Power supplies provide power to the PC motherboard as well as the Hard drives, floppy drives, CDroms or DVD drives connected to your system. Obviously, if you are planning to build your own computer... power will be critical to your success. It is important to have well regulated and efficient power supply so that the various components in the PC function properly.

Linear Power Supply :

Linear power supply is not efficient and hence not used in the PCs. The disadvantage of this power supply is that it uses large size components, such as large power diodes and large transformers. Hence, it is bulky and requires more space. It also dissipates great amount of heat and the regulation is poor.

In case of Linear power supplies, the input rectifiers rectify the AC input voltage. The output is a rectified AC voltage. This output voltage is filtered through a series of capacitors and inductors and finally the rectified DC power is obtained .In the process lot of power is lost in the input as well as the output state. Hence, the power supply is not efficient and can perform only upto 32% efficiency.

Switch Mode Power Supply :

Switch mode power supply functions very efficiently (90%). It consumes less space and can be packed into a small box. The components of which the power supply is made are much smaller in size as compared to the linear power supplies, hence, the overall size of the power supply is reduced. The power supply is suitable for supplying DC power to the mother board as well as the hard disks, floppy drives, CD Rom drives and DVD drives.

In case of Switch mode power supply the Switching mode principle is used to achieve high amount of efficiency. You can understand how the Switch mode power supply functions from the steps given below:

1) The AC Voltage is applied to the primary rectifier to get the Rectified AC voltage.

2) The AC Voltage is filtered by the Primary filter.

3) The filtered AC voltage is applied to the input of the solid state transistor switch to get the pulsating (chopped) DC output voltage. The switching transistor acts like a chopper, which breaks up the primary DC into chopped DC.

4) The chopped DC voltage is applied to a step down power transformer. This DC voltage has a long duty cycle. This duty cycle affects the final output DC voltage. If the duty cycle is large then the output voltage is large and if the duty cycle is short then the output voltage is low.

5) The AC voltage obtained at the secondary of the transformer is again rectified by the secondary rectifiers. The output is a rectified DC voltage.

6) The final rectified DC voltage is filtered by means of the output filtering circuits and final DC voltages are obtained. This voltage is applied to the load, ie the motherboard and the Hard drives, Floppy drives, CDRom drives or DVD drives.

7) The output voltage changes quite frequently. When the load increases the output voltage drops down. As soon as the load decreases the output voltage rises. In an ideal power supply, however, the output voltage must be maintained steady even if there is fluctuation in the load. This ensures that the PC components run smoothly. In switch mode power supply this is achieved by means of a feedback circuit. The output voltage is sensed and part of it is fed back to the input of the Switching transistor. The feedback circuit ensures that if the load increases the duty cycle decreases and if the load decreases the duty cycle increases. Hence, the voltage is kept constant even if there is fluctuation in the load.

Instead of using separate transistors as switching and sensing elements, modern power supplies use a single IC chip, which has inbuilt switching and sensing elements. This helps to achieve better and smaller power supply reducing the overall cost.

The Advantage of the Switch mode power supply is that there is less power consumption in the input section as well as the output section. Thus, the Switch mode power supply is more efficient than the linear power supply.

Another advantage is that the switch mode power supply produces regulated DC power suitable for the safe running of the components of the PC.

The Switch mode power supply is compact and occupies less space inside the PC.

The disadvantage of the Switch mode supply is that it produced excess noise and the output voltage always contains some amount of high frequency ripple.

Another disadvantage is that there are more components in a Switch mode power supply making it difficult to repair.

Power Supply Connections.

The AT – style power-supply provides four voltages to the motherboard. +12 V, -12V, +5V and -5V. The power is provided by means of two six pin connectors. One pin on the connectors is reserved for the power good signal.

The ATX and NLX style power supplies use only one 20 pin connector to supply power to the mother board. The power supplies provide 5 different voltages (+12 V,-12V, +5V,-5V and +3.3V). Additionally, logic signals are used to control the power. When the logic signal PS-On is low it turns on all of the main power outputs. When this signal is held high or left open the power-supply outputs turn off.

Besides powering the motherboard the power supplies provide power to the various types of drives present in the system. The Hard drives, floppy drives, CD Roms etc are provided DC power by means of connectors from the power supply. The power supply uses the heavy duty four wire connectors to provide +12V and +5V to each of the drives.