USING a 20-PIN COMPUTER POWER SUPPLY
WITH a 24-PIN MOTHERBOARD


and visa versa


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Constantly changing computer specifications often cause power supply compatibility issues when you do a replacement or an upgrade. The frequent questions are how to connect an PC power supply with 20-pin main connector to a 24-pin motherboard or visa versa, and will a 20 pin ATX plug work in 24 pin socket?

Connecting a 20-pin PSU to a 24-pin motherboard.



You can plug a 20-pin ATX connector straight into a 24-pin motherboard socket, leaving pins 11, 12, 23, and 24 unconnected. The four pins you leave empty are just redundant pins for higher current capability (see A Guide to Computer Power Supply Connectors and Pinouts). The power connector is polarized, so you can't insert it wrong into the socket unless you try really hard.

However, the power supply compatibility is not just a mechanical issue. The extra four pins have been added to the main power cable for a reason- to support higher power requirements.
If you exceed the current rating of the connector, it may overheat and even melt. Since the Molex power connector is rated 6 amps per pin, with 20-pin connector you should not draw more than 12A on 3.3V, 24A on +5V and 6A on +12V (note that pin 11 is often used for 3.3V remote sense). The 4-pin 12V2 connector P4 is rated 8A/pin, so you can additionally get up to 16A on +12V. A simple math shows that with 20-pin main connector and 4-pin 12V2 connector your maximum power could be 355 watt. This is enought for most PCs, but if your system needs more current on any of the rails and the motherboard does not have an auxiliary 6-pin power connector (which has been removed from ATX 2.0 spec in 2003), you would be better off going with an ATX12V v2.0 compliant power supply with 24-pin connector even if your old PSU has sufficient overall power rating. You may use various online PSU wattage calculators to estimate your requirements. Even if your system needs overall less than 355 watt, you still need to know current distribution on the motherboard and check if you get required amps from each rail of your power supply.


In other words, if the motherboard requires 3.3V current over 12 A or +5V current over 24 A, and it does not have an aux 6-pin connector, you should use a 24-pin PSU. Note that 20 to 24 pin adapter cables are totally useless since they don't change the fact that all the currents are passing through a 20-pin power supply connector. These adapters actually make things worse because they add extra resistance between PSU and the load causing more voltage drop.

If you use high-end graphics card with an additional 6-pin or 8-pin power connector, you may try to use an adapter that converts two 4-pin peripheral cables into a 6-pin PCI Express cable, but the PSU typically has to be >400W, or you may not have enough current rating on 12 volt output.

PSUs built prior to 2003 may not have a serial ATA cable. In this case, you may try using an IDE to SATA adapter, which attachs to one of 4-pin peripheral connectors and converts it into SATA connector with missing 3.3V rail. It may work if your SATA device does not use 3.3V, which is often the case.

 ATX connector 20/24 pin
Finally, an old PSU may cause a start up problem. Today's motherboards are using PWR_OK signal for proper timing. Upon initial power up it should be asserted TTL logic high by the power supply within 100 to 500 ms after +12 VDC, +5 VDC, and +3.3 VDC outputs reach their regulation bands. Conversely, it has to switch to a TTL low state when any of positive DC voltages falls below its under voltage threshold, or at least 16 ms after input power has been removed. An old cheap PSU may not have "Power OK" circuitry and might just connect pin 8 to +5V. Modern motherboards may have booting problems and may not work without properly functioning PWR_OK signal.



Connecting 24 pin PSU to 20 pin board.



Many ATX12V v2.0 compliant PC power supplies come with 24/20 connector (sometimes called 20+4 connector) for backward compatibility. This connector has the last four pins on a detachable section that slips out, so you can plug it into an older motherboard (see the picture above). Just don't plug the remaining 4-pin piece anywhere- it is not compatible with any other connectors! If your PSU happens to have a solid 24-pin connector, you can still try to plug it into 20-pin socket with the last four pins hanging over the edge of the board's connector, or buy a 24 to 20 pin adapter.
The only potential problem with using a new PSU with an old computer is most today's PSUs do not provide minus 5V output (they either don't have the white wire going to pin 20 at all, or leave it not connected). Even some of 20-pin PSUs may not have "-5V", which was made optional in ATX12V v1.2 spec as of 2002. The minus 5V output was used by ISA cards. It was phased out when ISA were replaced by PCI. If you have an old computer with ISA slots, it will need -5V. In this case, if you can't find a power supply with -5V rail, you can derive it from -12V by adding a negative linear regulator such as MC79M05. Since minus 5V bus can draw up to 0.3A current according to ATX spec v.1, this regulator will dissipate up to 7x0.3=2.1W, so you need to attach it to a little heatsink.
Note: don't use this information without independent verification (see the Disclaimer linked below).