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Switch mode
Practical power supplies

Dark secrets

Not everyone was happy about switch mode power supplies. They were and are difficult to design correctly and if you get things wrong they generate electrical interference that can find its way back into the mains to cause problems with TV and radio reception. Of course to day switch mode power supply design is fairly well mastered and such things don’t usually happen.

In the early days of the microcomputer the BBC was looking for a machine to call its own one of the specifications was that it shouldn’t have a switch mode power supply – presumably to avoid interference with TV reception, or more likely to rule out Apple from the contest!

The first Acorn design also had a switch mode power supply but this was quickly ripped out to be replaced by a linear supply of about the same size. This was fine but it ran hot enough to fry an egg and it was quickly replaced by a switch mode design soon after it became the BBC micro!

 

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Practical Power

It may be a strange thought that the personal computer revolution depended on an advance in something as basic as a power supply but it did and knowing this is important today.

The reason is that modern PC power supplies are complicated pieces of electronics. If they fail then don’t even think of trying to repair them! You may know a little electronics and think that a linear power supply is the easiest thing in the world to repair – true enough but a switch mode as found in a typical PC is next to impossible to repair and it is much cheaper to buy a new unit. The bad news is that there are eight standard types of power supply in use with the PC. In roughly chronological order we have:

  • PC/XT – the original big silver box
  • AT/Desk – big silver box with switch
  • AT/Tower – big silver box without switch
  • Baby AT – not quite so big box with switch
  • LPX – smaller than Baby AT
  • ATX – used in nearly all modern machines
  • NLX – same as ATX but for servers
  • SFX – very small and intended for micro systems

Fortunately you can still buy replacements for all of these types – just describe the unit in terms of its size, wattage and switch type.

Notice that ATX and later power supplies have 3.3V connections suitable for driving the newer processors without a wasteful additional power regulator on the motherboard. They also support “power-on” and “5V_standby” control lines enabling the machine to be switched on and off using nothing but the keyboard or software.

One final comment about sophistication. All PC power supplies after the PC/XT perform an initial diagnostic test and if they are working perfectly they set a “Power Good” control line that plugs into the motherboard. Without this signal the processor doesn’t run. So if you have a completely dead machine but the fans and disks are spinning it might be a loss of the “Power Good” signal causing the problem.


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