|
A power supply is electrically like a
stack of batteries. |
| Vocademy |
The power supply (Power Supply Unit or PSU) has dangerously high voltages. The typical computer technician is not trained to work on power supply circuits, so it is unlikely that you will need to do so. However, knowing how they work can help you diagnose if a problem is with the power supply or another circuit.
Early power supplies were called battery eliminators. This is because batteries powered early consumer electronic devices, particularly radio receivers. Users had to buy batteries frequently or have batteries recharged at radio shops. Plug-in power supplies emulated batteries and thus eliminated the need for them.
Modern power supplies still take the place of batteries as they convert the high-voltage alternating current of the power grid to the low-voltage direct current needed by virtually all electronic equipment, including computers.
A particular power supply may have a single DC output like a single battery or multiple outputs like multiple batteries. A computer power supply looks electrically like a stack of batteries. Early PCs needed four voltage levels, so their power supplies looked like four batteries—two seven-volt batteries and two five-volt batteries—stacked end-to-end.
|
|
|
The complete power supply consists of four power supply circuits, each acting like a battery, with the supplies connected positive to negative as the batteries in the battery stack model. Most computer power supplies deliver 24 volts divided between -12 volts (required for serial ports), -5 volts (required for older DRAM chips and not present on all modern power supplies), +5 volts (required for most of the circuits that don't use 3.3 volts), and +12 volts (required for hard disk, floppy disk, and optical disk motors and case fans).
To an electrician, ground is a specific concept. It is a solid connection to the Earth. In electronics, ground (often called common or com) is an arbitrary voltage that is a reference from which all other voltages are measured. It is much like measuring altitude. We measure altitude from sea level and call sea level zero feet or meters of altitude. If you go above sea level, you are at a positive altitude; if you go below sea level, you are at a negative altitude (in El Centro, CA, your GPS receiver will give you a negative altitude, as it will in Death Valley or on the shore of the Dead Sea). A negative voltage is not the opposite of a positive voltage; it is just a lower voltage than some other voltage designated as ground.
In the above diagram, the center of the stack of batteries is designated as ground. If you put the black probe of a voltmeter at the -12 volt wire (blue) and the red probe at +12 volts (yellow), it will read +24 volts. However, by convention, we measure the voltage in a computer referenced to the ground designated at the center of the imaginary battery stack. Therefore, you should put the voltmeter's black probe at that point—the black wire. Then, the blue wire measures -12 volts, and the yellow wire measures +12 volts. The +5 volt and -5 volt wires work the same way.
Newer computers need a +3.3 volt supply. This would likely be a separate 3.3-volt power supply, with the negative terminal connected to the designated ground or a second regulator on the +5-volt supply. The +5 volt supply could be split into a 3.3 and 1.7 volt supply, but this would be too complex to be a likely design.
Motherboards also have on-board regulators to provide other voltages or to have regulators close to critical components (such as the CPU).
All computer power supplies have switching regulators. Think of a linear power supply like someone controlling the pressure of a water supply by carefully holding a valve to the right opening, depending on demand. In a power supply, the main regulating transistor acts like a valve and is held to the correct impedance for the current demand at the moment. However, such a transistor dissipates a large amount of power, wasting energy and producing heat. A switching regulator is like turning the valve completely on and off at regular intervals, keeping a reservoir filled to the correct level. A transistor doing the same job for electrical current only produces power while transitioning from on to off. So, a switching regulator has a transistor that switches on and off rapidly, staying on for a longer part of the cycle when there is more current demand. Large capacitors (which act like reservoirs) and coils (which also act to smooth out the voltage changes) convert the switching current to a steady DC (see Power Supplies in the Analog Circuits class).
Linear power supplies require large heat sinks to dissipate the heat generated by the regulator. They also require large, heavy transformers to reduce the voltage before conversion to DC (rectification) and regulation to reduce the amount of power the regulator dissipates (power is voltage multiplied by current). Switching power supplies don't require these large, heavy, and expensive components and are therefore preferred for computer power supplies. A linear power supply that can deliver 300 watts would be at least half the size of an entire desktop computer and weigh more than six kilograms or 13 pounds.The power supply provides a Power-OK (PWR_OK) signal. This prevents the computer from starting before the voltages stabilize.
