Power supplies

Power supplies: theory, diagnostics and fixing.

Summary

Many pieces of electronic equipment include a power supply unit (often abbreviated to PSU) to convert the incoming electrical power from the mains or a battery to the form needed. This can involve the control and conversion of a relatively large amount of power in a fairly small space and some of the power will be wasted as heat. Heat can lead to failures.

Why is a power supply needed?

A power supply performs some or all of the following purposes:

• Voltage conversion:
  

  AC waveform
  The mains electricity supply is 240v, which is much too high for most electronic gadgets and is dangerous if fingers might be exposed to it, so a voltage reduction may be required. In a few cases (e.g. a microwave oven or the supply to a camera flash tube), conversion to a higher voltage is required.
• Isolation: Even though the voltage might be reduced, if there remains a direct electrical path to the mains it can still be dangerous. Professional power tools, especially if used outdoors, are often powered through an isolation transformer in order to break that path.
• Rectification:
  

  Recified AC
  The mains supply is AC, which means it reverses direction 100 times per second. Whilst this is fine for heating, some forms of lighting and most electric motors, electronics normally requires DC, which just goes in one direction. Rectification is the technical term for AC to DC conversion.
• Smoothing:
  

  Rectified and smoothed AC
  AC momentarily drops to zero as it reverses direction, and if you simply rectify it, it will still be dropping to zero 100 times per second. This may be fine for example if you're just using it charge a battery, but electronic equipment generally can't cope with this. In audio equipment, for instance, it would cause a very loud buzz which would obliterate the music. Smoothing stores electrical energy to fill in the gaps and smooth the flow, a bit like the way the silencer on a car smooths the exhaust flow to make it quieter.
• Regulation: If the input mains voltage varies, the rectified and smoothed output will vary in proportion. And unless large and expensive smoothing components are used, there will remain a certain amount of "ripple" which can cause an annoying hum in audio equipment. If the power is supplied by a battery its voltage will drop as it is exhausted. Electronic equipment often needs a supply which is not only very smooth but a precise voltage. Regulation provides this. There are two kinds of voltage regulator:
  • In a linear regulator, excess input power is simply absorbed and turned into heat, just a little if the excess is small or more if it's greater. This is a bit like keeping your foot on the accelerator of your car and controlling your speed with the brake. It's wasteful but cheap, nevertheless it may be good enough for low power devices unless they need to squeeze the maximum life out of a battery.
  • In a switching regulator the power is switched on and off up to a few million times per second, with the on-time continually adjusted to compensate for variations in the input or in the power drawn by the load. This is like a room thermostat which is on for a greater proportion of the time the colder it is outside. Just as the thermal inertia of the room smooths out the temperature variations as the thermostat switches on and off, the output of a switching regulator needs to be smoothed. However, the smoothing components can be very much smaller than are required for smoothing rectified mains as they only need to store energy for typically a few millionths instead of a hundredth of a second. A small coil is generally used which stores the energy as magnetism. A well designed switching regulator can be made very efficient, but is more complicated and more expensive that a linear regulator.

A switching-mode power supply as used with a laptop or in a phone charger generally includes all of the above elements but not in the same order. The raw mains input is rectified and roughly smoothed, giving mains voltage DC. This is then fed into a switching regulator. The switching regulator includes a transformer to provide isolation as well as to reduce the voltage to a more manageable level. Since the transformer is operating at a very high switching speed it can be much smaller than a mains transformer handling the same power, since it's transforming the power in very much smaller bites. Without a bulky mains transformer containing a lot of iron and copper, a switching-mode power supply can be made much lighter and more compact. In fact, it can be made cheaper as the cost of modern electronics is less than the cost of the iron and copper it replaces.

Theory and operation of power supplies

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Safety

Mains driven power supplies can be dangerous to work on unless you fully understand what you're doing. They often contain large capacitors which can store dangerous voltages even after you've disconected the mains supply.

External links

• External links as bullet points