The Switch-Mode Power Supply (SMPS) has 3 main functions:
- Convert A.C voltage taken from mains supply to into suitable D.C voltages and circulate appropriate power to each device.
- Monitor those output D.C voltages.
- Remove the heat that results from the consumption of electrical power.
SMPS first job is to accept energy from mains electricity supply via power cord connected to power receptible cord and convert it to D.C from raw A.C energy at several different voltages and store some of the energy so that it can bridge over small intervals in which the electrical input stops. Power Supply (SMPS) must store some enough power to at-least bride across those mini brownouts.
The Switch Mode Power Supply (SMPS) monitors the quality of D.C voltages and after output D.C voltages are stabilized at their nominal value(5%) which are suitable for different parts of PC it indicates to the motherboard that its output voltages are within range by raising a voltage on one of the wires going into the motherboard power supply connector. This wire is called Power-Good Line and rising of Power-Good Line’s voltage signals the CPU to start computing. This signal is called Power-Good Signal. In case the PG Signal goes down then the CPU will shut down.
There are 3 types of SMPS available namely:
- Advanced Technology (AT) – 12 pins connection to motherboard.
- Advanced Technology Extended (ATX) – 20 pins connection
- Balanced Technology Extended (BTX) – 24 pins connection
Here’s both AT and ATX SMPS wires giving their respective D.C voltages as the output and having different functions to perform. Common output DC voltages are +12V, +5V, -5V, -12V. Nowadays with increased CPU speeds, one way to prevent them from overheat and simply cooking themselves to death is to make them capable of running on lower voltages. Nowadays low output voltages used are +3.3V and 2.8V.
| Wire Color | AT Voltages | ATX Voltages | Function |
|---|---|---|---|
| Red | +5 V | +5 V | Data Transfer. R/W Data into Chips |
| Yellow | +12 V | +12 V | Running Motors |
| Blue | -12 V | -12 V | Backward Compatibility |
| White | -5 V | -5 V | Backward Compatibility |
| Black | 0 V | 0 V | Ground |
| Orange | +5 V (Power Good Signal) | +3.3 V | Specially for Running Processor (ATX) |
| Green | —— | +5 V | Power Supply ON/OFF |
| Purple | —— | +5 V | Auto Shutdown |
| Brown / Grey | —— | +5 V | Power Good Signal |
Besides SMPS holding some energy in reserve to bridge over mini brownouts, installing of special power supply containing both battery backup plus battery charger called UPS (Uninterrupted Power Supply) enables you to protect your PC from frequent brownouts or blackouts.
If you don’t have UPS then at least plug your PC into a power strip like Monster Digital PowerCenter that contains a really good surge arrester. These surge protecting devices can limit maximum voltages that come in from the mains power supply and protect your computer equipment.