Older power supplies had a switch near the power cord connection to switch between different power grid voltages. The switch is usually labeled for 115 V and 230 V. Configuring the switch to 230 V, where the power grid voltage is 115 V, would cause the power supply to supply insufficient voltage to run the computer. Configuring the switch to 115 V, where the power grid voltage is 230 V, would usually destroy the power supply.
Early power supplies had a mechanical power switch. This was usually a double-pole single-throw switch that cut the connection to both the hot and neutral wires from the grid power. Some power supplies had a single-pole, single-throw switch that only controlled the hot wire.
Modern power supplies are turned on and off by a signal from the motherboard and may or may not have a mechanical power switch. A power supply with no mechanical power switch is always in an on-state as long as it is plugged into the power grid., providing enough power for the motherboard to sense the pushbutton power switch.
Newer power supplies usually come with larger fans than older ones. The larger fan turns more slowly to produce the same cooling effect and is quieter.
The +12 output is often split among several wires (multiple +12 volt rails) to reduce the possibility of overheating wires with high current demand, even if there is a short circuit on the device being powered. Current is limited to 20 amps, and 18 amps should be available. Some manufacturers ignore this requirement, so not all power supplies have this safety feature.
For more information on the power connections to the motherboard, disk drives, video cards, and the front panel, see the respective chapters above. Here is a short review of the connectors.
 |
|
|
The rule for PC, XT, and AT power supplies is that the black wires go together on the motherboard connector.
 |
|
|
Power supplies no longer have connectors for floppy disks. The Molex connector is not used for SATA hard disks and optical drives.
 |
|
|
Many power supplies had no SATA power connectors during the transition from PATA to SATA drives. Adapters are available to adapt Molex connectors to SATA drives.
 |
|
|
The motherboard power connector can accommodate 20-pin or 24-pin motherboard power connectors. Four of the connections can be removed and left dangling.
 |
|
|
ATX power supplies and motherboards use a 20-pin or 24-pin power connector. Most ATX power supplies come with a motherboard connector with four detachable pins to fit 20-pin or 24-pin motherboard connectors. The connectors are keyed to make them difficult to plug in incorrectly.
 |
|
|
Pins 11, 12, 23, and 24 on the 24-pin connector supply extra power to the +12, +5 and +3.3 power rails. If the total system doesn't require extra power, a 20-pin power connector can be connected to a 24-pin motherboard connector. The connectors are keyed, so you cannot easily connect the power connector to the wrong end of the motherboard connector. Adapters are available to adapt 20-pin power connectors to 24-pin motherboard connectors, but they are unnecessary. However, upgrading the system to higher-performance graphics cards, etc., may require replacing the power supply.
Some manufacturers use their specific connectors that don't match the standard connector.
From 1996 to 2000, Dell used a motherboard connector identical to the standard ATX connector but wired differently. Connecting a Dell motherboard to a standard ATX power supply would destroy one or both. The same goes for connecting a standard ATX motherboard to a Dell power supply from that era. Double-check the specifications when working with Dell computers or parts from that era.
During the transition from AT to ATX standards, some power supplies had a six-pin auxiliary power connector that looked much like the PC/XT/AT power connectors. This provides 3.3 and 5 volts to motherboards that need it (usually AMD dual CPU motherboards).
Most motherboards have a separate connection for CPU power. This four-pin connector is recognized by having two yellow and two black wires.
 |
|
|
Adapter cables are available to adapt a Molex hard drive connector to an ATX CPU connector.
This is an 8-pin version of the 4-pin CPU power connector. It was required for motherboards with multiple CPUs. However, newer CPUs require less power, so this connector is rarely needed. The 8-pin CPU power connector on the motherboard can accept a 4-pin CPU power supply connector, and some are made to expect it. However, this doesn't always work.
Some high-performance video cards need more power than can be delivered through the expansion bus. These cards have one or more six or eight-pin PCIe power connectors.
 |
|
|
A graphics card can get 75 watts from the PCIe bus. A six-pin power connector can deliver 75 watts, and an eight-pin connector can deliver 150 watts. A graphics card will have a combination of six and eight-pin connectors as needed. Some PCIe power connectors have a six-and-two configuration to fit six or eight-pin connectors.
 |
|
|
Adapter cables are available to convert Molex hard drive power connectors to PCIe power connectors.
 |
|
|
Before buying a high-performance video card, be sure your power supply can match the power requirements and has the matching PCIe power connectors.
Most of the power connectors are covered above. However, there are some variations. If you find a different connector, use the usual sources of information to see how they are used.
—————————| Vocademy |