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A 20-pin power supply is a standard power source used in International Business Machines (IBM) Personal Computer (PC) compatibles from 1995 through 2003. Its outputs, cables and connectors are defined by the Advanced Technology Extended (ATX) computer form factor specification. The main power supply 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 usually provided. The 20-pin power supply also includes a few smaller connectors for hard drives, compact discs (CD) and digital video disc (DVD) drives and floppies.
As PCs grew more complex and power-hungry from 2000 through 2003, the ATX specification was extended. Specialty cables were added to carry more current for processors and new types of hard drives. The -5v DC supply—only used by a few 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 of the existing voltages. A 24-pin motherboard may still work if a 20-pin power supply connector is plugged into it—the plastic connector could melt if the motherboard or its peripherals draw a lot of current, however. A few more connectors have been defined since 2003 for powering processors and graphics cards, and the 6-pin auxiliary motherboard connector has been removed.
Many PCs from Dell™ use a 20-pin connection between the power supply and motherboard. Some also include a 6-pin auxiliary connection. These Dell™ connectors physically match the ATX connectors. The location and the number of lines for each voltage are different from ATX, however.
Serious damage will likely result 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. Individuals should check the model numbers with Dell™ to be certain before mixing power supplies and motherboards.
Prior to the 20-pin power supply, PC-compatibles followed the Advanced Technology (AT) specification. Its power supply uses two main connectors instead of one. The connectors are not always keyed to prevent backward insertion. An AT power supply does not include +3.3v DC outputs. There is also no motherboard-initiated "soft" shutdown available.
A PC-compatible laptop power supply is nothing like a 20-pin power supply. The latter usually provides 300 to 2,000 watts in stable DC voltages. A mobile power supply typically offers about +19v DC at 25 to 200 watts. Inside the laptop, this is converted and regulated to provide the needed power. Other types of computers often have their own unique power supply specifications.