The 20-pin power supply was used in IBM compatible PCs from 1995 to 2003, with outputs defined by the ATX specification. The 24-pin power supply was introduced in 2003, with added pins for more current. Dell PCs may use a 20-pin connection that is physically compatible with ATX but has different voltage lines. AT power supplies used two main connectors and did not include +3.3V DC outputs. Laptop power supplies are different from 20-pin power supplies and offer approximately +19V DC.
A 20-pin power supply is a standard power source used in International Business Machines (IBM) compatible personal computers (PCs) from 1995 to 2003. Its outputs, cables, and connectors are defined by the Advanced Technology computer form factor specification Extended (ATX) . The main power voltages are carried on a 20-pin connector that mates with an ATX motherboard. Switched-mode outputs of +3.3V, +5V, -5V, +12V, and -12V direct current (DC) are typically provided. The 20-pin power supply also includes some smaller connectors for hard drives, compact discs (CDs), and digital video discs (DVDs) drives and floppies.
As PCs became more complex and power hungry from 2000 to 2003, the ATX specification was extended. Special cables have been added to carry more current for processors and new types of hard drives. The -5V DC power supply, used by only some early Industry Standard Architecture (ISA) bus cards, was eventually phased out.
The 24-pin power supply was introduced in 2003. The first 20 pins of its new 24-pin connector are still compatible with the 20-pin power supply. The four added pins simply carry more current at most existing voltages. A 24-pin motherboard may still function if a 20-pin power connector is attached—the plastic connector may melt if the motherboard or its peripherals draw a lot of current, however. Since 2003, a few more connectors have been defined to power processors and graphics cards, and the 6-pin motherboard auxiliary connector has been removed.
Many Dell PCs use a 20-pin connection between the power supply and the motherboard. Some also include a 6-pin auxiliary connection. These Dell connectors physically correspond to ATX connectors. However, the location and number of lines for each voltage are different from ATX.
Serious damage is likely to occur if a proprietary Dell power supply is connected to an ATX motherboard or vice versa. Dell computers made since the early 2000s may actually follow the ATX specification. People should check model numbers with Dell to be certain before mixing power supplies and motherboards.
Before 20-pin power, compatible PCs followed the Advanced Technology (AT) specification. Its power supply uses two main connectors instead of one. Connectors are not always keyed to prevent rearward insertion. An AT power supply does not include +3.3V DC outputs. There is also no motherboard-initiated “soft” shutdown.
A PC compatible laptop power supply is not like a 20 pin power supply. The latter usually supplies 300 to 2,000 watts in stable DC voltages. A mobile power supply typically offers approximately +19V DC from 25 to 200 watts. Inside the laptop, this is converted and regulated to provide the required power. Other types of computers often have their own unique power specifications.
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