These power supplies obtain their power from the a.c. electricity supply of 230 V at 50 Hz in Europe, or 115 V at 60 Hz in America. Most electronic circuits obtain their operating power from d.c. voltage supplies. Some circuits, especially in portable equipment such as radios, mobile phones and lap-top computers which consume low power, use batteries to provide the d.c. voltages directly. Other equipment uses electronic power supplies to provide the required d.c. voltages.
There are three main types, as shown in picture above
- The simple unregulated ‘linear’ supply, shown in picture (a). This uses a transformer to convert the a.c. input to a lower voltage, usually in the range 6-24 V, followed by a rectifier to convert the low-voltage a.c. to d.c, and a capacitor which filters out most of the supply frequency ‘ripple’, leaving a fairly smooth d.c. output voltage.
- The regulated linear supply, shown in picture (b). This is basically an unregulated supply followed by an electronic regulator circuit to ‘regulate’ the output. The regulator uses analog circuit techniques to hold the output voltage constant in spite of changes in the a.c. supply voltage or in the output load current.
- The switched-mode supply, shown in picture (c). This type is widely used in television receivers and personal computers, and other equipment which uses a number of different voltage supplies. Its main advantage, in such equipment, is that the bulky and heavy supply frequency input transformer of the linear supply is replaced by much smaller, lighter and cheaper components.
The input a.c. supply is rectified directly to d.c, and this high-voltage d.c. is ‘chopped’ into a.c. at a rate just above the highest audible frequency, typically about 40 kHz. (Otherwise the chopping would be heard as a whistle) This a.c. voltage is converted to several different voltages by a transformer which, because it works at a much higher frequency, is much smaller and lighter than the linear-supply equivalent. (It also provides the electrical isolation needed for safety.) The transformer outputs are rectified and smoothed to provide the low-voltage d.c. supphes and, in computer monitors and TV sets using c.r.t. displays, an ‘extra-high-tension’ (EHT) supply, typically at 25 kV. Regulation can be achieved by sensing one of the output voltages and feeding-back a correction signal to the chopper circuit to adjust the power fed from the input supply to the transformer.
Uninterruptible Power Supply (UPS)
One further example of an electronic supply is the uninterruptible power supply (UPS). One type is shown in piture below. This has a quite different purpose to those described so far. It is intended to provide an a.c. supply at the same voltage and frequency as the local electricity supply, that is 230 V at 50 Hz or 115 V at 60 Hz. It is used in situations where a failure of the incoming a.c. supply would be disastrous. Examples are hospital operating theatres and intensive-care units, where a power cut would endanger lives; and computers, where even a momentary interruption can cause an expensive loss of all the current data in the computer’s working memory (the random-access memory or RAM).
During the incoming power failure, the UPS obtains its power from high-capacity storage batteries, and continues to supply the required a.c. without interruption. Clearly, the supply from the UPS will last only as long as the charge in the batteries, but even a few minutes will allow a computer operator to save all current work. Users such as hospitals may have sufficient battery capacity for 30min or more, and commonly have back-up engine-driven generators for longer power failures.
When the incoming power is restored, it re-charges the batteries, ready for the next time!