If you exceed the
current rating of the connector, it may overheat and melt. Since the
Molex power connector is rated 6A/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 20-pin PSU has sufficient overall power rating. You may use various power supply wattage calculators to estimate your
requirements. Even if your system needs less than 355 watt, you still have to check the individual rail current distribution on the motherboard.
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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 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.
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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 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 20-pin 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).
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