is possible if the power supply is not fully discharged before opening it up. And, if you miswire it, a fire or electrocution hazard may result.

Thankfully, it is not necessary to modify a computer PS in order to use one. Today, there are a multitude of breakout adapters available that accept the main system board connector (20 or 24 pins) and give access to all the “rails.” Two styles of adapter are generally available.

One style consolidates all the rails into 4 power pairs: +12v/GND, +5v/GND, +3.3v/GND and -12v/GND. The limitation of this type is that the consolidated rails may be current-limited, so check specs carefully before purchase.

This type of breakout consolidates like rails into 4 outputs. This particular one has fuses limiting current to 5 amps on each output. For smaller layouts, that limitation should be acceptable.

The other style is a pure breakout, with each rail individually exposed for connection. Here the current limitations are the limits of individual rails. Combining rails (connect all like rails into one power output for each voltage type) maximizes the current available for your layout.

This power supply breakout gives access to all power supply rails.

Of the two types, the latter is probably best because it gives you the greatest flexibility and current handling capacity.

UPDATED: 1/18/23

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